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Insect wings are deformable airfoils, in which deformations are mostly achieved by complicated interactions between their structural components. Due to the complexity of the wing design and technical challenges associated with testing the delicate wings, we know little about the properties of their components and how they determine wing response to flight forces. Here, we report an unusual structure from the hind-wing membrane of the beetle Pachnoda marginata . The structure, a transverse section of the claval flexion line, consists of two distinguishable layers: a bell-shaped upper layer and a straight lower layer. Our computational simulations showed that this is an effective one-way hinge, which is stiff in tension and upward bending but flexible in compression and downward bending. By systematically varying its design parameters in a computational model, we showed that the properties of the double-layer membrane hinge can be tuned over a wide range. This enabled us to develop a broad design space, which we later used for model selection. We used selected models in three distinct applications, which proved that the double-layer hinge represents a simple yet effective design strategy for controlling the mechanical response of structures using a single material and with no extra mass. The insect-inspired, one-way hinge is particularly useful for developing structures with asymmetric behavior, exhibiting different responses to the same load in two opposite directions. This multidisciplinary study not only advances our understanding of the biomechanics of complicated insect wings but also informs the design of easily tunable engineering hinges.

Charles Ellington graduated at Duke University, North Carolina, and came to Cambridge in 1973 to work for a PhD on insect flight dynamics. He developed novel methodology and software for the kinematic analysis of freely hovering insects and applied them to his own high-speed films of a range of species. He identified five new non-steady-state mechanisms for lift generation, was the first to develop a vortex theory for flapping flight and developed and extended the use of morphometric parameters in calculating the forces and power requirements of flight. He remained in Cambridge, married a colleague, joined the staff of the Department of Zoology, became a fellow of Downing College and continued to work on insect aerodynamics and energetics, publishing on flight muscle efficiency, the factors limiting flight performance and the aerodynamic implications of the origin of insect flight. Building a closed-circuit wind tunnel connected with a sensitive oxygen analyser, he studied with colleagues how the aerodynamics and metabolic power input of bumblebees vary with flight speed, challenging the orthodox theory that this should follow a U-shaped curve. Outstanding among later research was the discovery that hawkmoths, and by implication many other insects, gain high levels of lift by generating a vortex above the leading edge, stabilized by spiralling out along the span—a major focus of animal flight research ever since. His many administrative roles included editorship of theJournal of Experimental Biology. He became a British citizen in 1995, was elected FRS in 1998 and to a chair of animal mechanics in 1999. Awards include the Scientific Medal of the Zoological Society and the University of Cambridge Pilkington Prize for teaching excellence. He was diabetic throughout his adult life, and suffered progressive ill health following a heart attack in 1996. He took early retirement in 2010, lived quietly with his wife and two sons at home near Newmarket, and died in July 2019.

Odonata flight performance capabilities and behaviour and their body and wing form diversity are explored, and their interrelationships discussed theoretically and from observational evidence. Over...

The nature, occurrence, morphological basis and functions of insect wing deformation in flight are reviewed. The importance of relief in supporting the wing is stressed, and three types are recognized, namely corrugation, an M-shaped section and camber, all of which need to be overcome if wings are to bend usefully in the morphological upstroke. How this is achieved, and how bending, torsion and change in profile are mechanically interrelated, are explored by means of simple physical models which reflect situations that are visible in high speed photographs and films. The shapes of lines of transverse flexion are shown to reflect the timing and roles of bending, and their orientation is shown to determine the extent of the torsional component of the deformation process. Some configurations prove to allow two stable conditions, others to be monostable. The possibility of active remote control of wing rigidity by the thoracic musculature is considered, but the extent of this remains uncertain.

[][1] Charlie and The Flapper, the latter making its first public appearance at the Royal Society Soirees in 1991. These events are full-dress affairs; it can just be seen that the flapper is wearing a bow tie. Photo credit: R.J.W. By the mid-1970s, the study of insect flight

Insect wings lack internal muscles, and the orderly, necessary deformations which they undergo in flight and folding are in part remotely controlled, in part encoded in their structure. This factor is crucial in understanding their complex, extremely varied morphology. Models have proved particularly useful in clarifying the facilitation and control of wing deformation. Their development has followed a logical sequence from conceptual models through physical and simple analytical to numerical models. All have value provided their limitations are realized and constant comparisons made with the properties and mechanical behaviour of real wings. Numerical modelling by the finite element method is by far the most time–consuming approach, but has real potential in analysing the adaptive significance of structural details and interpreting evolutionary trends. Published examples are used to review the strengths and weaknesses of each category of model, and a summary is given of new work using finite element modelling to investigate the vibration properties and response to impact of hawkmoth wings.

Finite element analysis is used to model the automatic cambering of the locust hind wing during promotion: the umbrella effect. It was found that the model required a high degree of sophistication before replicating the deformations found in vivo. The model has been validated using experimental data and the deformations recorded both in vivo and ex vivo. It predicts that even slight modifications to the geometrical description used can lead to significant changes in the deformations observed in the anal fan. The model agrees with experimental data and produces deformations very close to those seen in free-flying locusts. The validated model may be used to investigate the varying geometries found in orthopteran anal fans and the stresses found throughout the wing when loaded.

As part of an investigation of the functional mechanics of the hind wing of the desert locust Schistocerca gregaria, the Young’s modulus of the membrane was measured using a newly developed universal materials test machine capable of testing very small specimens of cuticle, down to 1 mm gauge length. Strain was measured optically. Specimens were cut from various locations around the wing and tested under controlled temperature and humidity. The modulus of the membrane was typically between 1 and 5 GPa, but both this and the membrane thickness varied around the wing, with the remigium and the anal fan showing markedly different properties. The membrane was tested for chitin using two methods: a gas pyrolysis/mass spectrometry assay, and a gold-labelled immunoassay specific to chitin. None was detected, and the membrane may consist of epicuticle alone. The wings were examined for evidence of crystalline material using standard polarising microscopy and an advanced technique that distinguishes between three components of the polarised image. Birefringence was detected in the membrane of the anterior part of the wing, but vanished when the membrane was separated from the surrounding veins, suggesting that it was due to pre-stress rather than to ultrastructure. The implications are discussed.

Some principles governing the design of invertebrate paired propulsive appendages are discussed, with particular reference to the extent to which information encoded in their skeletal structure determines their instantaneous shape in locomotion. The hydrostatic paired fins of some cephalopods and marine gastropods, polychaete parapodia and onychophoran lobopodia rely entirely on musculature for shape control. The deformations of walking limbs, though still under muscular control, are strongly influenced by the nature and sequence of movement of the joints. Limbs adapted for walking in air are effectively point-loaded, and their rigid components need to resist axial forces as well as bending and torsional moments. Aquatic walking limbs have little axial loading, while swimming appendages and wings experience only bending and torsional moments, and can exploit these to assist in the deformations that are necessary to gain force asymmetry between half-strokes. Swimming appendages normally employ both muscles and drag, but the wings of insects lack internal muscles, and their changes in shape are largely complex aeroelastic responses to the constantly changing aerodynamic and inertial loads, moderated by muscles inserted at the base. For illustration, wings modelled as thin shells with flexible hinge-lines are used to investigate how transverse distal flexion, essential for controlling the angle of attack in the upstroke, is remotely controlled by the indirect flight muscles.

The Poisson's ratio of a material is strictly defined only for small strain linear elastic behavior. In practice, engineering strains are often used to calculate Poisson's ratio in place of the mathematically correct true strains with only very small differences resulting in the case of many engineering amterials. The engineering strain definition is often used even in the inelastic region, for example, in metals during plastic yielding. However, for highly nonlinear elastic materials, such as many biomaterials, smart materials and microstructured materials, this convenient extension may be misleading, and it becomes advantageous to define a strainvarying Poisson's function. This is analogous to the use of a tangent modulus for stiffness. An important recent application of such a Poisson's function is that of auxetic materials that demonstrate a negative Poisson's ratio and are often highly strain dependent. In this paper, the importance of the use of a Poisson's function in appropriate circumstances is demonstrated. Interpretation methods for coping with error-sensitive data or small strains are also described.

Brilliant iridescent colouring in male butterflies enables long–range conspecific communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra–scale microstructures to overall butterfly visibility, both in terms of reflected and transmitted intensities and viewing angles. Using a focused–laser technique, we investigated the absolute reflectivity and transmissivity associated with the single–scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable wide–angle visibility. Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100° in one plane and 15° in the other. We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected. Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single–scale microstructure to be n = (1.56+0.01) + (0.06 ±0.01)i. This figure is required for theoretical modelling of such microstructure systems.

Detailed comparison of patterns of folding in insect wings has shown that all those which fold transversely can be referred to two basic mechanisms, each consisting of four panels rotating hingewise about four folding lines, intersecting at a single point. The mechanisms, which have one degree of freedom, are in effect lever systems, whose velocity ratios change non-linearly as they operate. They are designated as ‘Internal9 (INT) and ‘External’ (EXT), and further specified by a plus or minus sign, derived from the convex or concave orientation of the folding lines. There are hence four possible combinations: INT-, INT+, EXT- and EXT+. The two types are kinematically distinct. As INT unfolds, the effector panels move slowly at first and accelerate towards the end of the movement, whereas in EXT the effector panels move fast initially, then decelerate rapidly to their their final position. Furthermore INT operates reversibly, whereas EXT usually needs to be closed elastically, or by an extraneous force. The principles of the two mechanisms are applicable to many other folding structures.

The `umbrella effect', a mechanism of camber generation in the hindwing fans of orthopteroid and dictyopteroid insects, is modelled geometrically and mechanically, treating the fan as a framework whose outer margin describes a truncated logarithmic spiral. Tension in the margin compresses the radial veins, causing them to undergo Euler buckling. The model describes the distribution of compression forces in the veins around the spiral, and predicts conditions which must be met, and departures from the model expected in real wings. Actual insects fit the model and predictions quite well. The functional implications, and those of departure from the model, are discussed.

Insect wings are mounted on hinges, restricting the extent to which their bases can be supinated for the upstroke. The forewings of many insects therefore include devices that allow the distal part of the wing to twist relative to the base under aerodynamic loading in the upstroke, but restrict such twisting in the downstroke where the effect would be detrimental. In the broad forewings of butterflies, this asymmetric resistance to aerodynamic twisting seems to be a consequence of the curved section of the leading edge. The wing can be modelled as a cantilevered, thin cambered plate. Torsional tests on the forewings of four butterfly species and on a paper wing of curved section confirm the effect. Differences between the results for the four species appear to fit their morphological and kinematic differences. The nature of the mechanism is outlined.

The bright green dorsal iridescence of the hindwings of Neurobasis chinensis chinensis males, very rare in Odonata, is known to play a significant role in their courtship behaviour. The mechanism responsible for such high contrast and spectrally pure colour has been investigated and found to be optical interference, producing structural colour from distinct laminations in the wing membrane cuticle. The ventral sides of these iridescent wings are dark brown in colour. In a single continuous membrane of wing cuticle, this is an effect that requires a specialized structure. It is accomplished through the presence of high optical absorption (kappa = 0.13) within two thick layers near the ventral surface of the wing, which leads to superior dorsal colour characteristics. By simultaneously fitting five sets of optical reflectivity and transmissivity spectra to theory, we were able to extract very accurate values of the complex refractive index for all three layer types present in the wing. The real parts of these are n = 1.47, 1.68 and 1.74. Although there is often similarly significant dorsal and ventral colour contrast in other structurally coloured natural systems, very few system designs comprise only a single continuous membrane.

Few mechanisms exist in nature that effect colour reflectivity, simultaneously high in spectral purity and in intensity, over a strictly limited portion of solid angle above a surface. Fewer still bring about such colour reflectivity with an angle dependence that is distinct from the colour transition associated with conventional multilayer interference. We have discovered that the ventral wings of the butterfly Ancyluris meliboeus exhibit these optical effects, and that they result from remarkable nano-scale architecture on the wing scales of the butterfly. This nano-structure is in the form of high-tilt multilayering that, as a result of abrupt termination of the multilayers, brings about diffraction concurrently with interference. The product is bright structural colour in a limited angular region over the ventral wing surface that enables remarkably strong flicker and colour contrast through minimal wing movement. The visibility effects associated with its colour, in terms of bright and dark zones of the observation hemisphere over the wing surface, are described. We suggest the purpose of the high-contrast ventral wing visibility associated with A. meliboeus is at-rest signalling; this is distinct from the dorsal wing visibility of other species such as those of the genus Morpho, the function of which is largely for in-flight signalling.

Detailed morphological investigation, mechanical testing and high-speed cinematography and stroboscopic examination of desert locusts, Schistocerca gregaria, in flight show that their hind wings are adapted to deform cyclically and automatically through the wing stroke and that the deformations are subtly dependent on the wings’ structure: their shape, venation and vein design and the local properties of the membrane. The insects predominantly fly fast forwards, generating most force on the downstroke, and the hind wings generate extra lift by peeling apart at the beginning of the downstroke and by developing a cambered section during the stroke’s translation phase through the ‘umbrella effect’ – an automatic consequence of the active extension of the wings’ expanded posterior fan. Bending experiments indicate that most of the hind wing is more rigid to forces from below than from above and demonstrate that the membrane acts as a stressed skin to stiffen the structure.

The wings of archaic Odonatoidea from the mid-Carboniferous of Argentina show features analogous to “smart” mechanisms in modern dragonflies that are associated with the agile, versatile flight necessary to catch prey in flight. These mechanisms act automatically in flight to depress the trailing edge and to facilitate wing twisting, in response to aerodynamic loading. The presence of similar features suggests that the earliest known odonatoids were already becoming adapted for high-performance flight in association with a predatory habit.

Insects are superb flying machines — many can hover and manoeuvre with great precision, all the time taking in information about their surroundings. These capabilities would be immensely useful in surveillance and industrial fault-location. A growing understanding of the principles of insect flight is paving the way to the realization of such micro-air-vehicles, although formidable engineering challenges remain.

Insect flight is a complicated business: the wings don't just flap up and down, but oscillate and twist to ensure that as much of the wing-stroke cycle is creating useful aerodynamic force. The latest finding is that transient bursts of lift are generated by the wings as they rotate at the top and bottom of each stroke, enhancing the lift achieved in other parts of the cycle.

The optical properties of transparent single membranes on the wings of the dragonfly Aeshna cyanea have been investigated. These membranes comprise one central thick cuticular layer covered dorsally and ventrally with typical odonatan wax pruinosity. Optical characterisation of individual membranes reveals they can support optical guided modes comprising differential polarisation reflection. We suggest this may offer an intraspecific signalling channel. The guided modes' characteristics depend on membrane thickness and the nature of the wax pruinosity. We accurately modelled multiple optical data sets simultaneously, thereby inaugurally quantifying the roughness of the pruinosity and the complex refractive indices of the wax and the odonatan cuticle.

The dazzling iridescence seen in some hummingbirds1 and tropical butterflies2 arises from natural optical phenomena, the brightest of which originate in nanoscale structures that produce ultra-high reflectivity and narrow-band spectral purity3. Here we investigate the coloration of male Ancyluris meliboeus Fabricius butterflies4, which have patches of unusual microstructure on their ventral wing scales. We find that this highly tilted, multilayered arrangement produces a bright iridescence of broad wavelength range and generates a strong flicker contrast from minimal wing movement.

Representatives of six butterfly species, flying freely in the field or in simulated field conditions, were filmed with a high-speed ciné camera and subjected to kinematic and morphometric analysis. This is the first detailed investigation on an insect performing the varied patterns of ‘natural’ flight. Kinematic parameters in representative sequences of selected flight modes were calculated and compared, and wing shapes were characterized using aspect ratio and non-dimensional moment parameters. The analyses and field observations of these and other butterflies suggest possible correlations between flight performance and wing shape. The behaviour of individual species conforms reasonably well with crude predictions based on aspect ratio, wing loading and wing inertia.

A structural engineering approach to the pleated wings of Odonata has been developed during a functional study of wing morphology in the group. The wing can be regarded as a folded plate structure within which each pleat-side acts as a deep plate-girder. Small cross-veins act as stiffeners within the girders, allowing the membrane to carry web shearing forces as pure tension, through a stressed-skin effect. Bending experiments confirm that the membrane significantly increases the rigidity of wing components. The properties of the membrane are unknown. It lacks birefringence, is very thin, and may be pure epicuticle. The advantages of stressed-skin construction are discussed, and possible modes of structural failure considered. The wing seems adapted to yield reversibly to unpredictable heavy loads.

Coupled investigations of insect wing movements and detailed wing morphology are in progress, and some functional principles underlying wing design are emerging. High speed cine and still photography and stroboscopy indicate that most wings undergo orderly deformation in flight. Common patterns are described and their significance discussed in the light of recent aerodynamic studies. Many aspects of wing morphology–venational features, relief, thickened areas, flexionlines and vein fractures–may be related to the control of three-dimensional shape while beating. It is usually possible to distinguish areas specialized for deformability, and for support and the limiting of deformation. Some structural adaptations for these roles are described and illustrated.

The functional wing morphology of the wings of the scorpion fly Panorpa germanica L. was investigated using a combination of light microscopy, high-speed cinematography, wing manipulation and mechanical testing. In rising forward flight the wings are flapped 40° out of phase along a shallow stroke plane, the forewings leading. Aerodynamic analysis suggests that unsteady effects are important in flight. During the downstroke, both wings are straight and cambered, the chord being parallel to the body axis, which is angled 45° upwards from horizontal. Both wings are supinated at lower stroke reversal, the hindwing to a much greater extent, and flex ventrally halfway along their length for the first half of the upstroke. Flexion is parallel to the chord in the hindwing, but is oblique in the forewing, so distal forewing areas are supinated relative to proximal areas. The behaviour of the wings is related to their structure. Spars at the leading and trailing edges of both wings support the wing during the downstroke, and flexion during the upstroke is facilitated by buckling of the weak ventral thyridium region. The oblique flexion seen in the forewing is due to its relatively longer leading edge spar. The differences between the wings are, in turn, related to their pitch control mechanisms. The forewing has a well-developed clavus, like that of the forewing of a locust, and pitch is altered by relative movement of this and the leading edge, but only within a narrow range. Oblique flexion is necessary to invert the aerofoil. The weaker and less well-developed clavus of the hindwing, more similar to that of the Diptera, allows a greater degree of supination, effected largely by wing inertia. No oblique flexion is necessary.