Extraordinary flight performance of the smallest beetles

Evolution of and structures involved in wing folding in featherwing beetles (Coleoptera: Ptiliidae)

The ability to fold the wings is an important phenomenon in insect evolution and a feature that attracts the attention of engineers who develop biomimetic technologies. Beetles of the family Ptiliidae (featherwing beetles) are unique among microinsects in their ability to fold their bristled wings under the elytra and unfold them before flight. The folding and unfolding of bristled wings and of the structures involved in these processes varies among ptiliids, but only one species, Acrotrichis sericans, has been analyzed in detail. In this study, we analyze in detail the wing folding pattern and the mechanism of the folding and unfolding of the wings in species of different lineages of Ptiliidae, using scanning electron, сonfocal laser scanning, and optical microscopy, and compare the wing-folding patterns of Ptiliidae with those of the sister group, Hydraenidae, to reconstruct the evolution of the involved structures. We confirm that the two subfamilies of Ptiliidae have two distinct patterns of wing folding: Nossidiinae has retained the ancestral ('agyrtid') asymmetrical pattern with overlapping wings and with folds at different angles to the wing axis, while Ptiliinae, which includes the smallest of all known beetles, has evolved a symmetrical pattern with non-overlapping wings and folds perpendicular to the wing axis, with one additional oblique fold in the genus Ptenidium. Ptiliids have a longer alacrista, which helps to lock the elytra at rest, and a more complex set of structures involved in wing folding on abdominal tergites. These genus-specific structures, which include setae and wing-folding patches on some of the tergites and the palisade fringe of setae on the posterior margin of tergite 7, help the insect to tuck the wing under the elytron and fold it after flight. The symmetrical wing-folding pattern is simpler than the wing folding patterns of most larger beetles. The obtained data on the mechanisms and patterns of the folding and unfolding of the wings in Ptiliidae elucidate the evolution of wing folding as an adaptation protecting the wings at rest. Structures involved in wing folding can be used as distinguishing characters in taxonomy. The wing-folding mechanisms of Ptiliidae may eventually be used for developing miniature biomimetic robots.

The Weis-Fogh Number Describes Resonant Performance Tradeoffs in Flapping Insects

Dimensionless numbers have long been used in comparative biomechanics to quantify competing scaling relationships and connect morphology to animal performance. While common in aerodynamics, few relate the biomechanics of the organism to the forces produced on the environment during flight. We discuss the Weis-Fogh number, N, as a dimensionless number specific to flapping flight, which describes the resonant properties of an insect and resulting tradeoffs between energetics and control. Originally defined by Torkel Weis-Fogh in his seminal 1973 paper, N measures the ratio of peak inertial to aerodynamic torque generated by an insect over a wingbeat. In this perspectives piece, we define N for comparative biologists and describe its interpretations as a ratio of torques and as the width of an insect's resonance curve. We then discuss the range of N realized by insects and explain the fundamental tradeoffs between an insect's aerodynamic efficiency, stability, and responsiveness that arise as a consequence of variation in N, both across and within species. N is therefore an especially useful quantity for comparative approaches to the role of mechanics and aerodynamics in insect flight.

Fluid-structure interactions of bristled wings: the trade-off between weight and drag

The smallest flying insects often have bristled wings resembling feathers or combs. We combined experiments and three-dimensional numerical simulations to investigate the trade-off between wing weight and drag generation. In experiments of bristled strips, a reduced physical model of the bristled wing, we found that the elasto-viscous number indicates when reconfiguration occurs in the bristles. Analysis of existing biological data suggested that bristled wings of miniature insects lie below the reconfiguration threshold, thus avoiding drag reduction. Numerical simulations of bristled strips showed that there exist optimal numbers of bristles that maximize the weighted drag when the additional volume due to the bristles is taken into account. We found a scaling relationship between the rescaled optimal numbers and the dimensionless bristle length. This result agrees qualitatively with and provides an upper bound for the bristled wing morphological data analysed in this study.

Size-dependent Scaling of Stingless Bee Flight Metabolism Reveals an Energetic Benefit to Small Body Size

Understanding the effect of body size on flight costs is critical for development of models of aerodynamics and animal energetics. Prior scaling studies that have shown that flight costs scale hypometrically have focused primarily on larger (> 100 mg) insects and birds, but most flying species are smaller. We studied the flight physiology of thirteen stingless bee species over a large range of body sizes (1-115 mg). Metabolic rate during hovering scaled hypermetrically (scaling slope = 2.11). Larger bees had warm thoraxes while small bees were nearly ecothermic; however, even controlling for body temperature variation, flight metabolic rate scaled hypermetrically across this clade. Despite having a lower mass-specific metabolic rate during flight, smaller bees could carry the same proportional load. Wingbeat frequency did not vary with body size, in contrast to most studies that find wingbeat frequency increases as body size decreases. Smaller stingless bees have greater relative forewing surface area which may help them reduce the energy requirements needed to fly. Further, we hypothesize that the relatively larger heads of smaller species may change their body pitch in flight. Synthesizing across all flying insects, we demonstrate that the scaling of flight metabolic rate changes from hypermetric to hypometric at approximately 58 mg body mass with hypermetic scaling below (slope = 1.2) and hypometric scaling (slope = 0.67) above 58 mg in body mass. The reduced cost of flight likely provides selective advantages for the evolution of small body size in insects. The biphasic scaling of flight metabolic rates and wingbeat frequencies in insects supports the hypothesis that the scaling of metabolic rate is closely related to the power requirements of locomotion and cycle frequencies.

Distinct forms of resonant optimality within insect indirect flight motors

Insect flight motors are extraordinary natural structures that operate efficiently at high frequencies. Structural resonance is thought to play a role in ensuring efficient motor operation, but the details of this role are elusive. While the efficiency benefits associated with resonance may be significant, a range of counterintuitive behaviours are observed. In particular, the relationship between insect wingbeat frequencies and thoracic natural frequencies is uncertain, with insects showing wingbeat frequency modulation over both short and long time scales. Here, we offer new explanations for this modulation. We show how, in linear and nonlinear models of an indirect flight motor, resonance is not a unitary state at a single frequency, but a complex cluster of distinct and mutually exclusive states, each representing a different form of resonant optimality. Additionally, by characterizing the relationship between resonance and the state of negative work absorption within the motor, we demonstrate how near-perfect resonant energetic optimality can be maintained over significant wingbeat frequency ranges. Our analysis leads to a new conceptual model of flight motor operation: one in which insects are not energetically restricted to a precise wingbeat frequency, but instead are robust to changes in thoracic and environmental properties-an illustration of the extraordinary robustness of these natural motors.

Bristled-wing design of materials, microstructures, and aerodynamics enables flapping flight in tiny wasps

Parasitoid wasps of the smallest flying insects with bristled wings exhibit sophisticated flight behaviors while challenging biomechanical limitations in miniaturization and low-speed flow regimes. Here, we investigate the morphology, material composition, and mechanical properties of the bristles of the parasitoid wasps Anagrus Haliday. The bristles are extremely stiff and exhibit a high-aspect-ratio conical tubular structure with a large Young's modulus. This leads to a marginal deflection and uniform structural stress distribution in the bristles while they experience high-frequency flapping–induced aerodynamic loading, indicating that the bristles are robust to fatigue. The flapping aerodynamics of the bristled wings reveal that the wing surfaces act as porous flat paddles to reduce the overall inertial load while utilizing a passive shear-based aerodynamic drag-enhancing mechanism to generate the requisite aerodynamic forces. The bristled wing may have evolved as a novel design that achieves multiple functions and provides innovative ideas for developing bioinspired engineering microdevices.

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Bristles are extremely stiff and exhibit a high-aspect-ratio conical tubular structure

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Bristles uniformalize structural stress distributions and are robust to loading fatigue

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Bristled wings are light, using less power to achieve novel aerodynamic force production

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Bristled wings may bring an innovative design for bioinspired engineering microdevices

Novel flight style and light wings boost flight performance of tiny beetles

Flight speed is positively correlated with body size in animals¹. However, miniature featherwing beetles can fly at speeds and accelerations of insects three times their size². Here we show that this performance results from a reduced wing mass and a previously unknown type of wing-motion cycle. Our experiment combines three-dimensional reconstructions of morphology and kinematics in one of the smallest insects, the beetle Paratuposa placentis (body length 395 μm). The flapping bristled wings follow a pronounced figure-of-eight loop that consists of subperpendicular up and down strokes followed by claps at stroke reversals above and below the body. The elytra act as inertial brakes that prevent excessive body oscillation. Computational analyses suggest functional decomposition of the wingbeat cycle into two power half strokes, which produce a large upward force, and two down-dragging recovery half strokes. In contrast to heavier membranous wings, the motion of bristled wings of the same size requires little inertial power. Muscle mechanical power requirements thus remain positive throughout the wingbeat cycle, making elastic energy storage obsolete. These adaptations help to explain how extremely small insects have preserved good aerial performance during miniaturization, one of the factors of their evolutionary success.

Three-dimensional reconstructions of morphology and flight mechanics of the beetle Paratuposa placentis reveal adaptations that enable extremely small insects to fly at speeds similar to those of much larger insects.

Global Size Pattern in a Group of Important Ecological Indicators (Diptera, Chironomidae) Is Driven by Latitudinal Temperature Gradients

Simple Summary

The size of animals is a result of the complex interactions between the evolution of a group, the environment in which the animal lives, and its physiology. It has been known for a long time that warm-blooded animals (such as birds or mammals) become larger in colder climates. This phenomenon is called “Bergmann’s rule”, and it is caused by the necessity of the animals to produce and preserve their heat in colder climates. This is easier for larger animals, as they have a lower ratio of body surface area to body volume. In cold-blooded animals, such as insects, similar patterns have been found in some cases, but their origin is less clear. In this paper, we show a strong negative relationship between size and temperature in a large group of aquatic insects (non-biting midges). We found that wings of non-biting midges are shorter by 32.4 µm for every 1 °C of mean annual temperature increase. This finding is important for use of non-biting midges in monitoring aquatic ecosystem health and tracking global climate change.

Abstract

Size is one of the most outwardly obvious characteristics of animals, determined by multiple phylogenetic and environmental variables. Numerous hypotheses have been suggested to explain the relationship between the body size of animals and their geographic latitude. Bergmann’s Rule, describing a positive relationship between the body size of endothermic animals and their geographic latitude, is especially well known. Whether or not insects exhibit a similar pattern has long been a subject for debate. We hypothesize that latitudinal size gradients are coupled to temperature variation affecting the metabolic rate of these merolimnic insects. We showcase a strong latitudinal size gradient in non-biting midges (Diptera: Chironomidae), based on the examination of 4309 specimens of these midges from around the world. Although phylogenetic position was a key predictor of wing length, we also found that wing length decreases by 32.4 µm per every 1 °C of mean annual temperature increase. This pattern was found across different taxa and could be detected in 20 of 24 genera studied. We discuss the reasons for this pattern origin and its palaeoecological implications.

Small brains for big science

As the study of the human brain is complicated by its sheer scale, complexity, and impracticality of invasive experiments, neuroscience research has long relied on model organisms. The brains of macaque, mouse, zebrafish, fruit fly, nematode, and others have yielded many secrets that advanced our understanding of the human brain. Here, we propose that adding miniature insects to this collection would reduce the costs and accelerate brain research. The smallest insects occupy a special place among miniature animals: despite their body sizes, comparable to unicellular organisms, they retain complex brains that include thousands of neurons. Their brains possess the advantages of those in insects, such as neuronal identifiability and the connectome stereotypy, yet are smaller and hence easier to map and understand. Finally, the brains of miniature insects offer insights into the evolution of brain design.

Cantilever-based differential pressure sensor with a bio-inspired bristled configuration

Inspired by the bristled wing configuration of tiny insects, we proposed a novel polyimide (PI) cantilever-based differential pressure (DP) sensor. This bristled PI cantilever with a thin metallic piezoresistor was designed to detect the pressure difference that induced the aerodynamic loading on the surface of the cantilever. Owing to the aerodynamic characteristics of the bristled cantilever, the DP-sensor with the bristled cantilever could not only retain a comparable sensitivity with that of the paddle cantilever under low differential pressures but also achieve a higher theoretical upper detection limit due to the enhanced leakage of the bristles. Experimental results indicated that the DP-sensor with bristled cantilevers extended the detection range by ∼30% in comparison with the DP-sensor with paddle cantilevers. The high sensitivity, wide detection range, and facile fabrication process of these bio-inspired DP-sensors make them promising for future applications.

Bat target tracking strategies for prey interception

Insectivorous bats capture their prey in flight with impressive success. They rely on the echoes of their own ultrasonic vocalization that yield acoustic snapshots, which enable target tracking on a rapid time scale. This task requires the use of intermittent information to navigate a dynamically changing environment. Bats may solve this challenging task by building internal models that estimate target velocity to anticipate the future location of a prey item. This has been recently tested empirically in perched bats tracking a target moving across their acoustic field. In this report, we build on past work to propose a new model that describes bat flight trajectories employing predictive strategies. Furthermore, we compare this model with a previous model of bat target interception that has also been employed by some visually guided animals: parallel navigation.

Abbreviations: HTTP, Hybrid Target Trajectory Prediction; CATD, Constant Absolute Target Direction; CB, Constant Bearing; PN, Parallel Navigation

Miniaturization re-establishes symmetry in the wing folding patterns of featherwing beetles

Most microinsects have feather-like bristled wings, a state known as ptiloptery, but featherwing beetles (family Ptiliidae) are unique among winged microinsects in their ability to fold such wings. An asymmetrical wing folding pattern, found also in the phylogenetically related rove beetles (Staphylinidae), was ancestral for Ptiliidae. Using scanning electron, confocal laser scanning, and optical microscopy, high-speed video recording, and 3D reconstruction, we analyze in detail the symmetrical wing folding pattern and the mechanism of the folding and unfolding of the wings in Acrotrichis sericans (Coleoptera: Ptiliidae) and show how some of the smaller featherwing beetles have reverted to strict symmetry in their wing folding. The wings are folded in three phases by bending along four lines (with the help of wing folding patches on the abdominal tergites) and locked under the closed elytra; they unfold passively in two phases, apparently with the help of the elasticity provided by resilin unevenly distributed in the wing and of convexities forming in the cross-sections of the unfolding wing, making it stiffer. The minimum duration of folding is 3.5 s; unfolding is much more rapid (minimum duration lowest recorded in beetles, 0.038 s). The folding ratio of A. sericans is 3.31 (without setae), which is greater than in any beetle in which it has been measured. The symmetrical wing folding pattern found in A. sericans and in all of the smallest ptiliids, in which ptiloptery is especially pronounced, is the only known example of symmetry re-established during miniaturization. This direction of evolution is remarkable because miniaturization is known to result in various asymmetries, while in this case miniaturization was accompanied by reversal to symmetry, probably associated with the evolution of ptiloptery. Our results on the pattern and mechanisms of wing folding and unfolding can be used in robotics for developing miniature biomimetic robots: the mechanisms of wing folding and unfolding in Ptiliidae present a challenge to engineers who currently work at designing ever smaller flying robots and may eventually produce miniature robots with foldable wings.