Flight respiration and energetics

Effects of age on oxidative stress and locomotion in the pollinator, Megachile rotundata

Despite numerous aging studies, the relationship between oxidative stress, aging, and decline in functions such as locomotion is still debated. Insects offer a promising model for analyzing the relationship between oxidative stress and aging, because they exhibit vast differences in lifespan that may be affected by the environment, social factors, levels of activity, and aging interventions. In this study, we explore the effects of aging on oxidative stress and locomotion using the pollinator, Megachile rotundata, a species that is very mobile and active in the adult stage. Across the adult lifespan of M. rotundata, we assessed changes in walking, flight, oxidative damage, and antioxidant defenses. Our results suggest that M. rotundata experience age-related declines in flight, but not walking. Additionally, we found that oxidative damage and antioxidant capacity initially increase with age and physical activity, but then levels are maintained. Overall, these data show that M. rotundata, like some other organisms, may not perfectly follow the free radical theory of aging.

Commentary: The microbial dependence continuum: Towards a comparative physiology approach to understand host reliance on microbes

Comparative physiologists often compare physiological traits across organisms to understand the selective pressures influencing their evolution in different environments. Traditionally focused on the organisms themselves, comparative physiology has more recently incorporated studies of the microbiome-the communities of microbes living in and on animals that influence host physiology. In this commentary, we describe the utility of applying a comparative framework to study the microbiome, particularly in understanding how hosts vary in their dependence on microbial communities for physiological function, a concept we term the "microbial dependence continuum". This hypothesis suggests that hosts exist on a spectrum ranging from high to low reliance on their microbiota. Certain physiological traits may be highly dependent on microbes for proper function in some species but microbially independent in others. Comparative physiology can elucidate the selective pressures driving species along this continuum. Here, we discuss the microbial dependence continuum in detail and how comparative physiology can be useful to study it. Then, we discuss two example traits, herbivory and flight, where comparative physiology has helped reveal the selective pressures influencing host dependence on microbial communities. Lastly, we discuss useful experimental approaches for studying the microbial dependence continuum in a comparative physiology context.

Altering developmental oxygen exposure influences thermoregulation and flight performance of Manduca sexta

Endothermic, flying insects are capable of some of the highest recorded metabolic rates. This high aerobic demand is made possible by the insect's tracheal system, which supplies the flight muscles with oxygen. Many studies focus on metabolic responses to acute changes in oxygen to test the limits of the insect flight metabolic system, with some flying insects exhibiting oxygen limitation in flight metabolism. These acute studies do not account for possible changes induced by developmental phenotypic plasticity in response to chronic changes in oxygen levels. The endothermic moth Manduca sexta is a model organism that is easy to raise and exhibits a high thorax temperature during flight (∼40°C). In this study, we examined the effects of developmental oxygen exposure during the larval, pupal and adult stages on the adult moth's aerobic performance. We measured flight critical oxygen partial pressure (Pcrit-), thorax temperature and thermoregulating metabolic rate to understand the extent of developmental plasticity as well as effects of developmental oxygen levels on endothermic capacity. We found that developing in hypoxia (10% oxygen) decreased thermoregulating thorax temperature when compared with moths raised in normoxia or hyperoxia (30% oxygen), when moths were warming up in atmospheres with 21-30% oxygen. In addition, moths raised in hypoxia had lower critical oxygen levels when flying. These results suggest that chronic developmental exposure to hypoxia affects the adult metabolic phenotype and potentially has implications for thermoregulatory and flight behavior.

Body-size-dependent effects of landscape-level resource energetics on pollinator abundance in woodland remnants

Land use change alters floral resource availability, thereby contributing to declines in important pollinators. However, the severity of land use impact varies by species, influenced by factors such as dispersal ability and resource specialization, both of which can correlate with body size. Here. we test whether floral resource availability in the surrounding landscape (the 'matrix') influences bee species' abundance in isolated remnant woodlands, and whether this effect varies with body size. We sampled quantitative flower-visitation networks within woodland remnants and quantified floral energy resources (nectar and pollen calories) available to each bee species both within the woodland and the matrix. Bee abundance in woodland increased with floral energy resources in the surrounding matrix, with strongest effects on larger-bodied species. Our findings suggest important but size-dependent effects of declining matrix floral resources on the persistence of bees in remnant woodlands, highlighting the need to incorporate landscape-level floral resources in conservation planning for pollinators in threatened natural habitats.

Investigating the thermal sensitivity of key enzymes involved in the energetic metabolism of three insect species

The metabolic responses of insects to high temperatures have been linked to their mitochondrial substrate oxidation capacity. However, the mechanism behind this mitochondrial flexibility is not well understood. Here, we used three insect species with different thermal tolerances (the honey bee, Apis mellifera; the fruit fly, Drosophila melanogaster; and the potato beetle, Leptinotarsa decemlineata) to characterize the thermal sensitivity of different metabolic enzymes. Specifically, we measured activity of enzymes involved in glycolysis (hexokinase, HK; pyruvate kinase, PK; and lactate dehydrogenase, LDH), pyruvate oxidation and the tricarboxylic acid cycle (pyruvate dehydrogenase, PDH; citrate synthase, CS; malate dehydrogenase, MDH; and aspartate aminotransferase, AAT), and the electron transport system (Complex I, CI; Complex II, CII; mitochondrial glycerol-3-phosphate dehydrogenase, mG3PDH; proline dehydrogenase, ProDH; and Complex IV, CIV), as well as that of ATP synthase (CV) at 18, 24, 30, 36, 42 and 45°C. Our results show that at high temperature, all three species have significantly increased activity of enzymes linked to FADH2 oxidation, specifically CII and mG3PDH. In fruit flies and honey bees, this coincides with a significant decrease of PDH and CS activity, respectively, that would limit NADH production. This is in line with the switch from NADH-linked substrates to FADH2-linked substrates previously observed with mitochondrial oxygen consumption. Thus, we demonstrate that even though the three insect species have a different metabolic regulation, a similar response to high temperature involving CII and mG3PDH is observed, denoting the importance of these proteins for thermal tolerance in insects.

Chronic cadmium exposure impairs flight behavior by dampening flight muscle carbon metabolism in bumblebees

Cadmium pollution affects the global ecosystem because cadmium can be transferred up the food chain. The bumblebee, Bombus terrestris, is an important insect pollinator. Their foraging activity on flowers exposes them to harmful heavy metals, which damages their health and leads to massive population declines. However, the effects of chronic exposure to heavy metals on the flight performance of bumblebees have not yet been characterized. Here, we studied variation in the flight capacity of bumblebees induced by chronic cadmium exposure at field-realistic concentrations using behavioral, physiological, and molecular approaches. Chronic cadmium exposure caused a significant reduction in the duration, distance, and mean velocity of bumblebee flight. Transcriptome analysis showed that the impairment of carbon metabolism and mitochondrial dysfunction in the flight muscle were the primary causes. Physiological, biochemical, and metabolomic analyses validated disruptions in energy metabolism, and impairments in mitochondrial respiratory chain complexes activities. Histological analysis revealed muscle fiber damage and mitochondrial loss. Exogenous decanoic acid or citric acid partially restored sustained flight ability of bumblebees by mitigating muscle fiber damage and increasing energy generation. These findings provide insights into how long-term cadmium stress affects the flight ability of insects and will aid human muscle or exercise-related disease research.

Heat tolerance in Drosophila melanogaster is influenced by oxygen conditions and mutations in cell size control pathways

Understanding metabolic performance limitations is key to explaining the past, present and future of life. We investigated whether heat tolerance in actively flying Drosophila melanogaster is modified by individual differences in cell size and the amount of oxygen in the environment. We used two mutants with loss-of-function mutations in cell size control associated with the target of rapamycin (TOR)/insulin pathways, showing reduced (mutant rictorΔ2) or increased (mutant Mnt1) cell size in different body tissues compared to controls. Flies were exposed to a steady increase in temperature under normoxia and hypoxia until they collapsed. The upper critical temperature decreased in response to each mutation type as well as under hypoxia. Females, which have larger cells than males, had lower heat tolerance than males. Altogether, mutations in cell cycle control pathways, differences in cell size and differences in oxygen availability affected heat tolerance, but existing theories on the roles of cell size and tissue oxygenation in metabolic performance can only partially explain our results. A better understanding of how the cellular composition of the body affects metabolism may depend on the development of research models that help separate various interfering physiological parameters from the exclusive influence of cell size. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Moderate mass loss enhances flight performance via alteration of flight kinematics and postures in a passerine bird

Many birds experience fluctuations in body mass throughout the annual life cycle. The flight efficiency hypothesis posits that adaptive mass loss can enhance avian flight ability. However, whether birds can increase additional wing loading following mass loss and how birds adjust flight kinematics and postures remain largely unexplored. We investigated physiological changes in body condition in breeding female Eurasian tree sparrows (Passer montanus) through a dietary restriction experiment and determined the changes in flight kinematics and postures. Body mass decreased significantly, but the external maximum load and mass-corrected total load increased significantly after 3 days of dietary restriction. After 6 days of dietary restriction (DR6), hematocrit, pectoralis and hepatic fat content, take-off speed, theoretical maximum range speed and maximum power speed declined significantly. Notably, the load capacity and power margin remained unchanged relative to the control group. The wing stroke amplitude and relative downstroke duration were not affected by the interaction between diet restriction and extra load. Wing stroke amplitude significantly increased after DR6 treatment, while the relative downstroke duration significantly decreased. The stroke plane angle significantly increased after DR6 treatment only in the load-free condition. In addition, the sparrows adjusted their body angle and stroke plane angle in response to the extra load, but stroke amplitude and wingbeat frequency remained unchanged. Therefore, birds can maintain and even enhance their flight performance by adjusting flight kinematics and postures after a short-term mass loss.

Heritability and preadult survivorship costs of ectoparasite resistance in the naturally occurring Drosophila-Gamasodes mite system

Our understanding of the evolutionary significance of ectoparasites in natural communities is limited by a paucity of information concerning the mechanisms and heritability of resistance to this ubiquitous group of organisms. Here, we report the results of artificial selection for increasing ectoparasite resistance in replicate lines of Drosophila melanogaster derived from a field-fresh population. Resistance, as ability to avoid infestation by naturally co-occurring Gamasodes queenslandicus mites, increased significantly in response to selection and realized heritability (SE) was estimated to be 0.11 (0.0090). Deployment of energetically expensive bursts of flight from the substrate was a main mechanism of host resistance that responded to selection, aligning with previously documented metabolic costs of fly behavioral defenses. Host body size, which affects parasitism rate in some fly-mite systems, was not shifted by selection. In contrast, resistant lines expressed significant reductions in larva-to-adult survivorship with increasing toxic (ammonia) stress, identifying an environmentally modulated preadult cost of resistance. Flies selected for resistance to G. queenslandicus were also more resistant to a different mite, Macrocheles subbadius, suggesting that we documented genetic variation and a pleiotropic cost of broad-spectrum behavioral immunity against ectoparasites. The results demonstrate significant evolutionary potential of resistance to an ecologically important class of parasites.

Toxic temperatures: Bee behaviours exhibit divergent pesticide toxicity relationships with warming

Climate change and agricultural intensification are exposing insect pollinators to temperature extremes and increasing pesticide usage. Yet, we lack good quantification of how temperature modulates the sublethal effects of pesticides on behaviours vital for fitness and pollination performance. Consequently, we are uncertain if warming decreases or increases the severity of different pesticide impacts, and whether separate behaviours vary in the direction of response. Quantifying these interactive effects is vital in forecasting pesticide risk across climate regions and informing pesticide application strategies and pollinator conservation. This multi-stressor study investigated the responses of six functional behaviours of bumblebees when exposed to either a neonicotinoid (imidacloprid) or a sulfoximine (sulfoxaflor) across a standardised low, mid, and high temperature. We found the neonicotinoid had a significant effect on five of the six behaviours, with a greater effect at the lower temperature(s) when measuring responsiveness, the likelihood of movement, walking rate, and food consumption rate. In contrast, the neonicotinoid had a greater impact on flight distance at the higher temperature. Our findings show that different organismal functions can exhibit divergent thermal responses, with some pesticide-affected behaviours showing greater impact as temperatures dropped, and others as temperatures rose. We must therefore account for environmental context when determining pesticide risk. Moreover, we found evidence of synergistic effects, with just a 3°C increase causing a sudden drop in flight performance, despite seeing no effect of pesticide at the two lower temperatures. Our findings highlight the importance of multi-stressor studies to quantify threats to insects, which will help to improve dynamic evaluations of population tipping points and spatiotemporal risks to biodiversity across different climate regions.

Behavioral state-dependent modulation of insulin-producing cells in Drosophila

Insulin signaling plays a pivotal role in metabolic control and aging, and insulin accordingly is a key factor in several human diseases. Despite this importance, the in vivo activity dynamics of insulin-producing cells (IPCs) are poorly understood. Here, we characterized the effects of locomotion on the activity of IPCs in Drosophila. Using in vivo electrophysiology and calcium imaging, we found that IPCs were strongly inhibited during walking and flight and that their activity rebounded and overshot after cessation of locomotion. Moreover, IPC activity changed rapidly during behavioral transitions, revealing that IPCs are modulated on fast timescales in behaving animals. Optogenetic activation of locomotor networks ex vivo, in the absence of actual locomotion or changes in hemolymph sugar levels, was sufficient to inhibit IPCs. This demonstrates that the behavioral state-dependent inhibition of IPCs is actively controlled by neuronal pathways and is independent of changes in glucose concentration. By contrast, the overshoot in IPC activity after locomotion was absent ex vivo and after starvation, indicating that it was not purely driven by feedforward signals but additionally required feedback derived from changes in hemolymph sugar concentration. We hypothesize that IPC inhibition during locomotion supports mobilization of fuel stores during metabolically demanding behaviors, while the rebound in IPC activity after locomotion contributes to replenishing muscle glycogen stores. In addition, the rapid dynamics of IPC modulation support a potential role of insulin in the state-dependent modulation of sensorimotor processing.

Non-canonical function of an Hif-1α splice variant contributes to the sustained flight of locusts

The hypoxia inducible factor (Hif) pathway is functionally conserved across metazoans in modulating cellular adaptations to hypoxia. However, the functions of this pathway under aerobic physiological conditions are rarely investigated. Here, we show that Hif-1α2, a locust Hif-1α isoform, does not induce canonical hypoxic responses but functions as a specific regulator of locust flight, which is a completely aerobic physiological process. Two Hif-1α splice variants were identified in locusts, a ubiquitously expressed Hif-1α1 and a muscle-predominantly expressed Hif-1α2. Hif-1α1 that induces typical hypoxic responses upon hypoxia exposure remains inactive during flight. By contrast, the expression of Hif-1α2, which lacks C-terminal transactivation domain, is less sensitive to oxygen tension but induced extensively by flying. Hif-1α2 regulates physiological processes involved in glucose metabolism and antioxidation during flight and sustains flight endurance by maintaining redox homeostasis through upregulating the production of a reactive oxygen species (ROS) quencher, DJ-1. Overall, this study reveals a novel Hif-mediated mechanism underlying prolonged aerobic physiological activity.

Overwintering in North American domesticated honeybees (Apis mellifera) causes mitochondrial reprogramming while enhancing cellular immunity

Many factors negatively impact domesticated honeybee (Apis mellifera) health causing a global decrease in their population year after year with major losses occurring during winter, and the cause remains thus far unknown. Here, we monitored for 12 months North American colonies of honeybees enduring important temperature variations throughout the year, to assess the metabolism and immune system of honeybees of summer and winter individuals. Our results show that in flight muscle, mitochondrial respiration via complex I during winter is drastically reduced compared to summer. However, the capacity for succinate and glycerol-3-phosphate (G3P) oxidation by mitochondria is increased during winter, resulting in higher mitochondrial oxygen consumption when complex I substrates, succinate and G3P were assessed altogether. Pyruvate kinase, lactate dehydrogenase, aspartate aminotransferase, citrate synthase and malate dehydrogenase tend to have reduced activity levels in winter unlike hexokinase, NADH dehydrogenase and pyruvate dehydrogenase. Transcript abundance of highly important immunity proteins like Vitellogenin and Defensin-1 were also increased in winter bees, and a stronger phagocytic response as well as a better hemocyte viability was observed during winter. Thus, a reorganization of substrate utilization favoring succinate and G3P while negatively affecting complex I of the ETS is occurring during winter. We suggest that this might be due to complex I transitioning to a dormant conformation through post-translational modification. Winter bees also have an increased response for antibacterial elimination in honeybees. Overall, this study highlights previously unknown cellular mechanisms between summer and winter honeybees that further our knowledge about this important species.

Postembryonic development of the tracheal system of beetles in the context of aptery and adaptations towards an arid environment

The tracheal system comprises one of the major adaptations of insects towards a terrestrial lifestyle. Many aspects such as the modifications towards wing reduction or a life in an arid climate are still poorly understood. To address these issues, we performed the first three-dimensional morphometric analyses of the tracheal system of a wingless insect, the desert beetle Gonopus tibialis and compared it with a flying beetle (Tenebrio molitor). Our results clearly show that the reduction of the flight apparatus has severe consequences for the tracheal system. This includes the reduction of the tracheal density, the relative volume of the trachea, the volume of the respective spiracles and the complete loss of individual tracheae. At the same time, the reduction of wings in the desert beetle allows modifications of the tracheal system that would be impossible in an animal with a functional flight apparatus such as the formation of a subelytral cavity as a part of the tracheal system, the strong elongation of the digestive tract including its tracheal system or the respiration through a single spiracle. Finally, we addressed when these modifications of the tracheal system take place during the development of the studied beetles. We can clearly show that they develop during pupation while the larvae of both species are almost identical in their tracheal system and body shape.

Effect of vibrational mating disruption on flight activity and oviposition to control the grapevine pest, Scaphoideustitanus

The increasing demand for safe and sustainably produced food is leading to the development of strategies of pest control alternative to chemicals. One innovative method is Vibrational Mating Disruption (VMD) to disrupt insect communication in plants. VMD was proven effective in preventing mating of the grapevine pest Scaphoideus titanus, vector of flavescence dorée. However, the stress induced by VMD on the target species has the potential to influence other crucial aspects of the insect biology and ethology. Therefore, the goal of this study was to understand side effects of VMD on the flight activity and oviposition of S. titanus. The results of our experiments conducted in the greenhouse showed that in the presence of a receptive female, males fly more if exposed to vibrations than in the silent control but not differently from singles males in silence. Surprisingly, we found that also females subjected to VMD fly more than in the silence. Regarding oviposition, we found that mated females exposed to vibrations and single females (unmated) laid significantly fewer eggs than mated females in silence. In conclusion, this study shows the potential of VMD to interfere, besides with mating, with other important biological aspects of the pest species.

Exercise and the Brain: Lessons From Invertebrate Studies

Benefits of physical exercise for brain functions are well documented in mammals, including humans. In this review, we will summarize recent research on the effects of species-specific intense locomotion on behavior and brain functions of different invertebrates. Special emphasis is made on understanding the biological significance of these effects as well as underlying cellular and molecular mechanisms. The results obtained in three distantly related clades of protostomes, Nematodes, Molluscs and Artropods, suggest that influence of intense locomotion on the brain could have deep roots in evolution and wide adaptive significance. In C. elegans, improved learning, nerve regeneration, resistance to neurodegenerative processes were detected after physical activity; in L. stagnalis—facilitation of decision making in the novel environment, in Drosophila—increased endurance, improved sleep and feeding behavior, in G. bimaculatus—improved orientation in conspecific phonotaxis, enhanced aggressiveness, higher mating success, resistance to some disturbing stimuli. Many of these effects have previously been described in mammals as beneficial results of running, suggesting certain similarity between distantly-related species. Our hypothesis posits that the above modulation of cognitive functions results from changes in the organism’s predictive model. Intense movement is interpreted by the organism as predictive of change, in anticipation of which adjustments need to be made. Identifying the physiological and molecular mechanisms behind these adjustments is easier in experiments in invertebrates and may lead to the discovery of novel neurobiological mechanisms for regulation and correction of cognitive and emotional status.

Effect of urbanization and its environmental stressors on the intraspecific variation of flight functional traits in two bumblebee species

The way urbanization shapes the intraspecific variation of pollinator functional traits is little understood. However, this topic is relevant for investigating ecosystem services and pollinator health. Here, we studied how urbanization affects the functional traits of workers in two bumblebee species (Bombus terrestris and B. pascuorum) sampled in 37 sites along a gradient of urbanization in North Italy (an area of 1800 km² including the metropolitan context of Milan and other surrounding capital districts). Namely, we investigated the effect of land use composition, configuration, air temperature, flower resource abundance, and air pollutants on the variation of traits related to flight performance and of stress during insect development (i.e., wing size, wing shape and size fluctuating asymmetry). The functional traits of the two bumblebees responded idiosyncratically to urbanization. Urban temperatures were associated with smaller wing sizes in B. pascuorum and with more accentuated fluctuating asymmetry of wing size in B. terrestris. Moreover, flower abundance correlated with bigger wings in B. terrestris and with less asymmetric wing size in B. pascuorum. Other traits did not vary significantly, and other urban variables played minor effects. These species-specific variation patterns highlight that environmental stressor linked to urbanization negatively impact the traits related to flight performance and development stability of these syntopic bumblebees, with possible consequences on the pollination service they provide.

Anna's hummingbird (Calypte anna) physiological response to novel thermal and hypoxic conditions at high elevations

Many species have not tracked their thermal niches upslope as predicted by climate change, potentially because higher elevations are associated with abiotic challenges beyond temperature. To better predict whether organisms can continue to move upslope with rising temperatures, we need to understand their physiological performance when subjected to novel high-elevation conditions. Here, we captured Anna's hummingbirds - a species expanding their elevational distribution in concordance with rising temperatures - from across their current elevational distribution and tested their physiological response to novel abiotic conditions. First, at a central aviary within their current elevational range, we measured hovering metabolic rate to assess their response to oxygen conditions and torpor use to assess their response to thermal conditions. Second, we transported the hummingbirds to a location 1200 m above their current elevational range limit to test for an acute response to novel oxygen and thermal conditions. Hummingbirds exhibited lower hovering metabolic rates above their current elevational range limit, suggesting lower oxygen availability may reduce performance after an acute exposure. Alternatively, hummingbirds showed a facultative response to thermal conditions by using torpor more frequently and for longer. Finally, post-experimental dissection found that hummingbirds originating from higher elevations within their range had larger hearts, a potential plastic response to hypoxic environments. Overall, our results suggest lower oxygen availability and low air pressure may be difficult challenges to overcome for hummingbirds shifting upslope as a consequence of rising temperatures, especially if there is little to no long-term acclimatization. Future studies should investigate how chronic exposure and acclimatization to novel conditions, as opposed to acute experiments, may result in alternative outcomes that help organisms better respond to abiotic challenges associated with climate-induced range shifts.

Flexible Thermal Sensitivity of Mitochondrial Oxygen Consumption and Substrate Oxidation in Flying Insect Species

Mitochondria have been suggested to be paramount for temperature adaptation in insects. Considering the large range of environments colonized by this taxon, we hypothesized that species surviving large temperature changes would be those with the most flexible mitochondria. We thus investigated the responses of mitochondrial oxidative phosphorylation (OXPHOS) to temperature in three flying insects: the honeybee (Apis mellifera carnica), the fruit fly (Drosophila melanogaster) and the Colorado potato beetle (Leptinotarsa decemlineata). Specifically, we measured oxygen consumption in permeabilized flight muscles of these species at 6, 12, 18, 24, 30, 36, 42 and 45°C, sequentially using complex I substrates, proline, succinate, and glycerol-3-phosphate (G3P). Complex I respiration rates (CI-OXPHOS) were very sensitive to temperature in honeybees and fruit flies with high oxygen consumption at mid-range temperatures but a sharp decline at high temperatures. Proline oxidation triggers a major increase in respiration only in potato beetles, following the same pattern as CI-OXPHOS for honeybees and fruit flies. Moreover, both succinate and G3P oxidation allowed an important increase in respiration at high temperatures in honeybees and fruit flies (and to a lesser extent in potato beetles). However, when reaching 45°C, this G3P-induced respiration rate dropped dramatically in fruit flies. These results demonstrate that mitochondrial functions are more resilient to high temperatures in honeybees compared to fruit flies. They also indicate an important but species-specific mitochondrial flexibility for substrate oxidation to sustain high oxygen consumption levels at high temperatures and suggest previously unknown adaptive mechanisms of flying insects’ mitochondria to temperature.

Flight-Fecundity Trade-offs: A Possible Mechanistic Link in Plant-Herbivore-Pollinator Systems

Plant-herbivore and plant-pollinator interactions are both well-studied, but largely independent of each other. It has become increasingly recognized, however, that pollination and herbivory interact extensively in nature, with consequences for plant fitness. Here, we explore the idea that trade-offs in investment in insect flight and reproduction may be a mechanistic link between pollination and herbivory. We first provide a general background on trade-offs between flight and fecundity in insects. We then focus on Lepidoptera; larvae are generally herbivores while most adults are pollinators, making them ideal to study these links. Increased allocation of resources to flight, we argue, potentially increases a Lepidopteran insect pollinator's efficiency, resulting in higher plant fitness. In contrast, allocation of resources to reproduction in the same insect species reduces plant fitness, because it leads to an increase in herbivore population size. We examine the sequence of resource pools available to herbivorous Lepidopteran larvae (maternally provided nutrients to the eggs, as well as leaf tissue), and to adults (nectar and nuptial gifts provided by the males to the females), which potentially are pollinators. Last, we discuss how subsequent acquisition and allocation of resources from these pools may alter flight-fecundity trade-offs, with concomitant effects both on pollinator performance and the performance of larval herbivores in the next generation. Allocation decisions at different times during ontogeny translate into costs of herbivory and/or benefits of pollination for plants, mechanistically linking herbivory and pollination.

Selection on dispersal drives evolution of metabolic capacities for energy production in female wing-polymorphic sand field crickets, Gryllus firmus

Life history and metabolism covary, but the mechanisms and individual traits responsible for these linkages remain unresolved. Dispersal capability is a critical component of life history that is constrained by metabolic capacities for energy production. Conflicting relationships between metabolism and life histories may be explained by accounting for variation in dispersal and maximal metabolic rates. We used female wing-polymorphic sand field crickets, Gryllus firmus, selected either for long wings (LW, flight-capable) or short wings (SW, flightless) to test the hypothesis that selection on dispersal capability drives the evolution of metabolic capacities. While resting metabolic rates were similar, long-winged crickets reached higher maximal metabolic rates than short-winged crickets, resulting in improved running performance. We further provided insight into the mechanisms responsible for covariation between life history and metabolism by comparing mitochondrial content of tissues involved in powering locomotion and assessing the function of mitochondria isolated from long- and short-winged crickets. Our results demonstrated that larger metabolic capacities in long-winged crickets were underpinned by increases in mitochondrial content of dorsoventral flight muscle and enhanced bioenergetic capacities of mitochondria within the fat body, a tissue responsible for fuel storage and mobilization. Thus, selection on flight capability correlates with increases in maximal, but not resting metabolic rates, through modifications of tissues powering locomotion at the cellular and organelle levels. This allows organisms to meet high energetic demands of activity for life history. Dispersal capability should therefore explicitly be considered as a potential factor driving the evolution of metabolic capacities.

Paradoxes of Hymenoptera flight muscles, extreme machines

In the Carboniferous, insects evolved flight. Intense selection drove for high performance and approximately 100 million years later, Hymenoptera (bees, wasps and ants) emerged. Some species had proportionately small wings, with apparently impossible aerodynamic challenges including a need for high frequency flight muscles (FMs), powered exclusively off aerobic pathways and resulting in extreme aerobic capacities. Modern insect FMs are the most refined and form large dense blocks that occupy 90% of the thorax. These can beat wings at 200 to 230 Hz, more than double that achieved by standard neuromuscular systems. To do so, rapid repolarisation was circumvented through evolution of asynchronous stimulation, stretch activation, elastic recoil and a paradoxically slow Ca2+ reuptake. While the latter conserves ATP, considerable ATP is demanded at the myofibrils. FMs have diminished sarcoplasmic volumes, and ATP is produced solely by mitochondria, which pack myocytes to maximal limits and have very dense cristae. Gaseous oxygen is supplied directly to mitochondria. While FMs appear to be optimised for function, several unusual paradoxes remain. FMs lack any significant equivalent to the creatine kinase shuttle, and myofibrils are twice as wide as those of within cardiomyocytes. The mitochondrial electron transport systems also release large amounts of reactive oxygen species (ROS) and respiratory complexes do not appear to be present at any exceptional level. Given that the loss of the creatine kinase shuttle and elevated ROS impairs heart function, we question how do FM shuttle adenylates at high rates and tolerate oxidative stress conditions that occur in diseased hearts?

Thermal and Oxygen Flight Sensitivity in Ageing Drosophila melanogaster Flies: Links to Rapamycin-Induced Cell Size Changes

Simple Summary

Cold-blooded organisms can become physiologically challenged when performing highly oxygen-demanding activities (e.g., flight) across different thermal and oxygen environmental conditions. We explored whether this challenge decreases if an organism is built of smaller cells. This is because small cells create a large cell surface, which is costly, but can ease the delivery of oxygen to cells’ power plants, called mitochondria. We developed fruit flies in either standard food or food with rapamycin (a human drug altering the cell cycle and ageing), which produced flies with either large cells (no supplementation) or small cells (rapamycin supplementation). We measured the maximum speed at which flies were flapping their wings in warm and hot conditions, combined with either normal or reduced air oxygen concentrations. Flight intensity increased with temperature, and it was reduced by poor oxygen conditions, indicating limitations of flying insects by oxygen supply. Nevertheless, flies with small cells showed lower limitations, only slowing down their wing flapping in low oxygen in the hot environment. Our study suggests that small cells in a body can help cold-blooded organisms maintain demanding activities (e.g., flight), even in poor oxygen conditions, but this advantage can depend on body temperature.

Abstract

Ectotherms can become physiologically challenged when performing oxygen-demanding activities (e.g., flight) across differing environmental conditions, specifically temperature and oxygen levels. Achieving a balance between oxygen supply and demand can also depend on the cellular composition of organs, which either evolves or changes plastically in nature; however, this hypothesis has rarely been examined, especially in tracheated flying insects. The relatively large cell membrane area of small cells should increase the rates of oxygen and nutrient fluxes in cells; however, it does also increase the costs of cell membrane maintenance. To address the effects of cell size on flying insects, we measured the wing-beat frequency in two cell-size phenotypes of Drosophila melanogaster when flies were exposed to two temperatures (warm/hot) combined with two oxygen conditions (normoxia/hypoxia). The cell-size phenotypes were induced by rearing 15 isolines on either standard food (large cells) or rapamycin-enriched food (small cells). Rapamycin supplementation (downregulation of TOR activity) produced smaller flies with smaller wing epidermal cells. Flies generally flapped their wings at a slower rate in cooler (warm treatment) and less-oxygenated (hypoxia) conditions, but the small-cell-phenotype flies were less prone to oxygen limitation than the large-cell-phenotype flies and did not respond to the different oxygen conditions under the warm treatment. We suggest that ectotherms with small-cell life strategies can maintain physiologically demanding activities (e.g., flight) when challenged by oxygen-poor conditions, but this advantage may depend on the correspondence among body temperatures, acclimation temperatures and physiological thermal limits.

Body size allometry impacts flight-related morphology and metabolic rates in the solitary bee Megachile rotundata

Body size is related to many aspects of life history, including foraging distance and pollination efficiency. In solitary bees, manipulating the amount of larval diet produces intraspecific differences in adult body size. The goal of this study was to determine how body size impacts metabolic rates, allometry, and flight-related morphometrics in the alfalfa leafcutting bee, Megachile rotundata. By restricting or providing excess food, we produced a range of body sizes, which allowed us to test the effect of body size on allometry, the power required for flight, and amount of energy produced, as measured indirectly through CO₂ emission. The power required during flight was predicted using the flight biomechanical formulas for wing loading and excess power index. We found larger bees had higher absolute metabolic rates at rest and during flight, but smaller bees had higher mass-specific metabolic rates at rest. During flight, bees did not have size-related differences in mass-specific metabolic rate. As bees increase in size, their thorax and abdomens become disproportionately larger, while their wings (area, and length) become disproportionately smaller. Smaller bees had more power available during flight as demonstrated by flight biomechanical formulas. Smaller body size was advantageous because of a reduced power requirement for flight with no metabolic cost.

Neuropeptide ACP facilitates lipid oxidation and utilization during long-term flight in locusts

Long-term flight depends heavily on intensive energy metabolism in animals; however, the neuroendocrine mechanisms underlying efficient substrate utilization remain elusive. Here, we report that the adipokinetic hormone/corazonin-related peptide (ACP) can facilitate muscle lipid utilization in a famous long-term migratory flighting species, Locusta migratoria. By peptidomic analysis and RNAi screening, we identified brain-derived ACP as a key flight-related neuropeptide. ACP gene expression increased notably upon sustained flight. CRISPR/Cas9-mediated knockout of ACP gene and ACP receptor gene (ACPR) significantly abated prolonged flight of locusts. Transcriptomic and metabolomic analyses further revealed that genes and metabolites involved in fatty acid transport and oxidation were notably downregulated in the flight muscle of ACP mutants. Finally, we demonstrated that a fatty-acid-binding protein (FABP) mediated the effects of ACP in regulating muscle lipid metabolism during long-term flight in locusts. Our results elucidated a previously undescribed neuroendocrine mechanism underlying efficient energy utilization associated with long-term flight.

Flight allows insects to find food or seek a better environment. Some insects have developed the ability of ‘long-term flight’, which allows them to make continuous journeys over large distances. For example, one locust species regularly crosses the Red Sea which is up to 300 km wide – a spectacular feat for insects only a few inches long.

However, flight is an energy-intensive activity, and insects’ muscles need the right sort of chemical fuel to work properly. Previous work has shown that this ‘fuel consumption’ is highly dynamic and happens in two stages. First, immediately after take-off, the muscles rapidly consume carbohydrates (sugars); then, during the prolonged phase of the flight, muscles switch to exclusively consume lipids (fats).

How the flight muscles ‘know’ when to start using fats for energy remains largely unclear. It has been suggested that this switch may involve hormone-like chemicals made in the brain called neuroendocrine peptides. Hou et al. therefore set out to test this hypothesis, using the locust species Locusta migratoria as a representative migratory insect.

Initial experiments used an abundance detection technique to determine which of the neuroendocrine peptides were active in adult locusts. Further analysis, looking specifically at locusts that had just been flying, revealed that the gene for a peptide called ACP became much more active after one hour of continuous flight. Further evidence that the ACP hormone could indeed be helping to power long-term flight came from locusts with a mutated, ‘switched-off’ version of the gene. These insects could only fly for half the time, and half the distance, compared to locusts that did not have mutations in the gene for ACP.

Biochemical studies of the ACP mutant locusts confirmed that their flight muscle cells could not transport and break down fatty acids normally. These experiments also showed that ACP was acting through a type of carrier protein called FABP, which is present in many different insects and normally ‘ferries’ lipids to the places they are needed.

These findings shed new light on the biological mechanisms that control long-term flight in migratory insects. The ability to move over long distances is key to the outbreak of locust plagues, which in turn cause widespread crop damage around the world. Hou et al. therefore hope that this knowledge will one day help develop effective strategies for locust pest control.

Oxygen Dependence of Flight Performance in Ageing Drosophila melanogaster

Similar to humans, insects lose their physical and physiological capacities with age, which makes them a convenient study system for human ageing. Although insects have an efficient oxygen-transport system, we know little about how their flight capacity changes with age and environmental oxygen conditions. We measured two types of locomotor performance in ageing Drosophila melanogaster flies: the frequency of wing beats and the capacity to climb vertical surfaces. Flight performance was measured under normoxia and hypoxia. As anticipated, ageing flies showed systematic deterioration of climbing performance, and low oxygen impeded flight performance. Against predictions, flight performance did not deteriorate with age, and younger and older flies showed similar levels of tolerance to low oxygen during flight. We suggest that among different insect locomotory activities, flight performance deteriorates slowly with age, which is surprising, given that insect flight is one of the most energy-demanding activities in animals. Apparently, the superior capacity of insects to rapidly deliver oxygen to flight muscles remains little altered by ageing, but we showed that insects can become oxygen limited in habitats with a poor oxygen supply (e.g., those at high elevations) during highly oxygen-demanding activities such as flight.

Chronic electrical stimulation reduces reliance on anaerobic metabolism in locust jumping muscle

Chronic electrical stimulation (CES) is a well-documented method for changing mammalian muscle from more fast-twitch to slow-twitch metabolic and contractile profiles. Although both mammalian and insect muscles have many similar anatomical and physiological properties, it is unknown if CES produces similar muscle plasticity changes in insects. To test this idea, we separated Schistocerca americana grasshoppers into two groups (n = 37 to 47): one that was subjected to CES for 180 min each day for five consecutive days and one group that was not. Each group was then electrically stimulated for a single time period (0, 5, 30, 60, or 180 min) before measuring jumping muscle lactate, a characteristic of fast-twitch type fibers. At each time point, CES led to a significantly reduced jumping muscle lactate concentration. Based on similar short-term CES mammalian studies, the reduction in lactate production was most likely due to a reduced reliance on anaerobic metabolism. Thus, longer stimulation periods should result in greater aerobic enzymatic activities, altered myosin ATPase, and shift fiber types. This is the first study to use electrical stimulation to explore insect muscle plasticity and our results show that grasshopper jumping muscle responds similarly to mammalian muscle.

Cytochrome c Oxidase at Full Thrust: Regulation and Biological Consequences to Flying Insects

Flight dispersal represents a key aspect of the evolutionary and ecological success of insects, allowing escape from predators, mating, and colonization of new niches. The huge energy demand posed by flight activity is essentially met by oxidative phosphorylation (OXPHOS) in flight muscle mitochondria. In insects, mitochondrial ATP supply and oxidant production are regulated by several factors, including the energy demand exerted by changes in adenylate balance. Indeed, adenylate directly regulates OXPHOS by targeting both chemiosmotic ATP production and the activities of specific mitochondrial enzymes. In several organisms, cytochrome c oxidase (COX) is regulated at transcriptional, post-translational, and allosteric levels, impacting mitochondrial energy metabolism, and redox balance. This review will present the concepts on how COX function contributes to flying insect biology, focusing on the existing examples in the literature where its structure and activity are regulated not only by physiological and environmental factors but also how changes in its activity impacts insect biology. We also performed in silico sequence analyses and determined the structure models of three COX subunits (IV, VIa, and VIc) from different insect species to compare with mammalian orthologs. We observed that the sequences and structure models of COXIV, COXVIa, and COXVIc were quite similar to their mammalian counterparts. Remarkably, specific substitutions to phosphomimetic amino acids at critical phosphorylation sites emerge as hallmarks on insect COX sequences, suggesting a new regulatory mechanism of COX activity. Therefore, by providing a physiological and bioenergetic framework of COX regulation in such metabolically extreme models, we hope to expand the knowledge of this critical enzyme complex and the potential consequences for insect dispersal.

Sex differences in postprandial blood glucose and body surface temperature are contingent on flight in the fruit bat, Cynopterus sphinx

The postprandial blood glucose level is very high for the body size in frugivorous bats. Like other homeotherms, bats release heat during digestion of dietary macronutrients. Despite males and females of the same species exhibiting different foraging behaviour, empirical support for sex differences in blood glucose and body surface temperature in fruit bats is poor. Moreover, while flight affects postprandial metabolism, whether such effects are different in each sex of fruit bats is unclear. Here, we studied these questions in the fruit bat, Cynopterus sphinx. We first assessed whether there are sex differences in the postprandial level of blood glucose and body surface temperature over time in rested bats. We then assessed whether flight affects outcomes of sex differences in both traits. We found that the estimated marginal means of both traits were generally higher in females than males, in rested bats. Notably, the sex difference in both traits was only significant at specific sampling time of the assay. Further, the trait means significantly differed between the sexes only in the rested, but not active, bats, meaning that signals of sex difference in metabolic traits eroded when bats were active. Taken together, our findings suggest that in C. sphinx, the sex specificity in the expression of metabolic traits is significantly dependent on physical activity.

Summary: The level of sex differences in metabolic traits is affected by flight in Cynopterus sphinx, a finding that has general implications for sex-specific life-history evolution in fruit bats.

Previous flight facilitates partner finding in female crickets

In the cricket Gryllus bimaculatus, flying occurs soon after the last imaginal molt and precedes the mating behavior in natural conditions. Here, we tested the hypothesis that flying may improve subsequent behavioral performance in a novel environment in female crickets. We developed a behavioral set-up to test female cricket responsiveness to male calling song as well as their ability to locate and find the source of the song. The male song was produced by a loudspeaker hidden behind the fabric wall of a spacious square arena. Forced flight prior to the test promoted female sexual searching behavior in the novel environment. After the flight, more females reached the hidden source zone, spent more time near the source and finally more of them climbed over the wall section immediately in front of the hidden loudspeaker. At the same time, their behavior in the arena did not differ from the control group when the calling song was not delivered, suggesting that flight exerts its behavioral effects by influencing sexual motivation. Our results support the suggestion that preceding intense locomotion facilitates sexual searching behavior of females in a novel environment.

De Novo Transcriptomic and Metabolomic Analyses Reveal the Ecological Adaptation of High-Altitude Bombus pyrosoma

Bombus pyrosoma is one of the most abundant bumblebee species in China, with a distribution range of very varied geomorphology and vegetation, which makes it an ideal pollinator species for research into high-altitude adaptation. Here, we sequenced and assembled transcriptomes of B. pyrosoma from the low-altitude North China Plain and the high-altitude Tibet Plateau. Subsequent comparative analysis of de novo transcriptomes from the high- and low-altitude groups identified 675 common upregulated genes (DEGs) in the high-altitude B. pyrosoma. These genes were enriched in metabolic pathways and corresponded to enzyme activities involved in energy metabolism. Furthermore, according to joint analysis with comparative metabolomics, we suggest that the metabolism of coenzyme A (CoA) and the metabolism and transport of energy resources contribute to the adaptation of high-altitude B. pyrosoma. Meanwhile, we found many common upregulated genes enriched in the Toll and immune deficiency (Imd)signaling pathways that act as important immune defenses in insects, and hypoxia and cold temperatures could induce the upregulation of immune genes in insects. Therefore, we suppose that the Toll and Imd signaling pathways also participated in the high-altitude adaptation of B. pyrosoma. Like other organisms, we suggest that the high-altitude adaptation of B. pyrosoma is controlled by diverse mechanisms.

Interactions with ectoparasitic mites induce host metabolic and immune responses in flies at the expense of reproduction-associated factors

Parasites cause harm to their hosts and represent pervasive causal agents of natural selection. Understanding host proximate responses during interactions with parasites can help predict which genes and molecular pathways are targets of this selection. In the current study, we examined transcriptional changes arising from interactions between Drosophila melanogaster and their naturally occurring ectoparasitic mite, Gamasodes queenslandicus. Shifts in host transcript levels associated with behavioural avoidance revealed the involvement of genes underlying nutrient metabolism. These genetic responses were reflected in altered body lipid and glycogen levels in the flies. Mite infestation triggered a striking immune response, while male accessory gland protein transcript levels were simultaneously reduced, suggesting a trade-off between host immune responses to parasite challenge and reproduction. Comparison of transcriptional analyses during mite infestation to those during nematode and parasitoid attack identified host genes similarly expressed in flies during these interactions. Validation of the involvement of specific genes with RNA interference lines revealed candidates that may directly mediate fly-ectoparasite interactions. Our physiological and molecular characterization of the Drosophila-Gamasodes interface reveals new proximate mechanisms underlying host-parasite interactions, specifically host transcriptional shifts associated with behavioural avoidance and infestation. The results identify potential general mechanisms underlying host resistance and evolutionarily relevant trade-offs.

The evolution of autotomy in leaf-footed bugs

Sacrificing body parts is one of many behaviors that animals use to escape predation. This trait, termed autotomy, is classically associated with lizards. However, several other taxa also autotomize, and this trait has independently evolved multiple times throughout Animalia. Despite having multiple origins and being an iconic antipredatory trait, much remains unknown about the evolution of autotomy. Here, we combine morphological, behavioral, and genomic data to investigate the evolution of autotomy within leaf-footed bugs and allies (Insecta: Hemiptera: Coreidae + Alydidae). We found that the ancestor of leaf-footed bugs autotomized and did so slowly; rapid autotomy (<2 min) then arose multiple times. The ancestor likely used slow autotomy to reduce the cost of injury or to escape nonpredatory entrapment but could not use autotomy to escape predation. This result suggests that autotomy to escape predation is a co-opted benefit (i.e., exaptation), revealing one way that sacrificing a limb to escape predation may arise. In addition to identifying the origins of rapid autotomy, we also show that across species variation in the rates of autotomy can be explained by body size, distance from the equator, and enlargement of the autotomizable appendage.

Pollutants and Their Interaction with Diseases of Social Hymenoptera

Many insect species, including social insects, are currently declining in abundance and diversity. Pollutants such as pesticides, heavy metals, or airborne fine particulate matter from agricultural and industrial sources are among the factors driving this decline. While these pollutants can have direct detrimental effects, they can also result in negative interactive effects when social insects are simultaneously exposed to multiple stressors. For example, sublethal effects of pollutants can increase the disease susceptibility of social insects, and thereby jeopardize their survival. Here we review how pesticides, heavy metals, or airborne fine particulate matter interact with social insect physiology and especially the insects’ immune system. We then give an overview of the current knowledge of the interactive effects of these pollutants with pathogens or parasites. While the effects of pesticide exposure on social insects and their interactions with pathogens have been relatively well studied, the effects of other pollutants, such as heavy metals in soil or fine particulate matter from combustion, vehicular transport, agriculture, and coal mining are still largely unknown. We therefore provide an overview of urgently needed knowledge in order to mitigate the decline of social insects.

Contrasting effects of altitude on species groups with different traits in a non-fragmented montane temperate forest

Temperature has strong effects on species composition and traits. These effects can differ within and between species groups. Thermoregulation and mobility are traits which can be strongly affected by altitudinal distribution. Our aim was to investigate the influence of altitude on the species richness, abundance and composition of species groups with different trophic, thermoregulatory and mobility traits. Carabids (Coleoptera; Carabidae), hoverflies (Diptera: Syrphidae) and birds (Aves: Passeriformes) were counted in three altitudinal belts with a total elevation difference of 700 m (from 300 m to 1000 m a.s.l.) in the same habitat type (non-fragmented temperate montane mixed beech and fir forest). We found that endotherms and more mobile species (i.e. birds) had a smaller turnover than ectotherms (i.e. hoverflies) and less mobile species (i.e. carabids), from which we can predict that the former species will undergo a less extreme shift than the latter in global warming scenarios. Species turnover across the altitudinal gradient increased from birds to hoverflies to carabid beetles. The effect of altitude on phenology was different between the studied ectotherm species groups (carabids and hoverflies). Hoverflies experience a phenological delay of species richness and abundance at higher altitudes in spring, but not at the end of summer, which implies that hoverfly phenology is affected by a change in temperature, while carabid beetle abundance exhibited a delay in phenology in summer at higher altitudes. We suggest that species that are expected to be most affected by climate change, such as ectotherms and species with poor dispersal ability should be prioritised as the best indicators for monitoring and conservation management purposes.

Exploring thermal flight responses as predictors of flight ability and geographic range size in Drosophila

Thermal flight performance curves (TFPCs) may be a useful proxy for determining dispersal on daily timescales in winged insect species. Few studies have assessed TFPCs across a range of species under standard conditions despite that they may be useful in predicting variation in performance, abundance or geographic range shifts with forecast climate variability. Indeed, the factors determining realized dispersal within and among flying insect species are generally poorly understood. To better understand how flight performance may be correlated with geographic range extent and potential latitudinal climate variability, we estimated the thermal performance curves of flight ability in 11 Drosophilidae species (in 4 °C increments across 16-28 °C) after standard laboratory rearing for two generations. We tested if key morphological, evolutionary or ecological factors (e.g. species identity, sex, body mass, wing loading, geographic range size) predicted traits of TFPCs (including optimum temperature, maximum performance, thermal breadth of performance) or flight ability (success/failure to fly). Although several parameters of TFPCs varied among species these were typically not statistically significant probably owing to the relatively small pool of species assessed and the limited trait variation detected. The best explanatory model of these flight responses across species included significant positive effects of test temperature and wing area. However, the rank of geographic distribution breadth and phylogeny failed to explain significant variation in most of the traits, except for thermal performance breadth, of thermal flight performance curves among these 11 species. Future studies that employ a wider range of Drosophilidae species, especially if coupled with fine-scale estimates of species' environmental niches, would be useful.

Approaches for testing hypotheses for the hypometric scaling of aerobic metabolic rate in animals

Hypometric scaling of aerobic metabolism [larger organisms have lower mass-specific metabolic rates (MR/g)] is nearly universal for interspecific comparisons among animals, yet we lack an agreed upon explanation for this pattern. If physiological constraints on the function of larger animals occur and limit MR/g, these should be observable as direct constraints on animals of extant species and/or as evolved responses to compensate for the proposed constraint. There is evidence for direct constraints and compensatory responses to O2 supply constraint in skin-breathing animals, but not in vertebrates with gas-exchange organs. The duration of food retention in the gut is longer for larger birds and mammals, consistent with a direct constraint on nutrient uptake across the gut wall, but there is little evidence for evolving compensatory responses to gut transport constraints in larger animals. Larger placental mammals (but not marsupials or birds) show evidence of greater challenges with heat dissipation, but there is little evidence for compensatory adaptations to enhance heat loss in larger endotherms, suggesting that metabolic rate (MR) more generally balances heat loss for thermoregulation in endotherms. Size-dependent patterns in many molecular, physiological, and morphological properties are consistent with size-dependent natural selection, such as stronger selection for neurolocomotor performance and growth rate in smaller animals and stronger selection for safety and longevity in larger animals. Hypometric scaling of MR very likely arises from different mechanisms in different taxa and conditions, consistent with the diversity of scaling slopes for MR.

Flight capacities of yellow-legged hornet (Vespa velutina nigrithorax, Hymenoptera: Vespidae) workers from an invasive population in Europe

The invasive yellow-legged hornet, Vespa velutina nigrithorax Lepeletier, 1836 (Hymenoptera: Vespidae), is native to Southeast Asia. It was first detected in France (in the southwest) in 2005. It has since expanded throughout Europe and has caused significant harm to honeybee populations. We must better characterize the hornet’s flight capacity to understand the species’ success and develop improved control strategies. Here, we carried out a study in which we quantified the flight capacities of V. velutina workers using computerized flight mills. We observed that workers were able to spend around 40% of the daily 7-hour flight tests flying. On average, they flew 10km to 30km during each flight test, although there was a large amount of variation. Workers sampled in early summer had lower flight capacities than workers sampled later in the season. Flight capacity decreased as workers aged. However, in the field, workers probably often die before this decrease becomes significant. During each flight test, workers performed several continuous flight phases of variable length that were separated by rest phases. Based on the length of those continuous flight phases and certain key assumptions, we estimated that V. velutina colony foraging radius is at least 700 m (half that in early summer); however, some workers are able to forage much farther. While these laboratory findings remain to be confirmed by field studies, our results can nonetheless help inform V. velutina biology and control efforts.

High postural costs and anaerobic metabolism during swimming support the hypothesis of a U-shaped metabolism-speed curve in fishes

Swimming performance is considered a key trait determining the ability of fish to survive. Hydrodynamic theory predicts that the energetic costs required for fishes to swim should vary with speed according to a U-shaped curve, with an expected energetic minimum at intermediate cruising speeds and increasing expenditure at low and high speeds. However, to date no complete datasets have shown an energetic minimum for swimming fish at intermediate speeds rather than low speeds. To address this knowledge gap, we used a negatively buoyant fish, the clearnose skate Raja eglanteria, and took two approaches: a classic critical swimming speed protocol and a single-speed exercise and recovery procedure. We found an anaerobic component at each velocity tested. The two approaches showed U-shaped, though significantly different, speed-metabolic relationships. These results suggest that (i) postural costs, especially at low speeds, may result in J- or U-shaped metabolism-speed curves; (ii) anaerobic metabolism is involved at all swimming speeds in the clearnose skate; and (iii) critical swimming protocols might misrepresent the true costs of locomotion across speeds, at least in negatively buoyant fish.

Evaluation of the impacts of climate change on disease vectors through ecological niche modelling

Vector-borne diseases are exceptionally sensitive to climate change. Predicting vector occurrence in specific regions is a challenge that disease control programs must meet in order to plan and execute control interventions and climate change adaptation measures. Recently, an increasing number of scientific articles have applied ecological niche modelling (ENM) to study medically important insects and ticks. With a myriad of available methods, it is challenging to interpret their results. Here we review the future projections of disease vectors produced by ENM, and assess their trends and limitations. Tropical regions are currently occupied by many vector species; but future projections indicate poleward expansions of suitable climates for their occurrence and, therefore, entomological surveillance must be continuously done in areas projected to become suitable. The most commonly applied methods were the maximum entropy algorithm, generalized linear models, the genetic algorithm for rule set prediction, and discriminant analysis. Lack of consideration of the full-known current distribution of the target species on models with future projections has led to questionable predictions. We conclude that there is no ideal 'gold standard' method to model vector distributions; researchers are encouraged to test different methods for the same data. Such practice is becoming common in the field of ENM, but still lags behind in studies of disease vectors.

Metabolic rate and hypoxia tolerance are affected by group interactions and sex in the fruit fly (Drosophila melanogaster): new data and a literature survey

Population density and associated behavioral adjustments are potentially important in regulating physiological performance in many animals. In r-selected species like the fruit fly (Drosophila), where population density rapidly shifts in unpredictable and unstable environments, density-dependent physiological adjustments may aid survival of individuals living in a social environment. Yet, how population density (and associated social behaviors) affects physiological functions like metabolism is poorly understood in insects. Additionally, insects often show marked sexual dimorphism (larger females). Thus, in this study on D. melanogaster, we characterized the effects of fly density and sex on both mass-specific routine oxygen consumption (V̇O₂) and hypoxia tolerance (P_Crit). Females had significantly lower routine V̇O₂ (∼4 µl O₂ mg⁻¹ h⁻¹) than males (∼6 µl O₂ mg⁻¹ h⁻¹) at an average fly density of 28 flies·respirometer chamber⁻¹. However, V̇O₂ was inversely related to fly density in males, with V̇O₂ ranging from 4 to 11 µl O₂ mg⁻¹ h⁻¹ at a density of 10 and 40 flies·chamber⁻¹, respectively (r²=0.58, P<0.001). Female flies showed a similar but less pronounced effect, with a V̇O₂ of 4 and 7 µl O₂ mg⁻¹ h⁻¹ at a density of 10 and 40 flies·chamber⁻¹, respectively (r²=0.43, P<0.001). P_Crit (∼5.5 to 7.5 kPa) varied significantly with density in male (r²=0.50, P<0.01) but not female (r²=0.02, P>0.5) flies, with higher fly densities having a lower P_Crit. An extensive survey of the literature on metabolism in fruit flies indicates that not all studies control for, or even report on, fly density and gender, both of which may affect metabolic measurements.

Summary: Technical advances allowing oxygen consumption measurement in individual fruit flies actually take them out of their normal highly social context, resulting in higher oxygen consumption rates than in natural groups.

Distribution and predictors of wing shape and size variability in three sister species of solitary bees

Morphological traits can be highly variable over time in a particular geographical area. Different selective pressures shape those traits, which is crucial in evolutionary biology. Among these traits, insect wing morphometry has already been widely used to describe phenotypic variability at the inter-specific level. On the contrary, fewer studies have focused on intra-specific wing morphometric variability. Yet, such investigations are relevant to study potential convergences of variation that could highlight micro-evolutionary processes. The recent sampling and sequencing of three solitary bees of the genus Melitta across their entire species range provides an excellent opportunity to jointly analyse genetic and morphometric variability. In the present study, we first aim to analyse the spatial distribution of the wing shape and centroid size (used as a proxy for body size) variability. Secondly, we aim to test different potential predictors of this variability at both the intra- and inter-population levels, which includes genetic variability, but also geographic locations and distances, elevation, annual mean temperature and precipitation. The comparison of spatial distribution of intra-population morphometric diversity does not reveal any convergent pattern between species, thus undermining the assumption of a potential local and selective adaptation at the population level. Regarding intra-specific wing shape differentiation, our results reveal that some tested predictors, such as geographic and genetic distances, are associated with a significant correlation for some species. However, none of these predictors are systematically identified for the three species as an important factor that could explain the intra-specific morphometric variability. As a conclusion, for the three solitary bee species and at the scale of this study, our results clearly tend to discard the assumption of the existence of a common pattern of intra-specific signal/structure within the intra-specific wing shape and body size variability.

Starvation stress during larval development facilitates an adaptive response in adult worker honey bees (Apis mellifera L.)

Most organisms are constantly faced with environmental changes and stressors. In diverse organisms, there is an anticipatory mechanism during development that can program adult phenotypes. The adult phenotype would be adapted to the predicted environment that occurred during organism maturation. However, whether this anticipatory mechanism is present in eusocial species is questionable because eusocial organisms are largely shielded from exogenous conditions by their stable nest environment. In this study, we tested whether food deprivation during development of the honey bee (Apis mellifera), a eusocial insect model, can shift adult phenotypes to better cope with nutritional stress. After subjecting fifth instar worker larvae to short-term starvation, we measured nutrition-related morphology, starvation resistance, physiology, endocrinology and behavior in the adults. We found that the larval starvation caused adult honey bees to become more resilient toward starvation. Moreover, the adult bees were characterized by reduced ovary size, elevated glycogen stores and juvenile hormone (JH) titers, and decreased sugar sensitivity. These changes, in general, can help adult insects survive and reproduce in food-poor environments. Overall, we found for the first time support for an anticipatory mechanism in a eusocial species, the honey bee. Our results suggest that this mechanism may play a role in honey bee queen–worker differentiation and worker division of labor, both of which are related to the responses to nutritional stress.