Dynamics of animal movement in an ecological context: dragonfly wing damage reduces flight performance and predation success

Insect Flight Energetics and the Evolution of Size, Form, and Function

Flying insects vary greatly in body size and wing proportions, significantly impacting their flight energetics. Generally, the larger the insect, the slower its flight wingbeat frequency. However, variation in frequency is also explained by differences in wing proportions, where larger-winged insects tend to have lower frequencies. These associations affect the energy required for flight. The correlated evolution of flight form and function can be further defined using a lineage of closely related bee species varying in body mass. The decline in flight wingbeat frequency with increasing size is paralleled by the flight mass-specific metabolic rate. The specific scaling exponents observed can be predicted from the wing area allometry, where a greater increase (hyperallometry) leads to a more pronounced effect on flight energetics, and hypoallometry can lead to no change in frequency and metabolic rate across species. The metabolic properties of the flight muscles also vary with body mass and wing proportions, as observed from the activity of glycolytic enzymes and the phospholipid compositions of muscle tissue, connecting morphological differences with muscle metabolic properties. The evolutionary scaling observed across species is recapitulated within species. The static allometry observed within the bumblebee Bombus impatiens, where the wing area is proportional and isometric, affects wingbeat frequency and metabolic rate, which is predicted to decrease with an increase in size. Intraspecific variation in flight muscle tissue properties is also related to flight metabolic rate. The role of developmental processes and phenotypic plasticity in explaining intraspecific differences is central to our understanding of flight energetics. These studies provide a framework where static allometry observed within species gives rise to evolutionary allometry, connecting the evolution of size, form, and function associated with insect flight.

Wing kinematics measurement and aerodynamics of hovering droneflies with wing damage

In this study, we performed successive unilateral and bilateral wing shearing to simulate wing damage in droneflies (Eristalis tenax) and measured the wing kinematics using high-speed photography technology. Two different shearing types were considered in the artificial wing damage. The aerodynamic force and power consumption were obtained by numerical method. Our major findings are the following. Different shearing methods have little influence on the kinematics, forces and energy consumption of insects. Following wing damage, among the potential strategies to adjust the three Euler angles of the wing, adjusting stroke angle (φ) in isolation, or combing the adjustment of stroke angle (φ) with pitch angle (ψ), contributed most to the change in vertical force. The balance of horizontal thrust can be restored by the adjustment of deviation angle (θ) under the condition of unilateral wing damage. Considering zero elastic energy storage, the mass-specific power (P₁) increases significantly following wing damage. However, the increase in mass-specific power with 100% elastic energy storage (P₂) is very small. The extra cost of the unilateral wing damage is that the energy consumption of the damaged wing and intact wing is highly asymmetrical when zero elastic energy storage is considered. The insects may alleviate the problems of increasing power consumption and asymmetric power distribution by storage and reuse of the negative inertial work of the wing.

Flies trade off stability and performance via adaptive compensation to wing damage

Physical injury often impairs mobility, which can have dire consequences for survival in animals. Revealing mechanisms of robust biological intelligence to prevent system failure can provide critical insights into how complex brains generate adaptive movement and inspiration to design fault-tolerant robots. For flying animals, physical injury to a wing can have severe consequences, as flight is inherently unstable. Using a virtual reality flight arena, we studied how flying fruit flies compensate for damage to one wing. By combining experimental and mathematical methods, we show that flies compensate for wing damage by corrective wing movement modulated by closed-loop sensing and robust mechanics. Injured flies actively increase damping and, in doing so, modestly decrease flight performance but fly as stably as uninjured flies. Quantifying responses to injury can uncover the flexibility and robustness of biological systems while informing the development of bio-inspired fault-tolerant strategies.

Damage from intraspecific combat is costly

When individuals engage in fights with conspecifics over access to resources, injuries can occur. Most theoretical models suggest that the costs associated with these injuries should influence an individual’s decision to retreat from a fight. Thus, damage from intraspecific combat is frequently noted and quantified. However, the fitness-related costs associated with this damage are not. Quantifying the cost of fighting-related damage is important because most theoretical models assume that it is the cost associated with the damage (not the damage itself) that should influence an individual’s decision to retreat. Here, we quantified the cost of fighting-related injuries in the giant mesquite bug, Thasus neocalifornicus. We demonstrate that experimentally simulated fighting injuries result in metabolic costs and costs to flight performance. We also show that flight costs are more severe when the injuries are larger. Overall, our results provide empirical support for the fundamental assumption that damage acquired during intraspecific combat is costly.

Flight activity and age cause wing damage in house flies

Wing damage attenuates aerial performance in many flying animals such as birds, bats and insects. Especially insect wings are fragile and light in order to reduce inertial power requirements for flight at elevated wing flapping frequencies. There is a continuing debate on the factors causing wing damage in insects including collisions with objects, mechanical stress during flight activity, and aging. This experimental study is engaged with the reasons and significance of wing damage for flight in the house fly Musca domestica. We determined natural wing area loss under two housing conditions and recorded flight activity and flight ability throughout the animals' lifetime. Our data show that wing damage occurs on average after 6 h of flight, is sex-specific, and depends on housing conditions. Statistical tests show that both physiological age and flight activity have similar significance as predictors for wing damage. Tests on freely flying flies showed that minimum wing area for active flight is approximately 10-34% below the initial area and requires a left-right wing area asymmetry of less than approximately 25%. Our findings broadly confirm predictions from simple aerodynamic theory based on mean wing velocity and area, and are also consistent with previous wing damage measurements in other insect species.

Flight Mill Experiments and Computer Simulations Indicate Islands Recruit More Capable Flyers of Moths

Understanding the traits related to species colonization and invasion, is a key question for both pest management and evolution. One of the key components is flight, which has been measured for a number of insect species through radar and tethered flight mill systems, but a general understanding of insect flight at a community level is lacking. In this study, we used flight mill experiments to quantify flight abilities of moth species, and simulation experiments to study which moths in mainland China have the potential for cross-island dispersal. We found that moths from superfamily Geometroidea (family Geometridae) have the weakest flight ability among the seven Lepidoptera superfamilies, which is characterized by the shortest longest single flight (LSF), the shortest time corresponding to the longest single flight (TLSF) (timecorrespondingtothelongestsingleflight), the lowest total distance flown (TDF), and the lowest average speed during the flight (VTDF). Surprisingly, the family Pyralidae (superfamily Pyraloidea) has the highest flight endurance of all 186 species of 12 families in this study, which is unexpected, given its small size and morphological traits yet it shows the longest LSF and TLSF. The comparison between species common to mainland and islands shows that flight distance (LSF) may be more important for species spread than flight speed. The results of mainland-island simulations show that when P(LSF>CD) (the proportion of individuals whose LSF is greater than the closest distance (CD) between mainland and island to the total number of individuals in the population) is less than 0.004, it is difficult for moth species to disperse to across islands without relying on external factors such as airflow. Over extended periods, with the immigration of species with strong flight abilities, islands are more likely to recruit species with stronger flight abilities.

Wings and halteres act as coupled dual oscillators in flies

The mechanics of Dipteran thorax is dictated by a network of exoskeletal linkages that, when deformed by the flight muscles, generate coordinated wing movements. In Diptera, the forewings power flight, whereas the hindwings have evolved into specialized structures called halteres, which provide rapid mechanosensory feedback for flight stabilization. Although actuated by independent muscles, wing and haltere motion is precisely phase-coordinated at high frequencies. Because wingbeat frequency is a product of wing-thorax resonance, any wear-and-tear of wings or thorax should impair flight ability. How robust is the Dipteran flight system against such perturbations? Here, we show that wings and halteres are independently driven, coupled oscillators. We systematically reduced the wing length in flies and observed how wing-haltere synchronization was affected. The wing-wing system is a strongly coupled oscillator, whereas the wing-haltere system is weakly coupled through mechanical linkages that synchronize phase and frequency. Wing-haltere link acts in a unidirectional manner; altering wingbeat frequency affects haltere frequency, but not vice versa. Exoskeletal linkages are thus key morphological features of the Dipteran thorax that ensure wing-haltere synchrony, despite severe wing damage.

The type of leg lost affects habitat use but not survival in a non-regenerating arthropod

Finding shelter and surviving encounters with predators are pervasive challenges for animals. These challenges may be exacerbated after individuals experience bodily damage. Certain forms of damage arise voluntarily in animals; for instance, some taxa release appendages (tails, legs, or other body parts) as a defensive strategy ("autotomy"). This behavior, however, may pose long-term negative consequences for habitat use and survival. Additionally, these putative consequences are expected to vary according to the function of the lost body part. We tested the effects of losing different functional leg types (locomotor or sensory) on future habitat use and survival in a Neotropical species of Prionostemma harvestmen (Arachnida: Opiliones) that undergo frequent autotomy but do not regrow limbs. Daytime surveys revealed that both eight-legged harvestmen and harvestmen missing legs roosted in similar frequencies across habitats (tree bark, mossy tree, or fern), and perched at similar heights. Mark-recapture data showed that harvestmen that lost sensory legs roosted in tree bark less frequently, but on mossy trees more frequently. On the contrary, we did not observe changes in habitat use for eight-legged animals or animals that lost locomotor legs. This change might be related to sensory exploration and navigation. Lastly, we found that recapture rates across substrates were not affected by the type of legs lost, suggesting that leg loss does not impact survival. This potential lack of effect might play a role in why a defensive strategy like autotomy is so prevalent in harvestmen despite the lack of regeneration.

Pendulum-based measurements reveal impact dynamics at the scale of a trap-jaw ant

Small organisms can produce powerful, sub-millisecond impacts by moving tiny structures at high accelerations. We developed and validated a pendulum device to measure the impact energetics of microgram-sized trap-jaw ant mandibles accelerated against targets at 10⁵ m s^-2 Trap-jaw ants (Odontomachus brunneus; 19 individuals, 212 strikes) were suspended on one pendulum and struck swappable targets that were either attached to an opposing pendulum or fixed in place. Mean post-impact kinetic energy (energy from a strike converted to pendulum motion) was higher with a stiff target (21.0-21.5 µJ) than with a compliant target (6.4-6.5 µJ). Target mobility had relatively little influence on energy transfer. Mean contact duration of strikes against stiff targets was shorter (3.9-4.5 ms) than against compliant targets (6.2-7.9 ms). Shorter contact duration was correlated with higher post-impact kinetic energy. These findings contextualize and provide an energetic explanation for the diverse, natural uses of trap-jaw ant strikes such as impaling prey, launching away threats and performing mandible-powered jumps. The strong effect of target material on energetic exchange suggests material interactions as an avenue for tuning performance of small, high acceleration impacts. Our device offers a foundation for novel research into the ecomechanics and evolution of tiny biological impacts and their application in synthetic systems.

Wing damage affects flight kinematics but not flower tracking performance in hummingbird hawkmoths

Wing integrity is crucial to the many insect species that spend distinct portions of their life in flight. How insects cope with the consequences of wing damage is therefore a central question when studying how robust flight performance is possible with such fragile chitinous wings. It has been shown in a variety of insect species that the loss in lift-force production resulting from wing damage is generally compensated by an increase in wing beat frequency rather than amplitude. The consequences of wing damage for flight performance, however, are less well understood, and vary considerably between species and behavioural tasks. One hypothesis reconciling the varying results is that wing damage might affect fast flight manoeuvres with high acceleration, but not slower ones. To test this hypothesis, we investigated the effect of wing damage on the manoeuvrability of hummingbird hawkmoths (Macroglossum stellatarum) tracking a motorised flower. This assay allowed us to sample a range of movements at different temporal frequencies, and thus assess whether wing damage affected faster or slower flight manoeuvres. We show that hummingbird hawkmoths compensate for the loss in lift force mainly by increasing wing beat amplitude, yet with a significant contribution of wing beat frequency. We did not observe any effects of wing damage on flight manoeuvrability at either high or low temporal frequencies.

Weapon performance drives weapon evolution

Many sexually selected traits function as weapons, and these weapons can be incredibly diverse. However, the factors underlying weapon diversity among species remain poorly understood, and a fundamental hypothesis to explain this diversity remains untested. Although weapons can serve multiple functions, an undeniably important function is their role in fights. Thus, a crucial hypothesis is that weapon diversification is driven by the evolution of weapon modifications that provide an advantage in combat (e.g. causing more damage). Here, we test this fighting-advantage hypothesis using data from 17 species of coreid bugs. We utilize the fact that male-male combat in coreids often results in detectable damage, allowing us to link different weapon morphologies to different levels of damage among species. We find that certain weapon morphologies inflict much more damage than others, strongly supporting the fighting-advantage hypothesis. Moreover, very different weapon morphologies can inflict similarly severe amounts of damage, leading to a weapon performance landscape with multiple performance peaks. This multi-peak pattern could potentially drive different lineages towards divergent weapon forms, further increasing weapon diversity among species. Overall, our results may help explain how sexually selected weapons have evolved into the diversity of forms seen today.

Wing kinematic and aerodynamic compensations for unilateral wing damage in a small phorid fly

To investigate the way in which very small insects compensate for unilateral wing damage, we measured the wing kinematics of a very small insect, a phorid fly (Megaselia scalaris), with 16.7% wing area loss in the outer part of the left wing and a normal counterpart, and we computed the aerodynamic forces and power expenditures of the phorid flies. Our major findings are the following. The phorid fly compensates for unilateral wing damage by increasing the stroke amplitude and the deviation angle of the damaged wing (the large deviation angle gives the wing a deep U-shaped wing path), unlike the medium and large insects studied previously, which compensate for the unilateral wing damage mainly by increasing the stroke amplitude of the damaged wing. The increased stroke amplitude and the deep U-shaped wing path give the damaged wing a larger wing velocity during its flapping motion and a rapid downward acceleration in the beginning of the upstroke, which enable the damaged wing to generate the required vertical force for weight support. However, the larger wing velocity of the damaged wing also generates larger horizontal and side forces, increasing the resultant aerodynamic force of the damaged wing. Due to the larger aerodynamic force and the smaller wing area, the wing loading of the damaged wing is 25% larger than that of the wings of the normal phorid fly; this may greatly shorten the life of the damaged wing. Furthermore, because the damaged wing has much larger angular velocity and produces larger aerodynamic moment compared with the intact wing of the damaged phorid fly, the aerodynamic power consumed by the damaged wing is 38% larger than that by the intact wing, i.e., the energy distribution between the damaged and intact wings is highly asymmetrical; this may greatly increase the muscle wastage of the damaged side.

Circulation in Insect Wings

Insect wings are living, flexible structures composed of tubular veins and thin wing membrane. Wing veins can contain hemolymph (insect blood), tracheae, and nerves. Continuous flow of hemolymph within insect wings ensures that sensory hairs, structural elements such as resilin, and other living tissue within the wings remain functional. While it is well known that hemolymph circulates through insect wings, the extent of wing circulation (e.g., whether flow is present in every vein, and whether it is confined to the veins alone) is not well understood, especially for wings with complex wing venation. Over the last 100 years, scientists have developed experimental methods including microscopy, fluorescence, and thermography to observe flow in the wings. Recognizing and evaluating the importance of hemolymph movement in insect wings is critical in evaluating how the wings function both as flight appendages, as active sensors, and as thermoregulatory organs. In this review, we discuss the history of circulation in wings, past and present experimental techniques for measuring hemolymph, and broad implications for the field of hemodynamics in insect wings.

Morphology, performance and fluid dynamics of the crayfish escape response

Sexual selection can result in an exaggerated morphology that constrains locomotor performance. We studied the relationship between morphology and the tail-flip escape response in male and female rusty crayfish (Faxonius rusticus), a species in which males have enlarged claws (chelae). We found that females had wider abdomens and longer uropods (terminal appendage of the tail fan) than males, while males possessed deeper abdomens and larger chelae, relative to total length. Chelae size was negatively associated with escape velocity, whereas longer abdomens and uropods were positively associated with escape velocity. We found no sex-specific differences in maximum force generated during the tail flip, but uropod length was strongly, positively correlated with tail-flip force in males. Particle image velocimetry (PIV) revealed that the formation of a vortex, rather than the expulsion of fluid between two closing body surfaces, generates propulsion in rusty crayfish. PIV also revealed that the pleopods (ventral abdominal appendages) contribute to the momentum generated by the tail. To our knowledge, this is the first confirmation of vortex formation in a decapod crustacean.

Insect wing damage: causes, consequences and compensatory mechanisms

The evolution of wings has played a key role in the success of insect species, allowing them to diversify to fill many niches. Insect wings are complex multifunctional structures, which not only have to withstand aerodynamic forces but also need to resist excessive stresses caused by accidental collisions. This Commentary provides a summary of the literature on damage-reducing morphological adaptations in wings, covering natural causes of wing collisions, their impact on the structural integrity of wings and associated consequences for both insect flight performance and life expectancy. Data from the literature and our own observations suggest that insects have evolved strategies that (i) reduce the likelihood of wing damage and (ii) allow them to cope with damage when it occurs: damage-related fractures are minimized because wings evolved to be damage tolerant and, in the case of wing damage, insects compensate for the reduced aerodynamic efficiency with dedicated changes in flight kinematics.

Interindividual variation in learning ability in honeybees

Performance on different cognitive tasks could either be positively correlated in an individual as a measure of general intelligence or costs related to specific aspects of cognition could give rise to specialized cognitive phenotypes. Social living offers the potential for individual specialization in learning and a cooperative group can benefit from a diversity of learning phenotypes. However, there is little empirical data regarding the nature of such interindividual variation in learning abilities in honeybees, a classic model of animal cognition. We tested for the presence of variation in learning abilities in the honeybee, Apis mellifera, and whether any component of learning has an influence on wing damage, a proxy for performance and survival. Our results show considerable interindividual variation in different types of learning abilities. At the individual level, while landmark and olfactory learning abilities are negatively correlated, olfactory learning shows a positive association with maneuverability performance, a measure which in turn shows a positive influence on wing damage, a proxy for survival. We discuss our results in the context of cognitive diversity and specialization in a social group.

Resilin in the flight apparatus of Odonata (Insecta)-cap tendons and their biomechanical importance for flight

In Odonata, a direct flight mechanism with specialized tendons evolved. One particular adaptation, the implementation of the rubber-like protein resilin in these cap tendons, might be of major importance. Although resilin was first described in one tendon of Odonata, to our knowledge no comprehensive study about the presence of resilin in the thorax exists yet. We investigated various species of Odonata, using µCT, dissection and fluorescence microscopy. Here we show a complete mapping of the odonatan pterothorax, regarding the presence of tendons and their properties. Thus, 20-21 cap tendons in the pterothorax of Odonata show the presence of resilin. While performing outstanding and often-aggressive flight manoeuvres, resilin can provide shock absorption against mechanical damage from strong impacts. It may further improve the wear and fatigue resistance owing to resilin's damping behaviour. Additionally, resilin in tendons can absorb and return kinetic energy to restore muscles to their original shape after contracting and help in maintaining self-oscillation of the flight muscles. Here, the material distribution within the direct flight system of Odonata and the biomechanical importance and possible function of resilin are discussed. These results are an important step towards the understanding of the complex form-material-function interplay of the insect cuticle.

Effects of natural wing damage on flight performance in Morpho butterflies: what can it tell us about wing shape evolution?

Flying insects frequently experience wing damage during their life. Such irreversible alterations of wing shape affect flight performance and ultimately fitness. Insects have been shown to compensate for wing damage through various behavioural adjustments, but the importance of damage location over the wings has been scarcely studied. Using natural variation in wing damage, here we tested how the loss of different wing parts affect flight performance. We quantified flight performance in two species of large butterflies, Morpho helenor and M. achilles, caught in the wild, and displaying large variation in the extent and location of wing damage. We artificially generated more severe wing damage in our sample to contrast natural vs. higher magnitude of wing loss. Wing shape alteration across our sample was quantified using geometric morphometrics to test the effect of different damage distributions on flight performance. Our results show that impaired flight performance clearly depends on damage location over the wings, pointing out a relative importance of different wing parts for flight. Deteriorated forewings leading edge most crucially affected flight performance, specifically decreasing flight speed and proportion of gliding flight. In contrast, most frequent natural damage such as scattered wing margin had no detectable effect on flight behaviour. Damages located on the hindwings – although having a limited effect on flight – were associated with reduced flight height, suggesting that fore- and hindwings play different roles in butterfly flight. By contrasting harmless and deleterious consequences of various types of wing damage, our study points at different selective regimes acting on morphological variations of butterfly wings.

Early swelling response to phytohemagglutinin is lower in older toads

The effects of age on performance of life-history traits are diverse, but a common outcome is senescence, an irreversible deterioration of physical and physiological capabilities of older individuals. Immune response is potentially bound to senescence. However, little is known about immune response ageing in amphibians. In this work, we test the hypothesis that amphibian early immune response is reduced in older individuals. To this end, we captured adult natterjack toads (Epidalea calamita) and inoculated them with phytohemagglutinin, an innocuous protein that triggers a skin-swelling immune response whose magnitude is directly proportional to the ability of the individual to mount an immune response. We measured early swelling immune response (corresponding to an innate-response stage) hourly, for six hours, and we calculated the area under the curve (AUC) for each individual's time series, as a measure of immune response magnitude incorporating time. We estimated toad age by means of phalanx skeletochronology. Swelling and AUC decreased with age. Therefore, in accordance with our predictions, early immune response seems subject to senescence in these toads. Reduced ability to get over infections due to senescence of immune respose might be-together with a worse functioning of other organs and systems-among the causes of lower survival of older specimens.

Effects of the parasite, Ophryocystis elektroscirrha, on wing characteristics important for migration in the monarch butterfly

There is mounting evidence that the longterm declines of overwintering monarchs in Mexico are exacerbated by losses during the fall migratory journey. Infection with the protozoan, Ophryocystis elektroscirrha (OE), is known to negatively impact migration success. Here we examine how infections affect specific wing traits of monarchs that are important for migratory success. We used a collection of infected and uninfected monarchs reared under identical conditions, and from the (deceased) specimens, measured wing area (larger monarchs are known to have greater migratory success), wing color (the shade of orange pigmentation in monarchs is a known predictor of migration and flight ability), and the physical density of wings (a measure of wing mass per unit area). We also measured the tear-resistance of wings, using an apparatus that measured the force needed to cause a tear in the wing. Results showed no effect of OE on overall wing size, nor on the shade of orange pigmentation, but a clear effect on measures of physical density and tensile strength. Wings of infected monarchs weighed less per unit area (by 6%), and there was a 20% reduction in tear-resistance of wings. All results were qualitatively similar in a follow-up investigation using freshly-killed specimens. Collectively, this indicates infected monarchs are more prone to wing damage, which would be costly during long-distance migration. As such, this would be one more way in which OE infections reduce migratory success. Given the toll of OE to the monarch population, especially during migration, it would be prudent to focus conservation efforts on mitigating human activities that spread this disease.

Can the narrow red bands of dragonflies be used to perceive wing interference patterns?

Despite numerous studies of selection on position and number of spectral vision bands, explanations to the function of narrow spectral bands are lacking. We investigate dragonflies (Odonata), which have the narrowest spectral bands reported, in order to investigate what features these narrow spectral bands may be used to perceive. We address whether it is likely that narrow red bands can be used to identify conspecifics by the optical signature from wing interference patterns (WIPs). We investigate the optical signatures of Odonata wings using hyperspectral imaging, laser profiling, ellipsometry, polarimetric modulation spectroscopy, and laser radar experiments. Based on results, we estimate the prospects for Odonata perception of WIPs to identify conspecifics in the spectral, spatial, intensity, polarization, angular, and temporal domains. We find six lines of evidence consistent with an ability to perceive WIPs. First, the wing membrane thickness of the studied Odonata is 2.3 μm, coinciding with the maximal thickness perceivable by the reported bandwidth. Second, flat wings imply that WIPs persist from whole wings, which can be seen at a distance. Third, WIPs constitute a major brightness in the visual environment only second after the solar disk. Fourth, WIPs exhibit high degree of polarization and polarization vision coincides with frontal narrow red bands in Odonata. Fifth, the angular light incidence on the Odonata composite eye provides all prerequisites for direct assessment of the refractive index which is associated with age. Sixth, WIPs from conspecifics in flight make a significant contribution even to the fundamental wingbeat frequency within the flicker fusion bandwidth of Odonata vision. We conclude that it is likely that WIPs can be perceived by the narrow red bands found in some Odonata species and propose future behavioral and electrophysiological tests of this hypothesis.

It's Not a Bug, It's a Feature: Functional Materials in Insects

Over the course of their wildly successful proliferation across the earth, the insects as a taxon have evolved enviable adaptations to their diverse habitats, which include adhesives, locomotor systems, hydrophobic surfaces, and sensors and actuators that transduce mechanical, acoustic, optical, thermal, and chemical signals. Insect-inspired designs currently appear in a range of contexts, including antireflective coatings, optical displays, and computing algorithms. However, as over one million distinct and highly specialized species of insects have colonized nearly all habitable regions on the planet, they still provide a largely untapped pool of unique problem-solving strategies. With the intent of providing materials scientists and engineers with a muse for the next generation of bioinspired materials, here, a selection of some of the most spectacular adaptations that insects have evolved is assembled and organized by function. The insects presented display dazzling optical properties as a result of natural photonic crystals, precise hierarchical patterns that span length scales from nanometers to millimeters, and formidable defense mechanisms that deploy an arsenal of chemical weaponry. Successful mimicry of these adaptations may facilitate technological solutions to as wide a range of problems as they solve in the insects that originated them.

Wing wear reduces bumblebee flight performance in a dynamic obstacle course

Previous work has shown that wing wear increases mortality in bumblebees. Although a proximate mechanism for this phenomenon has remained elusive, a leading hypothesis is that wing wear increases predation risk by reducing flight manoeuvrability. We tested the effects of simulated wing wear on flight manoeuvrability in Bombus impatiens bumblebees using a dynamic obstacle course designed to push bees towards their performance limits. We found that removing 22% wing area from the tips of both forewings (symmetric wear) caused a 9% reduction in peak acceleration during manoeuvring flight, while performing the same manipulation on only one wing (asymmetric wear) did not significantly reduce maximum acceleration. The rate at which bees collided with obstacles was correlated with body length across all treatments, but wing wear did not increase collision rate, possibly because shorter wingspans allow more room for bees to manoeuvre. This study presents a novel method for exploring extreme flight manoeuvres in flying insects, eliciting peak accelerations that exceed those measured during flight through a stationary obstacle course. If escape from aerial predation is constrained by acceleration capacity, then our results offer a potential explanation for the observed increase in bumblebee mortality with wing wear.

Flight of the dragonflies and damselflies

This work is a synthesis of our current understanding of the mechanics, aerodynamics and visually mediated control of dragonfly and damselfly flight, with the addition of new experimental and computational data in several key areas. These are: the diversity of dragonfly wing morphologies, the aerodynamics of gliding flight, force generation in flapping flight, aerodynamic efficiency, comparative flight performance and pursuit strategies during predatory and territorial flights. New data are set in context by brief reviews covering anatomy at several scales, insect aerodynamics, neuromechanics and behaviour. We achieve a new perspective by means of a diverse range of techniques, including laser-line mapping of wing topographies, computational fluid dynamics simulations of finely detailed wing geometries, quantitative imaging using particle image velocimetry of on-wing and wake flow patterns, classical aerodynamic theory, photography in the field, infrared motion capture and multi-camera optical tracking of free flight trajectories in laboratory environments. Our comprehensive approach enables a novel synthesis of datasets and subfields that integrates many aspects of flight from the neurobiology of the compound eye, through the aeromechanical interface with the surrounding fluid, to flight performance under cruising and higher-energy behavioural modes.

This article is part of the themed issue ‘Moving in a moving medium: new perspectives on flight’.

The probability of wing damage in the dragonfly Sympetrum vulgatum (Anisoptera: Libellulidae): a field study

Dragonfly wings resist millions of cycles of dynamic loading in their lifespan. During their operation, the wings are subjected to relatively high mechanical stresses. They further experience accidental collisions which result from the insects' daily activities, such as foraging, mating and fighting with other individuals. All these factors may lead to irreversible wing damage. Here, for the first time, we collected qualitative and quantitative data to systematically investigate the occurrence of damage in dragonfly wings in nature. The results obtained from the analysis of 119 wings from >30 individual Sympetrum vulgatum (Anisoptera: Libellulidae), collected at the second half of their flight period, indicate a high risk of damage in both fore- and hindwings. Statistical analyses show no significant difference between the extent of damage in fore- and hindwings, or between male and female dragonflies. However, we observe a considerable difference in the probability of damage in different wing regions. The wing damage is found to mainly result from two failure modes: wear and fracture.

Summary: This study provides the first qualitative and quantitative data on the occurrence of damage in the wings of the dragonfly Sympetrum vulgatum in nature. This article has an associated First Person interview with the first author of the paper as part of the supplementary information.

Asymmetry costs: effects of wing damage on hovering flight performance in the hawkmoth Manduca sexta

Flight performance is fundamental to the fitness of flying organisms. Whilst airborne, flying organisms face unavoidable wing wear and wing area loss. Many studies have tried to quantify the consequences of wing area loss to flight performance with varied results, suggesting that not all types of damage are equal and different species may have different means to compensate for some forms of wing damage with little to no cost. Here, we investigated the cost of control during hovering flight with damaged wings, specifically wings with asymmetric and symmetric reductions in area, by measuring maximum load lifting capacity and the metabolic power of hovering flight in hawkmoths (Manduca sexta). We found that while asymmetric and symmetric reductions are both costly in terms of maximum load lifting and hovering efficiency, asymmetric reductions are approximately twice as costly in terms of wing area lost. The moths also did not modulate flapping frequency and amplitude as predicted by a hovering flight model, suggesting that the ability to do so, possibly tied to asynchronous versus synchronous flight muscles, underlies the varied responses found in different wing clipping experiments.

Morphological variation of Aphidius ervi Haliday (Hymenoptera: Braconidae) associated with different aphid hosts

Background

Parasitoids are frequently used in biological control due to the fact that they are considered host specific and highly efficient at attacking their hosts. As they spend a significant part of their life cycle within their hosts, feeding habits and life history of their host can promote specialization via host-race formation (sequential radiation). The specialized host races from different hosts can vary morphologically, behaviorally and genetically. However, these variations are sometimes inconspicuous and require more powerful tools in order to detect variation such as geometric morphometrics analysis.

Methods

We examined Aphidius ervi, an important introduced biological control agent in Chile associated with a great number of aphid species, which are exploiting different plant hosts and habitats. Several combinations (biotypes) of parasitoids with various aphid/host plant combinations were analyzed in order to obtain measures of forewing shape and size. To show the differences among defined biotypes, we chose 13 specific landmarks on each individual parasitoid wing. The analysis of allometric variation calculated in wing shape and size over centroid size (CS), revealed the allometric changes among biotypes collected from different hosts. To show all differences in shape of forewings, we made seven biotype pairs using an outline-based geometric morphometrics comparison.

Results

The biotype A. pis_pea (Acyrthosiphon pisum on pea) was the extreme wing size in this study compared to the other analyzed biotypes. Aphid hosts have a significant influence in the morphological differentiation of the parasitoid forewing, splitting biotypes in two groups. The first group consisted of biotypes connected with Acyrthosiphon pisum on legumes, while the second group is composed of biotypes connected with aphids attacking cereals, with the exception of the R. pad_wheat (Rhopalosiphum padi on wheat) biotype. There was no significant effect of plant species on parasitoid wing size and shape.

Discussion

Although previous studies have suggested that the genotype of parasitoids is of greater significance for the morphological variations of size and shape of wings, this study indicates that the aphid host on which A. ervi develops is the main factor to alter the structure of parasitoid forewings. Bigger aphid hosts implied longer and broader forewings of A. ervi.

Flies compensate for unilateral wing damage through modular adjustments of wing and body kinematics

Using high-speed videography, we investigated how fruit flies compensate for unilateral wing damage, in which loss of area on one wing compromises both weight support and roll torque equilibrium. Our results show that flies control for unilateral damage by rolling their body towards the damaged wing and by adjusting the kinematics of both the intact and damaged wings. To compensate for the reduction in vertical lift force due to damage, flies elevate wingbeat frequency. Because this rise in frequency increases the flapping velocity of both wings, it has the undesired consequence of further increasing roll torque. To compensate for this effect, flies increase the stroke amplitude and advance the timing of pronation and supination of the damaged wing, while making the opposite adjustments on the intact wing. The resulting increase in force on the damaged wing and decrease in force on the intact wing function to maintain zero net roll torque. However, the bilaterally asymmetrical pattern of wing motion generates a finite lateral force, which flies balance by maintaining a constant body roll angle. Based on these results and additional experiments using a dynamically scaled robotic fly, we propose a simple bioinspired control algorithm for asymmetric wing damage.

Kinematic compensation for wing loss in flying damselflies

Flying insects can tolerate substantial wing wear before their ability to fly is entirely compromised. In order to keep flying with damaged wings, the entire flight apparatus needs to adjust its action to compensate for the reduced aerodynamic force and to balance the asymmetries in area and shape of the damaged wings. While several studies have shown that damaged wings change their flapping kinematics in response to partial loss of wing area, it is unclear how, in insects with four separate wings, the remaining three wings compensate for the loss of a fourth wing. We used high-speed video of flying blue-tailed damselflies (Ischnura elegans) to identify the wingbeat kinematics of the two wing pairs and compared it to the flapping kinematics after one of the hindwings was artificially removed. The insects remained capable of flying and precise maneuvering using only three wings. To compensate for the reduction in lift, they increased flapping frequency by 18±15.4% on average. To achieve steady straight flight, the remaining intact hindwing reduced its flapping amplitude while the forewings changed their stroke plane angle so that the forewing of the manipulated side flapped at a shallower stroke plane angle. In addition, the angular position of the stroke reversal points became asymmetrical. When the wingbeat amplitude and frequency of the three wings were used as input in a simple aerodynamic model, the estimation of total aerodynamic force was not significantly different (paired t-test, p=0.73) from the force produced by the four wings during normal flight. Thus, the removal of one wing resulted in adjustments of the motions of the remaining three wings, exemplifying the precision and plasticity of coordination between the operational wings. Such coordination is vital for precise maneuvering during normal flight but it also provides the means to maintain flight when some of the wings are severely damaged.

Recent developments in the study of insect flight

Here we review recent contributions to the study of insect flight, in particular those brought about by advances in experimental techniques. We focus particularly on the following areas: wing flexibility and deformation, the physiology and biophysics of asynchronous insect flight muscle, the aerodynamics of flight, and stability and maneuverability. This recent research reveals the importance of wing flexibility to insect flight, provides a detailed model of how asynchronous flight muscle functions and how it may have evolved, synthesizes many recent studies of insect flight aerodynamics into a broad-reaching summary of unsteady flight aerodynamics, and highlights new insights into the sources of flight stability in insects. The focus on experimental techniques and recently developed apparatus shows how these advancements have occurred and point the way towards future experiments.

Parental effects and flight behaviour in the burying beetle, Nicrophorus vespilloides

Parents play a key role in determining the phenotype of their offspring. However, relatively few studies have investigated whether parents can change their offspring's behaviour in a sustained way that persists into adulthood. With experiments on the burying beetle, Nicrophorus vespilloides, we investigated how the developmental environment created by parents affects their offspring's wing morphology in adulthood, and the correlated effects on adult flight behaviour. Burying beetles exhibit complex biparental care, but offspring can survive without parental provisioning. By removing parents just prior to hatching, while holding the nutritional environment constant, we investigated the downstream consequences for offspring morphology and behaviour. Larvae that developed in the absence of their parents had relatively long and more slender wings than those that developed in their parents' presence. Flight mill tests revealed that flight performance was dependent on the presence of parents during development but not on wing shape. Our results demonstrate that parents have long-lasting effects on the behaviour of their offspring, by influencing the morphology and flight behaviour of their young even after they have matured into adults.

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We investigated parental influence on offspring's morphology and flight behaviour.

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Parental care quality affects wing shape allometries and flight performance.

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Wing shape and body size do not affect flight performances in a flight mill.

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Parental care quality affects offspring's morphology and behaviour into adulthood.

Adaptive control of a millimeter-scale flapping-wing robot

Challenges for the controlled flight of a robotic insect are due to the inherent instability of the system, complex fluid-structure interactions, and the general lack of a complete system model. In this paper, we propose theoretical models of the system based on the limited information available from previous work and a comprehensive flight controller. The modular flight controller is derived from Lyapunov function candidates with proven stability over a large region of attraction. Moreover, it comprises adaptive components that are capable of coping with uncertainties in the system that arise from manufacturing imperfections. We have demonstrated that the proposed methods enable the robot to achieve sustained hovering flights with relatively small errors compared to a non-adaptive approach. Simple lateral maneuvers and vertical takeoff and landing flights are also shown to illustrate the fidelity of the flight controller. The analysis suggests that the adaptive scheme is crucial in order to achieve millimeter-scale precision in flight control as observed in natural insect flight.

Wing wear, but not asymmetry in wear, affects load-lifting capability in bumble bees Bombus impatiens

Wing wear is widespread in flying insects, but its effects on flight are controversial. In this research, we examine the separate and combined effects of wing area and wing area asymmetry on maximum load-lifting capability in bumble bees Bombus impatiens Cresson, 1863. Individual bees with experimentally induced forewing wear (0%–24% forewing area loss, 0%–38% forewing area asymmetry) were harnessed with a string to which small bead groups were attached and tested in a flight chamber to measure the maximum weight that they could lift incrementally. Wing wear significantly decreased load-lifting ability: the higher the mean wing area loss, the less mass a bee could lift (2.66 mg load reduction per 1% forewing area loss, which represents ∼1.6% of mean body mass or ∼5.2% of expected mean nectar load). However, wing area asymmetry, both alone and in combination with area loss, had no detectable effect on maximum lift. The clear cost of wing wear for bumble bees is a linear reduction in weight-lifting capability through loss of wing area. This relatively strong diminution of load lifting by wing wear, observed over the range of wing area losses naturally accrued by wild bees, provides a potential mechanism for declining foraging ability and survivorship of worker bees with wing wear. What remains to be explained is the utter insensitivity of maximum load lifted to forewing asymmetry.

Biomechanical strategies for mitigating collision damage in insect wings: structural design versus embedded elastic materials

The wings of many insects accumulate considerable wear and tear during their lifespan, and this irreversible structural damage can impose significant costs on insect flight performance and survivability. Wing wear in foraging bumblebees (and likely many other species) is caused by inadvertent, repeated collisions with vegetation during flight, suggesting the possibility that insect wings may display biomechanical adaptations to mitigate the damage associated with collisions. We used a novel experimental technique to artificially induce wing wear in bumblebees and yellowjacket wasps, closely related species with similar life histories but distinct wing morphologies. Wasps have a flexible resilin joint (the costal break) positioned distally along the leading edge of the wing, which allows the wing tip to crumple reversibly when it hits an obstacle, whereas bumblebees lack an analogous joint. Through experimental manipulation of its stiffness, we found that the costal break plays a critical role in mitigating collision damage in yellowjacket wings. However, bumblebee wings do not experience as much damage as would be expected based on their lack of a costal break, possibly due to differences in the spatial arrangement of supporting wing veins. Our results indicate that these two species utilize different wing design strategies for mitigating damage resulting from collisions. A simple inertial model of a flapping wing reveals the biomechanical constraints acting on the costal break, which may help explain its absence in bumblebee wings.

Habitat variation and wing coloration affect wing shape evolution in dragonflies

Habitats are spatially and temporally variable, and organisms must be able to track these changes. One potential mechanism for this is dispersal by flight. Therefore, we would expect flying animals to show adaptations in wing shape related to habitat variation. In this work, we explored variation in wing shape in relation to preferred water body (flowing water or standing water with tolerance for temporary conditions) and landscape (forested to open) using 32 species of dragonflies of the genus Trithemis (80% of the known species). We included a potential source of variation linked to sexual selection: the extent of wing coloration on hindwings. We used geometric morphometric methods for studying wing shape. We also explored the phenotypic correlation of wing shape between the sexes. We found that wing shape showed a phylogenetic structure and therefore also ran phylogenetic independent contrasts. After correcting for the phylogenetic effects, we found (i) no significant effect of water body on wing shape; (ii) male forewings and female hindwings differed with regard to landscape, being progressively broader from forested to open habitats; (iii) hindwings showed a wider base in wings with more coloration, especially in males; and (iv) evidence for phenotypic correlation of wing shape between the sexes across species. Hence, our results suggest that natural and sexual selection are acting partially independently on fore- and hindwings and with differences between the sexes, despite evidence for phenotypic correlation of wing shape between males and females.

Wing shape allometry and aerodynamics in calopterygid damselflies: a comparative approach

BACKGROUND

Wing size and shape have important aerodynamic implications on flight performance. We explored how wing size was related to wing shape in territorial males of 37 taxa of the damselfly family Calopterygidae. Wing coloration was also included in the analyses because it is sexually and naturally selected and has been shown to be related to wing shape. We studied wing shape using both the non-dimensional radius of the second moment of wing area (RSM) and geometric morphometrics. Lower values of the RSM result in less energetically demanding flight and wider ranges of flight speed. We also re-analyzed previously published data on other damselflies and dragonflies.

RESULTS

The RSM showed a hump-shaped relationship with wing size. However, after correcting for phylogeny using independent contrast, this pattern changed to a negative linear relationship. The basal genus of the study family, Hetaerina, was mainly driving that change. The obtained patterns were specific for the study family and differed from other damselflies and dragonflies. The relationship between the RSM and wing shape measured by geometric morphometrics was linear, but relatively small changes along the RSM axis can result in large changes in wing shape. Our results also showed that wing coloration may have some effect on RSM.

CONCLUSIONS

We found that RSM showed a complex relationship with size in calopterygid damselflies, probably as a result of other selection pressures besides wing size per se. Wing coloration and specific behavior (e.g. courtship) are potential candidates for explaining the complexity. Univariate measures of wing shape such as RSM are more intuitive but lack the high resolution of other multivariate techniques such as geometric morphometrics. We suggest that the relationship between wing shape and size are taxa-specific and differ among closely-related insect groups.

Wing flexibility enhances load-lifting capacity in bumblebees

The effect of wing flexibility on aerodynamic force production has emerged as a central question in insect flight research. However, physical and computational models have yielded conflicting results regarding whether wing deformations enhance or diminish flight forces. By experimentally stiffening the wings of live bumblebees, we demonstrate that wing flexibility affects aerodynamic force production in a natural behavioural context. Bumblebee wings were artificially stiffened in vivo by applying a micro-splint to a single flexible vein joint, and the bees were subjected to load-lifting tests. Bees with stiffened wings showed an 8.6 per cent reduction in maximum vertical aerodynamic force production, which cannot be accounted for by changes in gross wing kinematics, as stroke amplitude and flapping frequency were unchanged. Our results reveal that flexible wing design and the resulting passive deformations enhance vertical force production and load-lifting capacity in bumblebees, locomotory traits with important ecological implications.

Linking biomechanics and ecology through predator–prey interactions: flight performance of dragonflies and their prey

Aerial predation is a highly complex, three-dimensional flight behavior that affects the individual fitness and population dynamics of both predator and prey. Most studies of predation adopt either an ecological approach in which capture or survival rates are quantified, or a biomechanical approach in which the physical interaction is studied in detail. In the present study, we show that combining these two approaches provides insight into the interaction between hunting dragonflies (Libellula cyanea) and their prey (Drosophila melanogaster) that neither type of study can provide on its own. We performed >2500 predation trials on nine dragonflies housed in an outdoor artificial habitat to identify sources of variability in capture success, and analyzed simultaneous predator–prey flight kinematics from 50 high-speed videos. The ecological approach revealed that capture success is affected by light intensity in some individuals but that prey density explains most of the variability in success rate. The biomechanical approach revealed that fruit flies rarely respond to approaching dragonflies with evasive maneuvers, and are rarely successful when they do. However, flies perform random turns during flight, whose characteristics differ between individuals, and these routine, erratic turns are responsible for more failed predation attempts than evasive maneuvers. By combining the two approaches, we were able to determine that the flies pursued by dragonflies when prey density is low fly more erratically, and that dragonflies are less successful at capturing them. This highlights the importance of considering the behavior of both participants, as well as their biomechanics and ecology, in developing a more integrative understanding of organismal interactions.

Neuromuscular and biomechanical compensation for wing asymmetry in insect hovering flight

Wing damage is common in flying insects and has been studied using a variety of approaches to assess its biomechanical and fitness consequences. Results of these studies range from strong to nil effect among the variety of species, fitness measurements and damage modes studied, suggesting that not all damage modes are equal and that insects may be well adapted to compensate for some types of damage. Here, we examine the biomechanical and neuromuscular means by which flying insects compensate for asymmetric wing damage, which is expected to produce asymmetric flight forces and torques and thus destabilize the animal in addition to reducing its total wing size. We measured the kinematic and neuromuscular responses of hawkmoths (Manduca sexta) hovering in free flight with asymmetrically damaged wings via high-speed videography and extracellular neuromuscular activity recordings. The animals responded to asymmetric wing damage with asymmetric changes to wing stroke amplitude sufficient to restore symmetry in lift production. These asymmetries in stroke amplitude were significantly correlated with bilateral asymmetries in the timing of activation of the dorsal ventral muscle among and within trials. Correspondingly, the magnitude of wing asymmetry was significantly although non-linearly correlated with the magnitude of the neuromuscular response among individuals. The strongly non-linear nature of the relationship suggests that active neural compensation for asymmetric wing damage may only be necessary above a threshold, >12% asymmetry in wing second moment of area in this case, below which passive mechanisms may be adequate to maintain flight stability.

Resilin in dragonfly and damselfly wings and its implications for wing flexibility

Although there is mounting evidence that passive mechanical dynamics of insect wings play an integral role in insect flight, our understanding of the structural details underlying insect wing flexibility remains incomplete. Here, we use comparative morphological and mechanical techniques to illuminate the function and diversity of two mechanisms within Odonata wings presumed to affect dynamic wing deformations: flexible resilin vein-joints and cuticular spikes. Mechanical tests show that joints with more resilin have lower rotational stiffness and deform more in response to a load applied to an intact wing. Morphological studies of 12 species of Odonata reveal that resilin joints and cuticular spikes are widespread taxonomically, yet both traits display a striking degree of morphological and functional diversity that follows taxonomically distinct patterns. Interestingly, damselfly wings (suborder Zygoptera) are mainly characterized by vein-joints that are double-sided (containing resilin both dorsally and ventrally), whereas dragonfly wings (suborder Epiprocta) are largely characterized by single-sided vein-joints (containing resilin either ventrally or dorsally, but not both). The functional significance and diversity of resilin joints and cuticular spikes could yield insight into the evolutionary relationship between form and function of wings, as well as revealing basic principles of insect wing mechanical design.

What causes wing wear in foraging bumble bees?

Flying is an ecologically important behaviour in many insects, but it often results in permanent wing damage. Although wing wear in insects is often used as a method to determine insect age, and is associated with an increased risk of mortality, the causes of wing wear are unresolved. In this paper, we examine whether wing use while foraging explains wing wear in bumble bees (Bombus spp.). Wing wear may result from three distinct flight characteristics during foraging: time spent in flight, flight frequency and frequency of wing collisions with vegetation. To test these hypotheses for causes of wing wear, we recorded digital video of individually marked bumble bees foraging in nature on 12 different plant species that result in variation in these flight characteristics, and recaptured these individuals to photograph their wings over time. Bumble bees with a higher frequency of wing collisions showed an increased loss of wing area, which became more severe over time. Neither time in flight nor flight frequency was uniquely and significantly associated with wing wear. Therefore, the collision frequency hypothesis best explained wing wear in bumble bees. We conclude that wing use during foraging in bumble bees results in wing wear. Wing wear reflects behaviour, not simply age. Because wing wear has elsewhere been shown to increase mortality, this study provides an important mechanism linking foraging behaviour with lifespan.