Wetting Characteristics of Insect Wing Surfaces

Biological tiny structures have been observed on many kinds of surfaces such as lotus leaves,which have an effect on thecoloration of Morpho butterflies and enhance the hydrophobicity of natural surfaces.We investigated the micro-scale andnano-scale structures on the wing surfaces of insects and found that the hierarchical multiple roughness structures help in enhancingthe hydrophobicity.After examining 10 orders and 24 species of flying Pterygotan insects,we found that micro-scaleand nano-scale structures typically exist on both the upper and lower wing surfaces of flying insects.The tiny structures such asdenticle or setae on the insect wings enhance the hydrophobicity,thereby enabling the wings to be cleaned more easily.And thehydrophobic insect wings undergo a transition from Cassie to Wenzel states at pitch/size ratio of about 20.In order to examinethe wetting characteristics on a rough surface,a biomimetic surface with micro-scale pillars is fabricated on a silicon wafer,which exhibits the same behavior as the insect wing,with the Cassie-Wenzel transition occurring consistently around apitch/width value of 20.     

Variations in the arrangement of sensory bristles along butterfly wing margins

The surfaces of insect wings exhibit numerous sensilla, which have been suggested to have a behavioral function. Some evidence suggests that the sensory bristles along the wing margin of lepidopteran insects (butterflies and moths) are involved in the regulation of wing movement. We investigated the arrangement of sensory bristles along the wing margins of 62 species of papilionoid butterflies, using light-microscopic examination of mounts of whole wings after removing the scales surrounding the bristles. In the majority of the wings examined, bristles were located on the ventral wing surfaces and were continuously distributed along the wing margins, except in the vicinity of the wing bases. In some wings, bristles were also located on the dorsal wing surfaces, and were continuously or discontinuously distributed along the wing margins of different species. In a minority of the species studied, we observed bristle distribution in the vicinity of the wing base, discontinuous bristle distribution on both the dorsal and ventral wing surfaces, or an absence of bristles along the wing margins. This variation in the arrangement of bristles along the wing margins is discussed in relation to the reception and transmission of sensory information in the wings.     

The frequency of wing damage in a migrating butterfly

The ability to fly is crucial for migratory insects. Consequently, the accumulation of damage on the wings over time can affect survival, especially for species that travel long distances. We examined the frequency of irreversible wing damage in the migratory butterfly Vanessa cardui to explore the effect of wing structure on wing damage frequency, as well as the mechanisms that might mitigate wing damage. An exceptionally high migration rate driven by high precipitation levels in their larval habitats in the winter of 2018–2019 provided us with an excellent opportunity to collect data on the frequency of naturally occurring wing damage associated with long-distance flights. Digital images of 135 individuals of V. cardui were collected and analyzed in Germany. The results show that the hindwings experienced a greater frequency of damage than the forewings. Moreover, forewings experienced more severe damage on the lateral margin, whereas hindwings experienced more damage on the trailing margin. The frequency of wing margin damage was higher in the painted lady butterfly than in the migrating monarch butterfly and in the butterfly Pontia occidentalis following artificially induced wing collisions. The results of this study could be used in future comparative studies of patterns of wing damage in butterflies and other insects. Additional studies are needed to clarify whether the strategies for coping with wing damage differ between migratory and nonmigratory species.     

Patterns on the insect wing

The evolution of wings and the adaptive advantage they provide have allowed insects to become one of the most evolutionarily successful groups on earth. The incredible diversity of their shape, size, and color patterns is a direct reflection of the important role wings have played in the radiation of insects. In this review, we highlight recent studies on both butterflies and Drosophila that have begun to uncover the types of genetic variations and developmental mechanisms that control diversity in wing color patterns. In butterflies, these analyses are now possible because of the recent development of a suite of genomic and functional tools, such as detailed linkage maps and transgenesis. In one such study, extensive linkage mapping in Heliconius butterflies has shown that surprisingly few, and potentially homologous, loci are responsible for several major pattern variations on the wings of these butterflies. Parallel work on a clade of Drosophila has uncovered how cis-regulatory changes of the same gene correlate with the repeated gain and loss of pigmented wing spots. Collectively, our understanding of formation and evolution of color pattern in insect wings is rapidly advancing because of these recent breakthroughs in several different fields.     

Evolution of the wave: aerodynamic and aposematic functions of butterfly wing motion

Many unpalatable butterfly species use coloration to signal their distastefulness to birds, but motion cues may also be crucial to ward off predatory attacks. In previous research, captive passion-vine butterflies Heliconius mimetic in colour pattern were also mimetic in motion. Here, I investigate whether wing motion changes with the flight demands of different behaviours. If birds select for wing motion as a warning signal, aposematic butterflies should maintain wing motion independently of behavioural context. Members of one mimicry group (Heliconius cydno and Heliconius sapho) beat their wings more slowly and their wing strokes were more asymmetric than their sister-species (Heliconius melpomene and Heliconius erato, respectively), which were members of another mimicry group having a quick and steady wing motion. Within mimicry groups, wing beat frequency declined as its role in generating lift also declined in different behavioural contexts. In contrast, asymmetry of the stroke was not associated with wing beat frequency or behavioural context-strong indication that birds process and store the Fourier motion energy of butterfly wings. Although direct evidence that birds respond to subtle differences in butterfly wing motion is lacking, birds appear to generalize a motion pattern as much as they encounter members of a mimicry group in different behavioural contexts.     

Prey survival by predator intimidation: an experimental study of peacock butterfly defence against blue tits

Long-lived butterflies that hibernate as adults are expected to have well-developed antipredation devices as a result of their long exposure to natural enemies. The peacock butterfly, Inachis io, for instance, is a cryptic leaf mimic when resting, but shifts to active defence when disturbed, performing a repeated sequence of movements exposing major eyespots on the wings accompanied by a hissing noise. We studied the effect of visual and auditory defence by staging experiments in which wild-caught blue tits, Parus caeruleus, were presented with one of six kinds of experimentally manipulated living peacock butterflies as follows: butterflies with eyespots painted over and their controls (painted on another part of the wing), butterflies with their sound production aborted (small part of wings removed) and their controls, and butterflies with eyespots painted over and sound production aborted and their controls. The results showed that eyespots alone, or in combination with sound, constituted an effective defence; only 1 out of 34 butterflies with intact eyespots was killed, whereas 13 out of 20 butterflies without eyespots were killed. The killed peacocks were eaten, indicating that they are not distasteful. Hence, intimidation by bluffing can be an efficient means of defence for an edible prey.     

Control for small-speed lateral flight in a model insect

Controls required for small-speed lateral flight of a model insect were studied using techniques based on the linear theories of stability and control (the stability and control derivatives were computed by the method of computational fluid dynamics). The main results are as follows. (1) Two steady-state lateral motions can exist: one is a horizontal side translation with the body rolling to the same side of the translation by a small angle, and the other is a constant-rate yaw rotation (rotation about the vertical axis). (2) The side translation requires an anti-symmetrical change in the stroke amplitudes of the contralateral wings, and/or an anti-symmetrical change in the angles of attack of the contralateral wings, with the down- and upstroke angles of attack of a wing having equal change. The constant-rate yaw rotation requires an anti-symmetrical change in the angles of attack of the contralateral wings, with the down- and upstroke angles of attack of a wing having differential change. (3) For the control of the horizontal side translation, control input required for the steady-state motion has an opposite sign to that needed for initiating the motion. For example, to have a steady-state left side-translation, the insect needs to increase the stroke amplitude of the left wing and decrease that of the right wing to maintain the steady-state flight, but it needs an opposite change in stroke amplitude (decreasing the stroke amplitude of the left wing and increasing that of the right wing) to enter the flight.     

Species-specific color-pattern modifications of butterfly wings

We have previously shown that the systemic injection of sodium tungstate, a general protein-tyrosine phosphatase (PTPase) inhibitor, efficiently produces characteristic color-pattern modifications on the wings of the Painted Lady butterfly, Vanessa cardui. By using this method in the present study, we analyzed modification patterns of six species of Japanese butterflies. Whereas in Vanessa indica the black spots on the forewings reduced in size in response to the treatment, in Lycaena phlaeas the morphologically similar black spots enlarged in size. However, the metallic blue spots on the forewings of V. indica did enlarge in size, showing different behavior even within a single wing surface. The response patterns of Ypthima argus differed markedly from those of other species in that ectopic color-pattern elements were created. Colias erate showed minor modifications that coincidentally resembled the natural color-pattern of a closely related species, Colias palaeno. Through a comprehensive literature search, we confirmed the existence of naturally occurring aberrant color patterns with close similarities to the experimentally induced phenocopies in each of the modified species. Our results point out the possibility that a hypothetical transduction pathway with a PTPase for the scale-cell differentiation globally coordinates the wing-wide color-pattern development in butterflies.     

Comparative and functional morphology of wing coupling structures in Trichoptera: Annulipalpia

Several orders of morphologically four‐winged insects have evolved mechanisms that enforce a union between the mesothoracic and metathoracic wings (forewings and hindwings) during the wing beat cycle. Such mechanisms result in a morphologically tetrapterous insect flying as if it were functionally dipterous, and these mechanisms have been described for several insect orders. The caddisfly suborders Annulipalpia and Integripalpia (Trichoptera) each have evolved a wing coupling apparatus, with at least three systems having evolved within the suborder Annulipalpia. The comparative and inferred functional morphology of the putative wing coupling mechanisms is described for the annulipalpian families Hydropsychidae (subfamilies Macronematinae and Hydropsychinae), Polycentropodidae and Ecnomidae, and a novel form‐functional complex putatively involved with at‐rest forewing‐forewing coupling is described for Hydropsychidae: Smicrideinae. It is proposed that the morphology of the wing coupling apparatuses of Hydropsychinae and Macronematinae are apomorphies for those clades. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

Over-expression of Ultrabithorax alters embryonic body plan and wing patterns in the butterfly Bicyclus anynana

In insects, forewings and hindwings usually have different shapes, sizes, and color patterns. A variety of RNAi experiments across insect species have shown that the hox gene Ultrabithorax (Ubx) is necessary to promote hindwing identity. However, it remains unclear whether Ubx is sufficient to confer hindwing fate to forewings across insects. Here, we address this question by over-expressing Ubx in the butterfly Bicyclus anynana using a heat-shock promoter. Ubx whole-body over-expression during embryonic and larvae development led to body plan changes in larvae but to mere quantitative changes to adult morphology, respectively. Embryonic heat-shocks led to fused segments, loss of thoracic and abdominal limbs, and transformation of head limbs to larger appendages. Larval heat-shocks led to reduced eyespot size in the expected homeotic direction, but neither additional eyespots nor wing shape changes were observed in forewings as expected of a homeotic transformation. Interestingly, Ubx was found to be expressed in a novel, non-characteristic domain – in the hindwing eyespot centers. Furthermore, ectopic expression of Ubx on the pupal wing activated the eyespot-associated genes spalt and Distal-less, known to be directly repressed by Ubx in the fly?s haltere and leg primordia, respectively, and led to the differentiation of black wing scales. These results suggest that Ubx has been co-opted into a novel eyespot gene regulatory network, and that it is capable of activating black pigmentation in butterflies.     

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

Although there is mounting evidence that passive mechanical dynamics of insect wings play an integral role in insect flight, our understanding of the structural details underlying insect wing flexibility remains incomplete. Here, we use comparative morphological and mechanical techniques to illuminate the function and diversity of two mechanisms within Odonata wings presumed to affect dynamic wing deformations: flexible resilin vein‐joints and cuticular spikes. Mechanical tests show that joints with more resilin have lower rotational stiffness and deform more in response to a load applied to an intact wing. Morphological studies of 12 species of Odonata reveal that resilin joints and cuticular spikes are widespread taxonomically, yet both traits display a striking degree of morphological and functional diversity that follows taxonomically distinct patterns. Interestingly, damselfly wings (suborder Zygoptera) are mainly characterized by vein‐joints that are double‐sided (containing resilin both dorsally and ventrally), whereas dragonfly wings (suborder Epiprocta) are largely characterized by single‐sided vein‐joints (containing resilin either ventrally or dorsally, but not both). The functional significance and diversity of resilin joints and cuticular spikes could yield insight into the evolutionary relationship between form and function of wings, as well as revealing basic principles of insect wing mechanical design. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Thermoregulatory significance of wing melanization in Pieris butterflies (Lepidoptera: Pieridae): physics,posture, and pattern

Summary Pieris butterflies use a novel behavioral posture for thermoregulation called reflectance basking, in which the wings are used as solar reflectors to reflect radiation to the body. As a means of exploring the thermoregulatory significance of wing melanization patterns, I examine the relation of basking posture and wing color pattern to body temperature. A mathematical model of the reflectance process predicts certain combinations of dorsal wing melanization pattern and basking posture that maximize body temperature. Laboratory experiments and field observations show that this model correctly predicts qualitative differences in the relation of body temperature to basking posture based on differences in the extent of dorsal melanization on the wing margins, both between species and between sexes within species of Pieris. This is the first demonstration in insects that coloration of the entire wing surface can affect thermoregulation. Model and experimental results suggest that, in certain wing regions, increased melanization can reduce body temperature in Pieris; this effect of melanization is exactly the opposite of that found in other Pierid butterflies that use their wings as solar absorbers. These results are discussed in terms of the evolution of wing melanization pattern and thermoregulatory behavior in butterflies.     

Hox genes are essential for the development of eyespots in Bicyclus anynana butterflies

The eyespot patterns found on the wings of nymphalid butterflies are novel traits that originated first in hindwings and subsequently in forewings, suggesting that eyespot development might be dependent on Hox genes. Hindwings differ from forewings in the expression of Ultrabithorax (Ubx), but the function of this Hox gene in eyespot development as well as that of another Hox gene Antennapedia (Antp), expressed specifically in eyespots centers on both wings, are still unclear. We used CRISPR-Cas9 to target both genes in Bicyclus anynana butterflies. We show that Antp is essential for eyespot development on the forewings and for the differentiation of white centers and larger eyespots on hindwings, whereas Ubx is essential not only for the development of at least some hindwing eyespots but also for repressing the size of other eyespots. Additionally, Antp is essential for the development of silver scales in male wings. In summary, Antp and Ubx, in addition to their conserved roles in modifying serially homologous segments along the anterior–posterior axis of insects, have acquired a novel role in promoting the development of a new set of serial homologs, the eyespot patterns, in both forewings (Antp) and hindwings (Antp and Ubx) of B. anynana butterflies. We propose that the peculiar pattern of eyespot origins on hindwings first, followed by forewings, could be due to an initial co-option of Ubx into eyespot development followed by a later, partially redundant, co-option of Antp into the same network.     

Colour pattern homology and evolution in Vanessa butterflies (Nymphalidae: Nymphalini): eyespot characters

Ocelli are serially repeated colour patterns on the wings of many butterflies. Eyespots are elaborate ocelli that function in predator avoidance and deterrence as well as in mate choice. A phylogenetic approach was used to study ocelli and eyespot evolution in Vanessa butterflies, a genus exhibiting diverse phenotypes among these serial homologs. Forty‐four morphological characters based on eyespot number, arrangement, shape and the number of elements in each eyespot were defined and scored. Ocelli from eight wing cells on the dorsal and ventral surfaces of the forewing and hindwing were evaluated. The evolution of these characters was traced over a phylogeny of Vanessa based on 7750 DNA base pairs from 10 genes. Our reconstruction predicts that the ancestral Vanessa had 5 serially arranged ocelli on all four wing surfaces. The ancestral state on the dorsal forewing and ventral hindwing was ocelli arranged in two heterogeneous groups. On the dorsal hindwing, the ancestral state was either homogenous or ocelli arranged in two heterogeneous groups. On the ventral forewing, we determined that the ancestral state was organized into three heterogeneous groups. In Vanessa, almost all ocelli are individuated and capable of independent evolution relative to other colour patterns except for the ocelli in cells ?1 and 0 on the dorsal and ventral forewings, which appear to be constrained to evolve in parallel. The genus Vanessa is a good model system for the study of serial homology and the interaction of selective forces with developmental architecture to produce diversity in butterfly colour patterns.     

The redder the better: wing color predicts flight performance in monarch butterflies

The distinctive orange and black wings of monarchs (Danaus plexippus) have long been known to advertise their bitter taste and toxicity to potential predators. Recent work also showed that both the orange and black coloration of this species can vary in response to individual-level and environmental factors. Here we examine the relationship between wing color and flight performance in captive-reared monarchs using a tethered flight mill apparatus to quantify butterfly flight speed, duration and distance. In three different experiments (totaling 121 individuals) we used image analysis to measure body size and four wing traits among newly-emerged butterflies prior to flight trials: wing area, aspect ratio (length/width), melanism, and orange hue. Results showed that monarchs with darker orange (approaching red) wings flew longer distances than those with lighter orange wings in analyses that controlled for sex and other morphometric traits. This finding is consistent with past work showing that among wild monarchs, those sampled during the fall migration are darker in hue (redder) than non-migratory monarchs. Together, these results suggest that pigment deposition onto wing scales during metamorphosis could be linked with traits that influence flight, such as thorax muscle size, energy storage or metabolism. Our results reinforce an association between wing color and flight performance in insects that is suggested by past studies of wing melansim and seasonal polyphenism, and provide an important starting point for work focused on mechanistic links between insect movement and color.     

Assessing the Role of Wing Spots in Intraspecific Communication in the Cabbage White Butterfly (<Emphasis Type="Italic">Pieris rapae</Emphasis> L.) Using a Simple Device to Increase Butterfly Responses

Butterflies are regularly used as model systems for understanding the role of coloration in communication because of their highly variable and conspicuous phenotypes. Most research showing a role for color in communication has focused on aspects of brightness or hue of entire wings or large color patches. However, evidence is accumulating that butterflies sometimes use smaller wing pattern elements in communication. Here we provide evidence that both male and female cabbage white butterflies (Pieris rapae L.) discriminate among conspecifics on the basis of the number of small but conspicuous black wing spots of the dorsal forewing. Male butterflies were more interactive with model butterflies with two spots, which resemble female butterflies, than with model butterflies with only one spot, which resemble male butterflies. Female butterflies showed the opposite response, being more interactive with male-like (one-spot) models than with female-like (two-spot) models. Some of our experiments were conducted with an electronic device designed to create a realistic and controlled fluttering behavior of the models. We describe the design and function of this device and provide evidence that it increased butterfly responses compared to a non-fluttering model. This device could prove useful for others addressing questions of communication in butterflies or other flying insects.     

Aerodynamic performance of a hovering hawkmoth with flexible wings: a computational approach

Insect wings are deformable structures that change shape passively and dynamically owing to inertial and aerodynamic forces during flight. It is still unclear how the three-dimensional and passive change of wing kinematics owing to inherent wing flexibility contributes to unsteady aerodynamics and energetics in insect flapping flight. Here, we perform a systematic fluid-structure interaction based analysis on the aerodynamic performance of a hovering hawkmoth, Manduca, with an integrated computational model of a hovering insect with rigid and flexible wings. Aerodynamic performance of flapping wings with passive deformation or prescribed deformation is evaluated in terms of aerodynamic force, power and efficiency. Our results reveal that wing flexibility can increase downwash in wake and hence aerodynamic force: first, a dynamic wing bending is observed, which delays the breakdown of leading edge vortex near the wing tip, responsible for augmenting the aerodynamic force-production; second, a combination of the dynamic change of wing bending and twist favourably modifies the wing kinematics in the distal area, which leads to the aerodynamic force enhancement immediately before stroke reversal. Moreover, an increase in hovering efficiency of the flexible wing is achieved as a result of the wing twist. An extensive study of wing stiffness effect on aerodynamic performance is further conducted through a tuning of Young's modulus and thickness, indicating that insect wing structures may be optimized not only in terms of aerodynamic performance but also dependent on many factors, such as the wing strength, the circulation capability of wing veins and the control of wing movements.     

Aerodynamic Interactions Between Wing and Body of a Model Insect in Forward Flight and Maneuvers

The aerodynamic interactions between the body and the wings of a model insect in forward flight and maneuvers are studied using the method of numerically solving the Navier-Stokes equations over moving overset grids. Three cases are considered, including a complete insect, wing pair only and body only. By comparing the results of these cases, the interaction effect between the body and the wing pair can be identified. The changes in the force and moment coefficients of the wing pair due to the presence of the body are less than 4.5% of the mean vertical force coefficient of the model insect; the changes in the aerodynamic force coefficients of the body due to the presence of the wings are less than 5.0% of the mean vertical force coefficient of the model insect. The results of this paper indicate that in studying the aerodynamics and flight dynamics of a flapping insect in forward flight or maneuver, separately computing (or measuring) the aerodynamic forces and moments on the wing pair and on the body could be a good approximation.     

Moth wing scales slightly increase the absorbance of bat echolocation calls

Coevolutionary arms races between predators and prey can lead to a diverse range of foraging and defense strategies, such as countermeasures between nocturnal insects and echolocating bats. Here, we show how the fine structure of wing scales may help moths by slightly increasing sound absorbance at frequencies typically used in bat echolocation. Using four widespread species of moths and butterflies, we found that moth scales are composed of honeycomb-like hollows similar to sound-absorbing material, but these were absent from butterfly scales. Micro-reverberation chamber experiments revealed that moth wings were more absorbent at the frequencies emitted by many echolocating bats (40-60 kHz) than butterfly wings. Furthermore, moth wings lost absorbance at these frequencies when scales were removed, which suggests that some moths have evolved stealth tactics to reduce their conspicuousness to echolocating bats. Although the benefits to moths are relatively small in terms of reducing their target strengths, scales may nonetheless confer survival advantages by reducing the detection distances of moths by bats by 5-6%.