Surface-Skimming Stoneflies: A Possible Intermediate Stage in Insect Flight Evolution

Thoracic and abdominal outgrowths in early pterygotes: a clue to the common ancestor of winged insects?

One of the fundamental questions in insect evolution is the origin of their wings and primary function of ancestral wing precursors. Recent phylogenomic and comparative morphological studies broadly support a terrestrial ancestor of pterygotes, but an aquatic or semiaquatic ancestor cannot be ruled out. Here new features of the branchial system of palaeodictyopteran larvae of several different instars of Katosaxoniapteron brauneri gen. et sp. nov. (Eugereonoidea) from the late Carboniferous collected at Piesberg (Germany) are described, which consist of delicate dorsolateral and lamellate caudal abdominal gills that support an aquatic or at least semiaquatic lifestyle for these insects. Moreover, the similar form and surface microstructures on the lateral abdominal outgrowths and thoracic wing pads indicate that paired serial outgrowths on segments of both tagmata presumably functioned as ancestral type of gills resembling a protopterygote model. This is consistent with the hypothesis that the wing sheaths of later stage damselfly larvae in hypoxic conditions have a respiratory role similar to abdominal tracheal gills. Hence, the primary function and driving force for the evolution of the precursors of wing pads and their abdominal homologues could be respiration.

Reduced olfactory acuity in recently flightless insects suggests rapid regressive evolution

Background

Insects have exceptionally fast smelling capabilities, and some can track the temporal structure of odour plumes at rates above 100 Hz. It has been hypothesized that this fast smelling capability is an adaptation for flying. We test this hypothesis by comparing the olfactory acuity of sympatric flighted versus flightless lineages within a wing-polymorphic stonefly species.

Results

Our analyses of olfactory receptor neuron responses reveal that recently-evolved flightless lineages have reduced olfactory acuity. By comparing flighted versus flightless ecotypes with similar genetic backgrounds, we eliminate other confounding factors that might have affected the evolution of their olfactory reception mechanisms. Our detection of different patterns of reduced olfactory response strength and speed in independently wing-reduced lineages suggests parallel evolution of reduced olfactory acuity.

Conclusions

These reductions in olfactory acuity echo the rapid reduction of wings themselves, and represent an olfactory parallel to the convergent phenotypic shifts seen under selective gradients in other sensory systems (e.g. parallel loss of vision in cave fauna). Our study provides evidence for the hypothesis that flight poses a selective pressure on the speed and strength of olfactory receptor neuron responses and emphasizes the energetic costs of rapid olfaction.

Liquid-amplified zipping actuators for micro-air vehicles with transmission-free flapping

Flapping micro-air vehicles (MAVs) can access a wide range of locations, including confined spaces such as the inside of industrial plants and collapsed buildings, and offer high maneuverability and tolerance to disturbances. However, current flapping MAVs require transmission systems between their actuators and wings, which introduce energetic losses and additional mass, hindering performance. Here, we introduce a high-performance electrostatic flapping actuation system, the liquid-amplified zipping actuator (LAZA), which induces wing movement by direct application of liquid-amplified electrostatic forces at the wing root, eliminating the requirement of any transmission system and their associated downsides. The LAZA allows for accurate control of flapping frequency and amplitude, exhibits no variation in performance over more than 1 million actuation cycles, and delivers peak and average specific powers of 200 and 124 watts per kilogram, respectively, exceeding mammalian and insect flight muscle and on par with modern flapping MAV actuation systems. The inclusion of 50-millimeter-long passively pitching wings in a dragonfly-sized LAZA flapping system allowed the rectification of net directional thrust up to 5.73 millinewtons. This thrust was achieved while consuming only 243 milliwatts of electrical power, implying a thrust-to-power ratio of 23.6 newtons per kilowatt, similar to state-of-the-art flapping MAVs, helicopter rotors, and commercial drone motors. Last, a horizontally moving LAZA flapping system supported by a taut nylon wire was able to accelerate from at-rest and travel at speeds up to 0.71 meters per second. The LAZA enables lightweight, high-performance transmission-free flapping MAVs for long-term remote exploration and search-and-rescue missions.

Fossil dragonfly-type larva with lateral abdominal protrusions and implications on the early evolution of Pterygota

Aquatic larvae are known in three early branches of Pterygota: Ephemeroptera (mayflies), Plecoptera (stoneflies), and Odonata (dragonflies, damselflies). A common origin of these larvae has been suggested, yet also counterarguments have been put forward, for example, the different position of larval gills: laterally on the abdomen in Ephemeroptera, terminally in Odonata, variably in Plecoptera. We discuss recent fossil findings and report a new dragonfly-type larva from Kachin amber (Myanmar), which possesses ancestral characters such as a terminal filum, maintained in ephemeropterans, but lost in modern odonatan larvae. The new larva possesses lateral protrusions on the abdominal segments where in other lineages gills occur. Together with other fossils, such as a plecopteran retaining lateral gills on the abdomen, this indicates that lateral protrusions on the abdomen might have well been an ancestral feature, removing one important argument against the idea of an aquatic larva in the ground pattern of Pterygota.

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A new dragonfly-type larva was found in Kachin amber (Myanmar, 99 million years old).

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The larva possesses a terminal filum, which is not known in modern dragonfly larvae

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It also exhibits lateral abdominal protrusions where in other lineages gills occur

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This find makes an aquatic larva in the ground pattern of Pterygota more likely

Increasing 28 mitogenomes of Ephemeroptera, Odonata and Plecoptera support the Chiastomyaria hypothesis with three different outgroup combinations

BACKGROUND

The phylogenetic relationships of Odonata (dragonflies and damselflies) and Ephemeroptera (mayflies) remain unresolved. Different researchers have supported one of three hypotheses (Palaeoptera, Chiastomyaria or Metapterygota) based on data from different morphological characters and molecular markers, sometimes even re-assessing the same transcriptomes or mitochondrial genomes. The appropriate choice of outgroups and more taxon sampling is thought to eliminate artificial phylogenetic relationships and obtain an accurate phylogeny. Hence, in the current study, we sequenced 28 mt genomes from Ephemeroptera, Odonata and Plecoptera to further investigate phylogenetic relationships, the probability of each of the three hypotheses, and to examine mt gene arrangements in these species. We selected three different combinations of outgroups to analyze how outgroup choice affected the phylogenetic relationships of Odonata and Ephemeroptera.

METHODS

Mitochondrial genomes from 28 species of mayflies, dragonflies, damselflies and stoneflies were sequenced. We used Bayesian inference (BI) and Maximum likelihood (ML) analyses for each dataset to reconstruct an accurate phylogeny of these winged insect orders. The effect of outgroup choice was assessed by separate analyses using three outgroups combinations: (a) four bristletails and three silverfish as outgroups, (b) five bristletails and three silverfish as outgroups, or (c) five diplurans as outgroups.

RESULTS

Among these sequenced mitogenomes we found the gene arrangement IMQM in Heptageniidae (Ephemeroptera), and an inverted and translocated tRNA-Ile between the 12S RNA gene and the control region in Ephemerellidae (Ephemeroptera). The IMQM gene arrangement in Heptageniidae (Ephemeroptera) can be explained via the tandem-duplication and random loss model, and the transposition and inversion of tRNA-Ile genes in Ephemerellidae can be explained through the recombination and tandem duplication-random loss (TDRL) model. Our phylogenetic analysis strongly supported the Chiastomyaria hypothesis in three different outgroup combinations in BI analyses. The results also show that suitable outgroups are very important to determining phylogenetic relationships in the rapid evolution of insects especially among Ephemeroptera and Odonata. The mt genome is a suitable marker to investigate the phylogeny of inter-order and inter-family relationships of insects but outgroup choice is very important for deriving these relationships among winged insects. Hence, we must carefully choose the correct outgroup in order to discuss the relationships of Ephemeroptera and Odonata.

Waxing and Waning of Wings

A major challenge to Darwinian evolution is explaining 'rudimentary' organs. This is particularly relevant to birds: rudimentary wings occur in fossils, as well as in developing, molting, and flight-impaired birds. Evidence shows that young birds flap small wings to improve locomotion and transition to flight. Although small wings also occur in adults, their potential role in locomotion is rarely considered. Here we describe the prevalence of rudimentary wings in extant birds, and how wings wax and wane on many timescales. This waxing and waning is integral to the avian clade and offers a rich arena for exploring links between form, function, performance, behavior, ecology, and evolution. Although our understanding is nascent, birds clearly show that rudimentary structures can enhance performance and survival.

Honeybees use their wings for water surface locomotion

Honeybees display a unique biolocomotion strategy at the air-water interface. When water's adhesive force traps them on the surface, their wetted wings lose ability to generate aerodynamic thrust. However, they adequately locomote, reaching a speed up to 3 body lengths·s^-1 Honeybees use their wetted wings as hydrofoils for their water surface propulsion. Their locomotion imparts hydrodynamic momentum to the surrounding water in the form of asymmetric waves and a deeper water jet stream, generating ∼20-μN average thrust. The wing kinematics show that the wing's stroke plane is skewed, and the wing supinates and pronates during its power and recovery strokes, respectively. The flow under a mechanical model wing mimicking the motion of a bee's wing further shows that nonzero net horizontal momentum is imparted to the water, demonstrating net thrust. Moreover, a periodic acceleration and deceleration of water are observed, which provides additional forward movement by "recoil locomotion." Their water surface locomotion by hydrofoiling is kinematically and dynamically distinct from surface skimming [J. H. Marden, M. G. Kramer, Science 266, 427-430 (1994)], water walking [J. W. M. Bush, D. L. Hu, Annu. Rev. Fluid Mech. 38, 339-369 (2006)], and drag-based propulsion [J. Voise, J. Casas, J. R. Soc. Interface 7, 343-352 (2010)]. It is postulated that the ability to self-propel on a water surface may increase the water-foraging honeybee's survival chances when they fall on the water.

Evolutionary history of Polyneoptera and its implications for our understanding of early winged insects

Polyneoptera represents one of the major lineages of winged insects, comprising around 40,000 extant species in 10 traditional orders, including grasshoppers, roaches, and stoneflies. Many important aspects of polyneopteran evolution, such as their phylogenetic relationships, changes in their external appearance, their habitat preferences, and social behavior, are unresolved and are a major enigma in entomology. These ambiguities also have direct consequences for our understanding of the evolution of winged insects in general; for example, with respect to the ancestral habitats of adults and juveniles. We addressed these issues with a large-scale phylogenomic analysis and used the reconstructed phylogenetic relationships to trace the evolution of 112 characters associated with the external appearance and the lifestyle of winged insects. Our inferences suggest that the last common ancestors of Polyneoptera and of the winged insects were terrestrial throughout their lives, implying that wings did not evolve in an aquatic environment. The appearance of the first polyneopteran insect was mainly characterized by ancestral traits such as long segmented abdominal appendages and biting mouthparts held below the head capsule. This ancestor lived in association with the ground, which led to various specializations including hardened forewings and unique tarsal attachment structures. However, within Polyneoptera, several groups switched separately to a life on plants. In contrast to a previous hypothesis, we found that social behavior was not part of the polyneopteran ground plan. In other traits, such as the biting mouthparts, Polyneoptera shows a high degree of evolutionary conservatism unique among the major lineages of winged insects.

Genotyping-by-sequencing supports a genetic basis for wing reduction in an alpine New Zealand stonefly

Wing polymorphism is a prominent feature of numerous insect groups, but the genomic basis for this diversity remains poorly understood. Wing reduction is a commonly observed trait in many species of stoneflies, particularly in cold or alpine environments. The widespread New Zealand stonefly Zelandoperla fenestrata species group (Z. fenestrata, Z. tillyardi, Z. pennulata) contains populations ranging from fully winged (macropterous) to vestigial-winged (micropterous), with the latter phenotype typically associated with high altitudes. The presence of flightless forms on numerous mountain ranges, separated by lowland fully winged populations, suggests wing reduction has occurred multiple times. We use Genotyping by Sequencing (GBS) to test for genetic differentiation between fully winged (n = 62) and vestigial-winged (n = 34) individuals, sampled from a sympatric population of distinct wing morphotypes, to test for a genetic basis for wing morphology. While we found no population genetic differentiation between these two morphotypes across 6,843 SNP loci, we did detect several outlier loci that strongly differentiated morphotypes across independent tests. These findings indicate that small regions of the genome are likely to be highly differentiated between morphotypes, suggesting a genetic basis for wing reduction. Our results provide a clear basis for ongoing genomic analysis to elucidate critical regulatory pathways for wing development in Pterygota.

Flight in Ground Effect Dramatically Reduces Aerodynamic Costs in Bats

Most flying animals, from insects to seabirds [1], perform flights close to ground or water when taking off or landing [2], drinking, and feeding [3-5] or when traveling near water surfaces [1, 6, 7]. When flying close to a surface within approximately one wingspan, the surface acts as an aerodynamic mirror, interrupting the downwash [8, 9], resulting in increased pressure underneath the wing and suppression of wingtip vortex development [10]. This aerodynamic interaction lowers the energy added to the air by the animal, reducing the cost of flying. Modeling suggests that flapping wings in ground effect can affect the expected power savings compared to gliding flight, either positively or negatively, depending on the wing motion [11-13]. Although aerodynamic theory predicts substantial power reductions when animals fly in ground effect [4-6, 9, 11, 12], quantitative measurements of savings are lacking. Here, we show, through wake-based power measurements, that Daubenton's bats utilize 29% less aerodynamic power when flying in compared to out of ground effect, which is twice the predicted savings. Contrary to theoretical predictions [4-6, 9, 11, 12] we find no variation in savings with distance above ground when in ground effect. Given alterations in kinematics with ground proximity, we hypothesize that modulation of wing kinematics raises the achievable benefit from ground effect relative to current model predictions. The savings from ground effect are comparable to formation flight [14, 15] but are not limited to large bird species. Instead, ground effect is experienced by most flying animals and may have facilitated the evolution of powered animal flight.

A century and a half of research on the evolution of insect flight

The gill and paranotal lobe theories of insect wing evolution were both proposed in the 1870s. For most of the 20th century, the paranotal lobe theory was more widely accepted, probably due to the fundamentally terrestrial tracheal respiratory system; in the 1970s, some researchers advocated for an elaborated gill ("pleural appendage") theory. Lacking transition fossils, neither theory could be definitively rejected. Winged insects are abundant in the fossil record from the mid-Carboniferous, but insect fossils are vanishingly rare earlier, and all earlier fossils are from primitively wingless insects. The enigmatic, isolated mandibles of Rhyniognatha (early Devonian) hint that pterygotes may have been present much earlier, but the question remains open. In the late 20th century, researchers used models to study the interaction of body and protowing size on solar warming and gliding abilities, and stability and glide effectiveness of many tiny adjustable winglets versus a single, large pair of immobile winglets. Living stoneflies inspired the surface-skimming theory, which provides a mechanism to bridge between aquatic gills and flapping wings. The serendipitously discovered phenomenon of directed aerial descent suggests a likely route to the early origin of insect flight. It provides a biomechanically feasible sequence from guided falls to fully-powered flight.

Locomotion of arthropods in aquatic environment and their applications in robotics

Many bio-inspired robots have been developed so far after careful investigation of animals' locomotion. To successfully apply the locomotion of natural counterparts to robots for efficient and improved mobility, it is essential to understand their principles. Although a lot of research has studied either animals' locomotion or bio-inspired robots, there have only been a few attempts to broadly review both of them in a single article. Among the millions of animal species, this article reviewed various forms of aquatic locomotion in arthropods including relevant bio-inspired robots. Despite some previous robotics research inspired by aquatic arthropods, we found that many less-investigated or even unexplored areas are still present. Therefore, this article has been prepared to identify what types of new robotics research can be carried out after drawing inspiration from the aquatic locomotion of arthropods and to provide fruitful insights that may lead us to develop an agile and efficient aquatic robot.

The effects of temperature, activity and convection on the plastron PO2 of the aquatic bug Aphelocheirus aestivalis (Hemiptera; Aphelocheiridae)

The aquatic bug Aphelocheirus aestivalis (Fabricius 1794) utilises a plastron, a thin bubble layer on the surface of its body to extract O₂ from the water. Millions of tiny hairs keep the bubble from collapsing, enabling the bug to remain submerged indefinitely. The development of fibre optic O₂-probes has allowed measurements of O₂ pressure (PO₂) surrounding the plastron, and within the plastron although only for short periods. Here we developed methods to continuously measure plastron PO₂, and investigate how it is affected by temperature (15, 20, 25°C), activity, and water circulation. We also made measurements of water PO₂, temperature and velocity in the field and swimming velocity at the treatment temperatures. Results show that plastron PO₂ is inversely related to temperature, associated with differences in metabolic demand, and that small bouts of activity or changes in water convection result in rapid changes in plastron PO₂. A model was developed to calculate the conditions under which Aphelocheirus would exist without becoming O₂-limited in relation to water temperature, PO₂ and boundary layer thickness. This suggests that Aphelocheirus at one of two field sites may have a reduced metabolic scope even in well convected water in association with low PO₂ and moderate temperature, and that in well convected, air-saturated water, bugs may have a reduced metabolic scope where water temperatures are between 20 and 25°C. If exposed to 5kPa PO₂, Aphelocheirus cannot sustain resting metabolic rate even in well-convected water and would die at temperatures above approximately 25°C.

The unusual tracheal system within the wing membrane of a dragonfly

Some consider that the first winged insects had living tissue inside the wing membrane, resembling larval gills or developing wing pads. However, throughout the developmental process of the wing membrane of modern insects, cells and tracheoles in the lumen between dorsal and ventral cuticle disappear and both cuticles become fused. This process results in the rather thin rigid stable structure of the membrane. The herewith described remarkable case of the dragonfly Zenithoptera lanei shows that in some highly specialized wings, the membrane can still be supplemented by tracheae. Such a characteristic of the wing membrane presumably represents a strong specialization for the synthesis of melanin-filled nanolayers of the cuticle, nanospheres inside the wing membrane and complex arrangement of wax crystals on the membrane surface, all responsible for unique structural coloration.

Surface tension dominates insect flight on fluid interfaces

Flight on the 2D air-water interface, with body weight supported by surface tension, is a unique locomotion strategy well adapted for the environmental niche on the surface of water. Although previously described in aquatic insects like stoneflies, the biomechanics of interfacial flight has never been analysed. Here, we report interfacial flight as an adapted behaviour in waterlily beetles (Galerucella nymphaeae) which are also dexterous airborne fliers. We present the first quantitative biomechanical model of interfacial flight in insects, uncovering an intricate interplay of capillary, aerodynamic and neuromuscular forces. We show that waterlily beetles use their tarsal claws to attach themselves to the interface, via a fluid contact line pinned at the claw. We investigate the kinematics of interfacial flight trajectories using high-speed imaging and construct a mathematical model describing the flight dynamics. Our results show that non-linear surface tension forces make interfacial flight energetically expensive compared with airborne flight at the relatively high speeds characteristic of waterlily beetles, and cause chaotic dynamics to arise naturally in these regimes. We identify the crucial roles of capillary-gravity wave drag and oscillatory surface tension forces which dominate interfacial flight, showing that the air-water interface presents a radically modified force landscape for flapping wing flight compared with air.

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.

Wings versus legs in the avian bauplan: development and evolution of alternative locomotor strategies

Wings have long been regarded as a hallmark of evolutionary innovation, allowing insects, birds, and bats to radiate into aerial environments. For many groups, our intuitive and colloquial perspective is that wings function for aerial activities, and legs for terrestrial, in a relatively independent manner. However, insects and birds often engage their wings and legs cooperatively. In addition, the degree of autonomy between wings and legs may be constrained by tradeoffs, between allocating resources to wings versus legs during development, or between wing versus leg investment and performance (because legs must be carried as baggage by wings during flight and vice versa). Such tradeoffs would profoundly affect the development and evolution of locomotor strategies, and many related aspects of animal ecology. Here, we provide the first evaluation of wing versus leg investment, performance and relative use, in birds-both across species, and during ontogeny in three precocial species with different ecologies. Our results suggest that tradeoffs between wing and leg modules help shape ontogenetic and evolutionary trajectories, but can be offset by recruiting modules cooperatively. These findings offer a new paradigm for exploring locomotor strategies of flying organisms and their extinct precursors, and thereby elucidating some of the most spectacular diversity in animal history.

Wing base structural data support the sister relationship of megaloptera and neuroptera (insecta: neuropterida)

The phylogenetic status and the monophyly of the holometabolous insect order Megaloptera has been an often disputed and long unresolved problem. The present study attempts to infer phylogenetic relationships among three orders, Megaloptera, Neuroptera, and Raphidioptera, within the superorder Neuropterida, based on wing base structure. Cladistic analyses were carried out based on morphological data from both the fore- and hindwing base. A sister relationship between Megaloptera and Neuroptera was recovered, and the monophyly of Megaloptera was corroborated. The division of the order Megaloptera, the traditional higher classification, into Corydalidae (Corydalinae + Chauliodinae) and Sialidae, was also supported by our wing base data analyses.

Evolution of insect wings and development - new details from Palaeozoic nymphs

The nymphal stages of Palaeozoic insects differ significantly in morphology from those of their modern counterparts. Morphological details for some previously reported species have recently been called into question. Palaeozoic insect nymphs are important, however - their study could provide key insights into the evolution of wings, and complete metamorphosis. Here we review past work on these topics and juvenile insects in the fossil record, and then present both novel and previously described nymphs, documented using new imaging methods. Our results demonstrate that some Carboniferous nymphs - those of Palaeodictyopteroidea - possessed movable wing pads and appear to have been able to perform simple flapping flight. It remains unclear whether this feature is ancestral for Pterygota or an autapomorphy of Palaeodictyopteroidea. Further characters of nymphal development which were probably in the ground pattern of Pterygota can be reconstructed. Wing development was very gradual (archimetaboly). Wing pads did not protrude from the tergum postero-laterally as in most modern nymphs, but laterally, and had well-developed venation. The modern orientation of wing pads and the delay of wing development into later developmental stages (condensation) appears to have evolved several times independently within Pterygota: in Ephemeroptera, Odonatoptera, Eumetabola, and probably several times within Polyneoptera. Selective pressure appears to have favoured a more pronounced metamorphosis between the last nymphal and adult stage, ultimately reducing exploitation competition between the two. We caution, however, that the results presented herein remain preliminary, and the reconstructed evolutionary scenario contains gaps and uncertainties. Additional comparative data need to be collected. The present study is thus seen as a starting point for this enterprise.

Reanalysis and experimental evidence indicate that the earliest trace fossil of a winged insect was a surface-skimming neopteran

A recent description and analysis of an imprint fossil from the Carboniferous concluded that it was made by a mayfly landing in sediment at the edge of water. Here, I reanalyze that trace fossil and supply experimental evidence regarding wing traces and behavior. The thorax of the trace maker lacked structures characteristic of mayflies, but closely matches a modern neopteran insect family (Taeniopterygidae, Plecoptera) little changed from Early Permian fossils. Edges of the folded wings of live Taeniopteryx leave marks on sediment closely matching marks in the trace fossil. Faint marks lateral to and beyond the reach of meso- and metathoracic legs match the location where wings of surface-skimming Taeniopteryx stoneflies lightly touch the sediment when these insects skim onto wet ground at shorelines. Dimensions of the thorax of the trace indicate relatively weak flight ability compared to fossils from the Early Permian, making doubtful the hypothesis that the trace maker was flight capable. Ultimately, this fossil best fits a scenario in which a neopteran insect skimmed across the surface of water, then folded its wings. Surface skimming as a precursor to the evolution of flight in insects is supported by this fossil evidence of skimming behavior in a Carboniferous insect.

From extant to extinct: locomotor ontogeny and the evolution of avian flight

Evolutionary transformations are recorded by fossils with transitional morphologies, and are key to understanding the history of life. Reconstructing these transformations requires interpreting functional attributes of extinct forms by exploring how similar features function in extant organisms. However, extinct-extant comparisons are often difficult, because extant adult forms frequently differ substantially from fossil material. Here, we illustrate how postnatal developmental transitions in extant birds can provide rich and novel insights into evolutionary transformations in theropod dinosaurs. Although juveniles have not been a focus of extinct-extant comparisons, developing juveniles in many groups transition through intermediate morphological, functional and behavioral stages that anatomically and conceptually parallel evolutionary transformations. Exploring developmental transitions may thus disclose observable, ecologically relevant answers to long puzzling evolutionary questions.

Damping in flapping flight and its implications for manoeuvring, scaling and evolution

Flying animals exhibit remarkable degrees of both stability and manoeuvrability. Our understanding of these capabilities has recently been improved by the identification of a source of passive damping specific to flapping flight. Examining how this damping effect scales among different species and how it affects active manoeuvres as well as recovery from perturbations provides general insights into the flight of insects, birds and bats. These new damping models offer a means to predict manoeuvrability and stability for a wide variety of flying animals using prior reports of the morphology and flapping motions of these species. Furthermore, the presence of passive damping is likely to have facilitated the evolution of powered flight in animals by providing a stability benefit associated with flapping.

Contrasting micro/nano architecture on termite wings: two divergent strategies for optimising success of colonisation flights

Many termite species typically fly during or shortly after rain periods. Local precipitation will ensure water will be present when establishing a new colony after the initial flight. Here we show how different species of termite utilise two distinct and contrasting strategies for optimising the success of the colonisation flight. Nasutitermes sp. and Microcerotermes sp. fly during rain periods and adopt hydrophobic structuring/‘technologies’ on their wings to contend with a moving canvas of droplets in daylight hours. Schedorhinotermes sp. fly after rain periods (typically at night) and thus do not come into contact with mobile droplets. These termites, in contrast, display hydrophilic structuring on their wings with a small scale roughness which is not dimensionally sufficient to introduce an increase in hydrophobicity. The lack of hydrophobicity allows the termite to be hydrophilicly captured at locations where water may be present in large quantities; sufficient for the initial colonization period. The high wettability of the termite cuticle (Schedorhinotermes sp.) indicates that the membrane has a high surface energy and thus will also have strong attractions with solid particles. To investigate this the termite wings were also interacted with both artificial and natural contaminants in the form of hydrophilic silicon beads of various sizes, 4 µm C₁₈ beads and three differently structured pollens. These were compared to the superhydrophobic surface of the planthopper (Desudaba psittacus) and a native Si wafer surface. The termite cuticle demonstrated higher adhesive interactions with all particles in comparison to those measured on the plant hopper.

Paddling mode of forward flight in insects

By analyzing high-speed video of the fruit fly, we discover a swimminglike mode of forward flight characterized by paddling wing motions. We develop a new aerodynamic analysis procedure to show that these insects generate drag-based thrust by slicing their wings forward at low angle of attack and pushing backwards at a higher angle. Reduced-order models and simulations reveal that the law for flight speed is determined by these wing motions but is insensitive to material properties of the fluid. Thus, paddling is as effective in air as in water and represents a common strategy for propulsion through aquatic and aerial environments.

Late Carboniferous paleoichnology reveals the oldest full-body impression of a flying insect

Insects were the first animals to evolve powered flight and did so perhaps 90 million years before the first flight among vertebrates. However, the earliest fossil record of flying insect lineages (Pterygota) is poor, with scant indirect evidence from the Devonian and a nearly complete dearth of material from the Early Carboniferous. By the Late Carboniferous a diversity of flying lineages is known, mostly from isolated wings but without true insights into the paleoethology of these taxa. Here, we report evidence of a full-body impression of a flying insect from the Late Carboniferous Wamsutta Formation of Massachusetts, representing the oldest trace fossil of Pterygota. Through ethological and morphological analysis, the trace fossil provides evidence that its maker was a flying insect and probably was representative of a stem-group lineage of mayflies. The nature of this current full-body impression somewhat blurs distinctions between the systematics of traces and trace makers, thus adding to the debate surrounding ichnotaxonomy for traces with well-associated trace makers.

Animal aloft: the origins of aerial behavior and flight

Diverse taxa of animals exhibit remarkable aerial capacities, including jumping, mid-air righting, parachuting, gliding, landing, controlled maneuvers, and flapping flight. The origin of flapping wings in hexapods and in 3 separate lineages of vertebrates (pterosaurs, bats, and birds) greatly facilitated subsequent diversification of lineages, but both the paleobiological context and the possible selective pressures for the evolution of wings remain contentious. Larvae of various arboreal hemimetabolous insects, as well as many adult canopy ants, demonstrate the capacity for directed aerial descent in the absence of wings. Aerial control in the ancestrally wingless archaeognathans suggests that flight behavior preceded the origins of wings in hexapods. In evolutionary terms, the use of winglets and partial wings to effect aerial righting and maneuvers could select for enhanced appendicular motions, and ultimately lead to powered flight. Flight behaviors that involve neither flapping nor wings are likely to be much more widespread than is currently recognized. Further characterization of the sensory and biomechanical mechanisms used by these aerially capable taxa can potentially assist in reconstruction of ancestral winged morphologies and facilitate our understanding of the origins of flight.

How micro/nanoarchitecture facilitates anti-wetting: an elegant hierarchical design on the termite wing

The termite is an insect which is a weak flier, has a large wing area in relation to its body mass, and many species typically fly during rain or storm periods. Water droplets placed on these insects' wings will spontaneously roll off the surface. Here we show how the intricate hierarchical array design of these insect wings achieves anti-wetting properties with water bodies of various sizes by reducing contact area and thus adhesion. To repel large droplets, the termite uses an array of hairs with a specially designed nanoarchitecture, which we demonstrate is critical for this function. By coating single hairs with a polymer of varying thicknesses (with a similar hydrophobicity to insect cuticle), we demonstrate that hairs of the same chemistry and with the complete nanoarchitecture show the greatest resistance to penetrating water bodies. The wings also consist of an underlying non-wetting membrane substructure comprising an array of star-shaped microstructures which minimize interaction with micro-sized droplets of water. The sophisticated micro/nanostructured hierarchy on the termite wing membrane not only results in non-wetting at different length scales but also demonstrates a design for weight and material minimization while achieving this state. Elucidating the function of such structures has implications for understanding insect biology and the evolution of wings.

Advances in comparative physiology from high-speed imaging of animal and fluid motion

Since the time of Muybridge and Marey in the last half of the nineteenth century, studies of animal movement have relied on some form of high-speed or stop-action imaging to permit analysis of appendage and body motion. In the past ten years, the advent of megapixel-resolution high-speed digital imaging with maximal framing rates of 250 to 100,000 images per second has allowed new views of musculoskeletal function in comparative physiology that now extend to imaging flow around moving animals and the calculation of fluid forces produced by animals moving in fluids. In particular, the technique of digital particle image velocimetry (DPIV) has revolutionized our ability to understand how moving animals generate fluid forces and propel themselves through air and water. DPIV algorithms generate a matrix of velocity vectors through the use of image cross-correlation, which can then be used to calculate the force exerted on the fluid as well as locomotor work and power. DPIV algorithms can also be applied to images of moving animals to calculate the velocity of different regions of the moving animal, providing a much more detailed picture of animal motion than can traditional digitizing methods. Although three-dimensional measurement of animal motion is now routine, in the near future model-based kinematic reconstructions and volumetric analyses of animal-generated fluid flow patterns will provide the next step in imaging animal biomechanics and physiology.

The innovation triad: an EvoDevo agenda

This article introduces a special issue on evolutionary innovation and morphological novelty, two interrelated themes that have received a remarkable increase of attention over the past few years. We begin with a discussion of the question of whether innovation and novelty represent distinct evolutionary problems that require a distinct conceptualization. We argue that the mechanisms of innovation and their phenotypic results--novelty--can only be properly addressed if they are distinguished from the standard evolutionary themes of variation and adaptation, and we present arguments for making such a distinction. We propose that origination, the first formation of biological structures, is another distinct problem of morphological evolution, and that together with innovation and novelty it constitutes a conceptual complex we call the innovation triad. We define a problem agenda of the triad, which separates the analysis of the initiating conditions from the mechanistic realization of innovation, and we discuss the theoretical problems that arise from treating innovation as distinct from variation. Further, we categorize the empirical approaches that address themes of the innovation triad in recognizing four major strands of research: the morphology and systematics program, the gene regulation program, the epigenetic program, and the theoretical biology program. We provide examples of each program, giving priority to contributions in the present issue. In conclusion, we observe that the innovation triad is one of the defining topics of EvoDevo research and may represent its most pertinent contribution to evolutionary theory. We point out that an inclusion of developmental systems properties into evolutionary theory represents a shift of explanatory emphasis from the external factors of natural selection to the internal dynamics of developmental systems, complementing adaptation with emergence, and contingency with inherency.

Aquatic turning performance by the whirligig beetle: constraints on maneuverability by a rigid biological system

Turning performance is constrained by morphology, where the flexibility of the body and the mobility and position of the control surfaces determine the level of performance. The use of paddling appendages in conjunction with the rigid bodies of aquatic arthropods could potentially limit their turning performance. Whirligig beetles (Coleoptera: Gyrinidae) are rigid-bodied, but these aquatic insects can swim rapidly in circular patterns. Turning performance of swimming whirligig beetles (Dineutes horni) was assessed by videotaping beetles in a small (115 mm diameter) arena at 500 frames s(-1) and 1000 frames s(-1). Curved trajectories were executed as continuous powered turns. Asymmetrical paddling of the outboard legs was used to power the turn. Turns were produced also by abduction of the inboard elytra and vectored thrust generated from sculling of the wing at 47.14 Hz. The abducted elytra increased drag and acted as a pivot. Swimming speeds varied from 0.06 m s(-1) to 0.55 m s(-1) (4.7-44.5 L s(-1)). Relative minimum radius was 24% of body length. Maximum rate of turn was 4428 degrees s(-1) with maximum centripetal acceleration of 2.86 g. Turning radius was weakly associated with swimming velocity, although minimum values of the radius showed no correlation with velocity. Turning rate was also related indirectly to radius and directly to centripetal acceleration. Compared to vertebrates with flexible bodies, the relative turning radius of whirligig beetles is constrained by a rigid body and use of drag-based propulsive mechanisms. However, these mechanisms permit continuous turning, and the size of the beetle permits higher turn rates with lower centripetal accelerations.

Mechanics and aerodynamics of insect flight control

Insects have evolved sophisticated fight control mechanisms permitting a remarkable range of manoeuvres. Here, I present a qualitative analysis of insect flight control from the perspective of flight mechanics, drawing upon both the neurophysiology and biomechanics literatures. The current literature does not permit a formal, quantitative analysis of flight control, because the aerodynamic force systems that biologists have measured have rarely been complete and the position of the centre of gravity has only been recorded in a few studies. Treating the two best-known insect orders (Diptera and Orthoptera) separately from other insects, I discuss the control mechanisms of different insects in detail. Recent experimental studies suggest that the helicopter model of flight control proposed for Drosophila spp. may be better thought of as a facultative strategy for flight control, rather than the fixed (albeit selected) constraint that it is usually interpreted to be. On the other hand, the so-called 'constant-lift reaction' of locusts appears not to be a reflex for maintaining constant lift at varying angles of attack, as is usually assumed, but rather a mechanism to restore the insect to pitch equilibrium following a disturbance. Differences in the kinematic control mechanisms used by the various insect orders are related to differences in the arrangement of the wings, the construction of the flight motor and the unsteady mechanisms of lift production that are used. Since the evolution of insect flight control is likely to have paralleled the evolutionary refinement of these unsteady aerodynamic mechanisms, taxonomic differences in the kinematics of control could provide an assay of the relative importance of different unsteady mechanisms. Although the control kinematics vary widely between orders, the number of degrees of freedom that different insects can control will always be limited by the number of independent control inputs that they use. Control of the moments about all three axes (as used by most conventional aircraft) has only been proven for larger flies and dragonflies, but is likely to be widespread in insects given the number of independent control inputs available to them. Unlike in conventional aircraft, however, insects' control inputs are likely to be highly non-orthogonal, and this will tend to complicate the neural processing required to separate the various motions.

Molecular phylogenetic analysis of evolutionary trends in stonefly wing structure and locomotor behavior

Insects in the order Plecoptera (stoneflies) use a form of two-dimensional aerodynamic locomotion called surface skimming to move across water surfaces. Because their weight is supported by water, skimmers can achieve effective aerodynamic locomotion even with small wings and weak flight muscles. These mechanical features stimulated the hypothesis that surface skimming may have been an intermediate stage in the evolution of insect flight, which has perhaps been retained in certain modern stoneflies. Here we present a phylogeny of Plecoptera based on nucleotide sequence data from the small subunit rRNA (18S) gene. By mapping locomotor behavior and wing structural data onto the phylogeny, we distinguish between the competing hypotheses that skimming is a retained ancestral trait or, alternatively, a relatively recent loss of flight. Our results show that basal stoneflies are surface skimmers, and that various forms of surface skimming are distributed widely across the plecopteran phylogeny. Stonefly wings show evolutionary trends in the number of cross veins and the thickness of the cuticle of the longitudinal veins that are consistent with elaboration and diversification of flight-related traits. These data support the hypothesis that the first stoneflies were surface skimmers, and that wing structures important for aerial flight have become elaborated and more diverse during the radiation of modern stoneflies.

Surface-skimming stoneflies and mayflies: the taxonomic and mechanical diversity of two-dimensional aerodynamic locomotion

The best supported hypothesis for the evolutionary origin of insect wings is that they evolved from articulated, leg-derived respiratory structures of aquatic ancestors. However, there are no fossils of the immediate ancestors of winged insects, and it is difficult to imagine how a functional transition from gills to wings could have occurred. Recent studies of surface-skimming locomotion in stoneflies and mayflies offer a plausible solution by showing how rudimentary wings and muscle power can be used to accomplish two-dimensional aerodynamic locomotion on the surface of water. Here we extend that line of research by examining the phylogenetic distribution and mechanistic diversity of surface skimming in stoneflies, along with a limited examination of mayflies. These investigations reveal both a broad taxonomic occurrence and a fine gradation of mechanically distinct forms. Distinct forms of wing-flapping surface skimming include (1) stoneflies that flap their wings weakly while maintaining their body in contact with the water and undulating their abdomen laterally in a swimming-like motion, (2) stoneflies that skim while elevating their body above the water and maintaining all six legs on the surface, (3) stoneflies and mayflies that skim with only four legs on the water surface, (4) stoneflies that skim with only their two hind legs on the surface, and (5) stoneflies that, beginning with a series of leg motions nearly identical to hind-leg skimmers, use their hind legs to jump from the water into the air to initiate flapping flight. Comparisons across these forms of skimming show that wing-beat amplitude, horizontal velocity, and the verticality of aerodynamic force production increase as the body orientation becomes more upright and contact with the water is minimized. These behaviors illustrate a mechanical pathway by which flying insects could have evolved from swimming ancestors via a series of finely graded intermediate stages. The phylogenetic distribution of skimming and flight in stoneflies does not indicate any clear directionality toward either greater or lesser aerodynamic abilities; however, the broad and apparently basal phylogenetic distribution of skimming taxa supports the hypothesis that the common ancestor of stoneflies was a surface skimmer. This may also be true for the common ancestor of stoneflies and mayflies, that is, the first winged insects. We combine these data with fossil evidence to form a synthetic model for the evolution of flying insects from surface skimmers.

Flight respiration and energetics

We use a comparative approach to examine some of the physiological traits that make flight possible. Comparisons of related fliers and runners suggest that fliers generally have higher aerobic metabolic capacities than runners but that the difference is highly dependent on the taxa studied. The high metabolic rates of fliers relative to runners, especially in insects, are correlated with high locomotory muscle cycle frequencies and low efficiencies of conversion of metabolic power to mechanical power. We examine some factors that produce variation in flight respiration and energetics. Air temperature strongly affects the flight metabolic rate of some insects and birds. Flight speed interacts with flier mass, so that small fliers tend to exhibit a J-shaped power curve and larger fliers a U-shaped power curve. As body size increases, mass-specific aerobic flight metabolism decreases in most studies, but mass-specific power output is constant or increases, leading to an increase in efficiency with size. Intraspecific studies have revealed specific genetically based effects on flight metabolism and power output and multiple ecological correlates of flight capabilities.

The evolutionary physiology of animal flight: paleobiological and present perspectives

Recent geophysical analyses suggest the presence of a late Paleozoic oxygen pulse beginning in the late Devonian and continuing through to the late Carboniferous. During this period, plant terrestrialization and global carbon deposition resulted in a dramatic increase in atmospheric oxygen levels, ultimately yielding concentrations potentially as high as 35% relative to the contemporary value of 21%. Such hyperoxia of the late Paleozoic atmosphere may have physiologically facilitated the initial evolution of insect flight metabolism. Widespread gigantism in late Paleozoic insects and other arthropods is also consistent with enhanced oxygen flux within diffusion-limited tracheal systems. Because total atmospheric pressure increases with increased oxygen partial pressure, concurrently hyperdense conditions would have augmented aerodynamic force production in early forms of flying insects. By the late Permian, evolution of decompositional microbial and fungal communities, together with disequilibrium in rates of carbon deposition, gradually reduced oxygen concentrations to values possibly as low as 15%. The disappearance of giant insects by the end of the Permian is consistent with extinction of these taxa for reasons of asphyxiation on a geological time scale. As with winged insects, the multiple historical origins of vertebrate flight in the late Jurassic and Cretaceous correlate temporally with periods of elevated atmospheric oxygen. Much discussion of flight performance in Archaeopteryx assumes a contemporary atmospheric composition. Elevated oxygen levels in the mid- to late Mesozoic would, however, have facilitated aerodynamic force production and enhanced muscle power output for ancestral birds, as well as for precursors to bats and pterosaurs.