The evolution of autotomy in leaf-footed bugs

Sexually selected weapons can wear out, decreasing their effectiveness in combat

Sexual selection has resulted in some of the most elaborate traits seen in animals, many of which are used as weapons. These weapons can be incredibly diverse, even within species. Such morphological variation has largely been attributed to the environment in which individuals are reared and their genetics. However, variation in weapon form could also be the result of a weapon wearing out from usage. This mechanism has received relatively little attention. In this study, we demonstrate that sexually selected weapons can wear out from repeated use, providing experimental evidence that weapon usage can contribute to the diversity of weapon shapes observed within species. In a second experiment, we demonstrate that having a worn-out weapon decreases an individual's fighting ability. This finding illustrates that the shape of a weapon can have an important role in determining contest outcomes. Overall, these results suggest that individuals are limited in the number of times they can effectively use their weapons, which may be one factor (among others) influencing how frequently an animal engages in a fight.

Interleg coordination in free-walking bug Erthesina fullo (Hemiptera: Pentatomidae)

Insects can adapt their walking patterns to complex and varied environments and retain the ability to walk even after significant changes in their physical attributes, such as amputation. Although the interleg coordination of intact insects has been widely described in previous studies, the adaptive walking patterns in free-walking insects with amputation of 1 or more legs are still unclear. The pentatomid bug Erthesina fullo exhibits a tripod gait, when walking freely on horizontal substrates, like many other insects. In this study, amputations were performed on this species to investigate changes in interleg coordination. The walking parameters were analyzed, such as the locations of touchdown and liftoff, cycle period, walking speed, and head displacement of intact and amputated insects. The results show that E. fullo displays adaptive interleg coordination in response to amputations. With 1 amputated leg, bugs changed to a 3-unit gait, whereas with 2 amputated legs they employed a wave gait. These data are helpful in exploring the motion mode control in walking insects and provide the theoretical basis for the gait control strategy of robots, when leg failure occurs.

Shake it off: exploring drivers and outcomes of autotomy in marine invertebrates

Autotomy refers to self-amputation where the loss of a limb or organ is generally said to be (1) in response to stressful external stimuli; (2) voluntary and nervously mediated; (3) supported by adaptive features that increase efficiency and simultaneously mediate the cost; and (4) morphologically delineated by a predictable breakage plane. It is estimated that this phenomenon has evolved independently nine different times across the animal kingdom, appearing in many different taxa, including vertebrate and invertebrate as well as aquatic and terrestrial animals. Marine invertebrates use this behaviour in a diversity of manners that have yet to be globally reviewed and critically examined. Here, published data from marine invertebrate taxa were used to explore instances of injury as an evolutionary driver of autotomy. Findings suggest that phyla (e.g. Echinodermata and Arthropoda) possibly experiencing high rates of injury (tissue damage or loss) are more likely to be able to perform autotomy. Additionally, this review looks at various morphological, physiological and environmental conditions that have either driven the evolution or maintained the behaviour of autotomy in marine invertebrates. Finally, the use of autotomic abilities in the development of more sustainable and less ecologically invasive fisheries is explored.

Limb loss and specialized leg dynamics in tiny water-walking insects

The air-water of the planet's water bodies, such as ponds, lakes and streams, presents an uncertain ecological niche with predatory threats from above and below. As Microvelia move across the water surface in small ponds, they face potential injury from attacks by birds, fish, and underwater invertebrates. Thus, our study investigates the effects of losing individual or pairs of tarsi on the Microvelia's ability to walk on water. Removal of both hind tarsi causes Microvelia spp. to rock their bodies (yaw) while running across the water surface at ±19°, compared to ±7° in non-ablated specimens. This increase in yaw, resulting from the removal of hind tarsi, indicates that Microvelia use their hind legs as 'rudders' to regulate yaw, originating from the contralateral middle legs' strokes on the water's surface through an alternating tripod gait. Ablation of the ipsilateral middle and hind tarsi disrupts directionality, making Microvelia turn in the direction of their intact limbs. This loss of directionality does not occur with the removal of contralateral middle and hind tarsi. However, Microvelia lose their ability to use the alternating tripod gait to walk for water walking on the day of contralateral ablation. Remarkably, by the next day Microvelia adapt and regain the ability to walk on water using the alternating tripod gait. Our findings elucidate the specialized leg dynamics within the alternating tripod gait of Microvelia spp., and their adaptability to tarsal loss. This research could guide the development and design strategies of small, adaptive, and resilient micro-robots that can adapt to controller malfunction or actuator damage for walking on water and terrestrial surfaces.

A Flexible Escape Skin Bioinspired by the Defensive Behavior of Shedding Scales

Artificial skins with functions such as sensing, variable stiffness, actuation, self-healing, display, adhesion, and camouflage have been developed and widely used, but artificial skins with escape function are still a research gap. In nature, every species of animal can use its innate skills and functions to escape capture. Inspired by the behavior of fish-scale geckoes escaping predation by shedding scales when grasped or touched, we propose a flexible escape skin by attaching artificial scales to a flexible film. Experiments demonstrate that the escape skin has significant effects in reducing escape force, escaping from harmful force environments, and resisting mechanical damage. Furthermore, we enabled active control of escape force and skin hardness by changing temperature, increasing the adaptability of the escape skin to the surrounding. Our study helps lay the foundation for engineering systems that depend on escape skin to improve robustness.

The evolution of multi-component weapons in the superfamily of leaf-footed bugs

Sexually selected weapons, such as the antlers of deer, claws of crabs, and tusks of beaked whales, are strikingly diverse across taxa and even within groups of closely related species. Phylogenetic comparative studies have typically taken a simplified approach to investigate the evolution of weapon diversity, examining the gains and losses of entire weapons, major shifts in size or type, or changes in location. Less understood is how individual weapon components evolve and assemble into a complete weapon. We addressed this question by examining weapon evolution in the diverse, multi-component hind-leg and body weapons of leaf-footed bugs, superfamily Coreoidea (Hemiptera: Heteroptera). Male leaf-footed bugs use their morphological weapons to fight for access to mating territories. We used a large multilocus dataset comprised of ultraconserved element loci for 248 species and inferred evolutionary transitions among component states using ancestral state estimation. Our results suggest that weapons added components over time with some evidence of a cyclical evolutionary pattern-gains of components followed by losses and then gains again. Furthermore, our best estimate indicated that certain trait combinations evolved repeatedly across the phylogeny, suggesting that they function together in battle or that they are genetically correlated. This work reveals the remarkable and dynamic evolution of weapon form in the leaf-footed bugs and provides insights into weapon assembly and disassembly over evolutionary time.

Petal-shaped femoral lobes facilitate gliding in orchid mantises

To glide in forest canopies, arboreal vertebrates evolved various skin-derived aerodynamic structures, such as patagial membranes or webbing, but no comparable structure has been reported from wingless arboreal arthropods.1,2,3 Orchid mantises (Hymenopus coronatus) have been traditionally considered a textbook example of flower mimicry for ∼200 years due to their highly expanded, petal-shaped femoral lobes. However, the empirical evidence substantiating the petal-mimicry function of the femoral lobes has not been entirely conclusive.4,5,6 Observational and experimental evidence suggests that these lobes do not contribute to flower mimicry for luring pollinators6,7 and likely serve other functions.7,8 After observing their aerial escape initiated with active jumping, we hypothesized that orchid mantises can glide and that their femoral lobes are used for gliding. Through behavioral investigations and morphological analyses, we show that orchid mantis nymphs are excellent gliders, exhibiting the shallowest gliding trajectories observed in terrestrial invertebrates.9,10,11,12,13 The lobe extensions on their femoral segments are cambered airfoils, which increase the mantis projected area by ∼36% and play a vital role in the aerodynamic underpinning of the observed gliding. Despite a 165-fold increase in body mass throughout ontogeny, older female mantis nymphs maintained a persistent gliding capability. We further showed a notable 40%-56% reduction in wing loading attributed to the positive size allometry of these lobes, indicating a clear promotion of gliding throughout ontogeny. This is the first documentation of gliding-adapted "leg wings" in a wingless arthropod. The evolution of such structures is potentially common among arboreal arthropods and demands a systematic re-examination.

Snow flies self-amputate freezing limbs to sustain behavior at sub-zero temperatures

Temperature profoundly impacts all living creatures. In spite of the thermodynamic constraints on biology, some animals have evolved to live and move in extremely cold environments. Here, we investigate behavioral mechanisms of cold tolerance in the snow fly (Chionea spp.), a flightless crane fly that is active throughout the winter in boreal and alpine environments of the northern hemisphere. Using thermal imaging, we show that adult snow flies maintain the ability to walk down to an average body temperature of -7°C. At this supercooling limit, ice crystallization occurs within the snow fly's hemolymph and rapidly spreads throughout the body, resulting in death. However, we discovered that snow flies frequently survive freezing by rapidly amputating legs before ice crystallization can spread to their vital organs. Self-amputation of freezing limbs is a last-ditch tactic to prolong survival in frigid conditions that few animals can endure. Understanding the extreme physiology and behavior of snow insects holds particular significance at this moment when their alpine habitats are rapidly changing due to anthropogenic climate change. VIDEO ABSTRACT.

Selection on weapon allometry in the wild

Allometry is the scaling relationship between a trait and body size. This relationship can often explain considerable morphological variation within and among species. Nevertheless, much remains unknown about the factors that underlie allometric patterns. For example, when different allometric relationships are observed amongst closely related species, these differences are regularly considered to be products of selection. However, directional selection on allometry (particularly the slope) has rarely been tested and observed in natural populations. Here, we investigate selection on the scaling relationship between weapon size and body size (i.e., weapon allometry) in a wild population of giant mesquite bugs, Pachylis neocalifornicus (previously Thasus neocalifornicus). Males in this species use their weapons (enlarged femurs) to compete with one another over access to resources and females. We found that large males with relatively large weapons successfully secured access to mates. However, we also found that small males with relatively small weapons could access mates as well. These two patterns together can increase the allometric slope of the sexually selected weapon, suggesting a straightforward process by which the allometric slope can evolve.

Behavioural Indicators of Pain and Suffering in Arthropods and Might Pain Bite Back?

Pain in response to tissue damage functions to change behaviour so that further damage is minimised whereas healing and survival are promoted. This paper focuses on the behavioural criteria that match the function to ask if pain is likely in the main taxa of arthropods. There is evidence consistent with the idea of pain in crustaceans, insects and, to a lesser extent, spiders. There is little evidence of pain in millipedes, centipedes, scorpions, and horseshoe crabs but there have been few investigations of these groups. Alternative approaches in the study of pain are explored and it is suggested that studies on traumatic mating, agonistic interactions, and defensive venoms might provide clues about pain. The evolution of high cognitive ability, sensory systems, and flexible decision-making is discussed as well as how these might influence the evolution of pain-like states.

Juvenile leg autotomy predicts adult male morph in a New Zealand harvestman with weapon polymorphism

Intraspecific weapon polymorphisms that arise via conditional thresholds may be affected by juvenile experience such as predator encounters, yet this idea has rarely been tested. The New Zealand harvestman Forsteropsalis pureora has three male morphs: majors (alphas and betas) are large-bodied with large chelicerae used in male-male contests, while minors (gammas) are small-bodied with small chelicerae and scramble to find mates. Individuals use leg autotomy to escape predators and there is no regeneration of the missing leg. Here, we tested whether juvenile experience affects adult morph using leg autotomy scars as a proxy of predator encounters. Juvenile males that lost at least one leg (with either locomotory or sensory function) had a 45 times higher probability of becoming a minor morph at adulthood than intact juvenile males. Leg loss during development may affect foraging, locomotion, and/or physiology, potentially linking a juvenile's predator encounters to their final adult morph and future reproductive tactic.

Low hybridization temperatures improve target capture success of invertebrate loci: a case study of leaf-footed bugs (Hemiptera: Coreoidea)

Target capture is widely used in phylogenomic, ecological and functional genomic studies. Bait sets that allow capture from a diversity of species can be advantageous, but high-sequence divergence from baits can limit yields. Currently, only four experimental comparisons of a critical target capture parameter, hybridization temperature, have been published. These have been in vertebrates, where bait divergences are typically low, and none include invertebrates where bait-target divergences may be higher. Most invertebrate capture studies use a fixed, high hybridization temperature to maximize the proportion of on-target data, but many report low locus recovery. Using leaf-footed bugs (Hemiptera: Coreoidea), we investigate the effect of hybridization temperature on capture success of ultraconserved elements targeted by (i) baits developed from divergent hemipteran genomes and (ii) baits developed from less divergent coreoid transcriptomes. Lower temperatures generally resulted in more contigs and improved recovery of targets despite a lower proportion of on-target reads, lower read depth and more putative paralogues. Hybridization temperatures had less of an effect when using transcriptome-derived baits, which is probably due to lower bait-target divergences and greater bait tiling density. Thus, accommodating low hybridization temperatures during target capture can provide a cost-effective, widely applicable solution to improve invertebrate locus recovery.

Evolution of stridulatory mechanisms: vibroacoustic communication may be common in leaf-footed bugs and allies (Heteroptera: Coreoidea)

Intra- and interspecific communication is crucial to fitness via its role in facilitating mating, territoriality and defence. Yet, the evolution of animal communication systems is puzzling-how do they originate and change over time? Studying stridulatory morphology provides a tractable opportunity to deduce the origin and diversification of a communication mechanism. Stridulation occurs when two sclerotized structures rub together to produce vibratory and acoustic (vibroacoustic) signals, such as a cricket 'chirp'. We investigated the evolution of stridulatory mechanisms in the superfamily Coreoidea (Hemiptera: Heteroptera), a group of insects known for elaborate male fighting behaviours and enlarged hindlegs. We surveyed a large sampling of taxa and used a phylogenomic dataset to investigate the evolution of stridulatory mechanisms. We identified four mechanisms, with at least five evolutionary gains. One mechanism, occurring only in male Harmostini (Rhopalidae), is described for the first time. Some stridulatory mechanisms appear to be non-homoplastic apomorphies within Rhopalidae, while others are homoplastic or potentially homoplastic within Coreidae and Alydidae, respectively. We detected no losses of these mechanisms once evolved, suggesting they are adaptive. Our work sets the stage for further behavioural, evolutionary and ecological studies to better understand the context in which these traits evolve and change.

Metabolomics approach to assess the effect of siphonal autotomy on metabolic characteristics of razor clam Solen grandis

Autotomy appendages are fundamental evolutionary adaptations to escape predation. The siphon is an important foraging organ for bivalves. Here, we report the first demonstration of autotomy of the siphon in marine bivalves (razor clam Solen grandis) and the effect of siphonal autotomy in S. grandis on foraging and metabolic characteristics. In this study, the feeding rate and digestive enzyme activities upon siphonal autotomy in razor clams were investigated. Moreover, endogenous metabolites pre/post-autotomy of the siphon were investigated using liquid chromatography tandem-mass spectrometry (LC–MS). The feeding rate and digestive enzyme activities decreased significantly after siphonal autotomy in S. grandis (P < 0.05), suggesting that autotomy of the siphon negatively affected its foraging. These results might be related to the reduction in the foraging radius. Additionally, the effect of autotomy was investigated on a total of 34 differentially abundant metabolites, and pathway analysis indicated that 32 differentially enriched metabolic pathways were worthy of attention. Further integrated key metabolic pathway analysis showed that glycine, serine and threonine metabolism; taurine and hypotaurine metabolism; biotin metabolism; vitamin B6 and thiamine metabolism were significantly relevant pathways in S. grandis pre/post-autotomy of the siphon. The downregulation of glycine, taurine, and hypotaurine is expected to indicate a shortage of intermediate compounds and energy in S. grandis. Therefore, to provide the required energy and materials for siphon regeneration in S. grandis, we anticipated that it would be necessary to supplement these as exogenous metabolites from the daily diet.

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.

Nothing in Cognitive Neuroscience Makes Sense Except in the Light of Evolution

Evolutionary theory should be a fundamental guide for neuroscientists. This would seem a trivial statement, but I believe that taking it seriously is more complicated than it appears to be, as I argue in this article. Elsewhere, I proposed the notion of “bounded functionality” As a way to describe the constraints that should be considered when trying to understand the evolution of the brain. There are two bounded-functionality constraints that are essential to any evolution-minded approach to cognitive neuroscience. The first constraint, the bricoleur constraint, describes the evolutionary pressure for any adaptive solution to re-use any relevant resources available to the system before the selection situation appeared. The second constraint, the satisficing constraint, describes the fact that a trait only needs to behave more advantageously than its competitors in order to be selected. In this paper I describe how bounded-functionality can inform an evolutionary-minded approach to cognitive neuroscience. In order to do so, I resort to Nikolaas Tinbergen’s four questions about how to understand behavior, namely: function, causation, development and evolution. The bottom line of assuming Tinbergen’s questions is that any approach to cognitive neuroscience is intrinsically tentative, slow, and messy.

Trade-offs between weapons and testes do not manifest at high social densities

Social conditions can alter the allocation of resources to reproductive traits. For example, an increase in social density during development is frequently associated with an increase in the testes mass of males. Sperm competition theory assumes that increased investment in testes should come at the expense of investing into precopulatory traits, such as sexually selected weaponry. However, much remains unknown about the role of the social context on the concurrent, relative investment in both testes and weapons. We found that the leaf-footed cactus bug, Narnia femorata (Hemiptera: Coreidae), grew nearly 20% larger testes when raised in high social densities. In addition to manipulating social density, we used autotomy (limb loss) to limit investment in their hindlimb weapon during development. At low densities, we found that those that lost a weapon during development grew larger testes by adulthood, supporting previous work demonstrating a weapons-testes trade-off. However, at high social densities, males that dropped a hindlimb did not grow larger testes, though testes were already large at this density. These results underscore the importance of the social context to resource allocation patterns within the individual.

Do sexually selected weapons drive diversification?

Sexual selection is often thought to promote speciation. This expectation is largely driven by the fact that sexually selected traits can influence mating patterns and contribute to reproductive isolation. Indeed, some comparative studies have shown that clades with sexually selected traits have increased rates of speciation and diversification. However, these studies have almost exclusively focused on one mechanism of sexual selection: female choice. Another widespread mechanism is male-male competition. Few empirical studies (if any) have investigated the role of this alternative mechanism in driving diversification. Nevertheless, recent reviews have suggested that male-male competition can increase speciation rates. Here, we investigated whether traits associated with precopulatory male-male competition (i.e., sexually selected weapons) have promoted speciation and diversification in insects. We focused on three clades with both weapons and suitable phylogenies: leaf-footed and broad-headed bugs (Coreidae+Alydidae; ∼2850 species), stick insects and relatives (Phasmatodea; ∼3284 species), and scarab beetles (Scarabaeoidea; ∼39,717 species). We found no evidence that weapon-bearing lineages in these clades have higher rates of speciation or diversification than their weaponless relatives. Thus, our results suggest that precopulatory male-male competition may not have strong, general effects on speciation and diversification in insects, a group encompassing ∼60% of all described species.

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.

Morphological diversity of acoustic and electric communication systems of mochokid catfish

Mochokid catfish offer a distinct opportunity to study a communication system transitioning to a new signaling channel because some produce sounds and others electric discharges. Both signals are generated using an elastic spring system (ESS), which includes a protractor muscle innervated by motoneurons within the protractor nucleus that also has a motoneuron afferent population. Synodontis grandiops and S. nigriventris produce sounds and electric discharges, respectively, and their ESSs show several morphological and physiological differences. The extent to which these differences explain different signal types remains unclear. Here, we compare ESS morphologies and behavioral phenotypes among five mochokids. S. grandiops and S. nigriventris were compared with Synodontis eupterus that is known to produce both signal types, and representative members of two sister genera, Microsynodontis cf. batesii and Mochokiella paynei, for which no data were available. We provide support for the hypothesis that peripheral and central components of the ESS are conserved among mochokids. We also show that the two nonsynodontids are only sonic, consistent with sound production being an ancestral character for mochokids. Even though the three sound producing‐only species differ in some ESS characters, several are similar and likely associated with only sound production. We propose that the ability of S. eupterus to generate both electric discharges and sounds may depend on a protractor muscle intermediate in morphology between sound producing‐only and electric discharge‐only species, and two separate populations of protractor motoneurons. Our results further suggest that an electrogenic ESS in synodontids is an exaptation of a sound producing ESS.

Mary CM. Leaf-footed bugs possess multiple hidden contrasting color signals, but only one is associated with increased body size

Antipredatory displays that incorporate hidden contrasting coloration are found in a variety of different animals. These displays are seen in organisms that have drab coloration at rest, but when disturbed reveal conspicuous coloration. Examples include the bright abdomens of mountain katydids and the colorful underwings of hawk moths. Such hidden displays can function as secondary defenses, enabling evasion of a pursuant predator. To begin to understand why some species have these displays while others do not, we conducted phylogenetic comparative analyses to investigate factors associated with the evolution of hidden contrasting coloration in leaf-footed bugs. First, we investigated whether hidden contrasting coloration was associated with body size because these displays are considered to be more effective in larger organisms. We then investigated whether hidden contrasting coloration was associated with an alternative antipredatory defense, in this case rapid autotomy. We found that leaf-footed bugs with hidden contrasting coloration tended to autotomize more slowly, but this result was not statistically significant. We also found that the presence of a body size association was dependent upon the form of the hidden color display. Leaf-footed bugs that reveal red/orange coloration were the same size, on average, as species without a hidden color display. However, species that reveal white patches on a black background were significantly larger than species without a hidden color display. These results highlight the diversity of forms that hidden contrasting color signal can take, upon which selection may act differently.