Wettability gradient on the elytra in the aquatic beetle Cybister chinensis and its role in angular position of the beetle at water-air interface

Structure and mechanical properties of ladybird elytra as biological sandwich panels

The armour of the ladybird, elytra, protect the body from injury and are well-adapted to flight. However, experimental methods to decipher their mechanical performances had been challenging due to the small size, making it unclear how the elytra balance mass and strength. Here, we provide insights to the relationship between the microstructure and multifunctional properties of the elytra by means of structural characterization, mechanical analysis and finite element simulations. Micromorphology analysis on the elytron revealed the thickness ratio of the upper lamination, middle layer and lower lamination is approximately 51:139:7. The upper lamination had multiple cross fibre layers and the thickness of each fibre layer is not the same. In addition, the tensile strength, elastic modulus, fracture strain, bending stiffness and hardness of elytra were obtained through in-situ tensile and nanoindentation-bending under the influence of multiple loading conditions, which also serve as references for finite element models. The finite element model revealed that structural factors such as thickness of each layer, angle of fibre layer and trabeculae are key to affecting the mechanical properties, but the effect is different. When the thickness of upper, middle and lower layers is the same, the tensile strength provided by unit mass of the model is 52.78% lower than that provided by elytra. These findings broaden the relationship between the structural and mechanical properties of the ladybird elytra, and are expected to inspire the development of sandwich structures in biomedical engineering.

Staying Dry and Clean: An Insect's Guide to Hydrophobicity

Insects demonstrate a wide diversity of microscopic cuticular and extra-cuticular features. These features often produce multifunctional surfaces which are greatly desired in engineering and material science fields. Among these functionalities, hydrophobicity is of particular interest and has gained recent attention as it often results in other properties such as self-cleaning, anti-biofouling, and anti-corrosion. We reviewed the historical and contemporary scientific literature to create an extensive review of known hydrophobic and superhydrophobic structures in insects. We found that numerous insects across at least fourteen taxonomic orders possess a wide variety of cuticular surface chemicals and physical structures that promote hydrophobicity. We discuss a few bioinspired design examples of how insects have already inspired new technologies. Moving forward, the use of a bioinspiration framework will help us gain insight into how and why these systems work in nature. Undoubtedly, our fundamental understanding of the physical and chemical principles that result in functional insect surfaces will continue to facilitate the design and production of novel materials.

Dimorphic cocoons of the robin moth, Hyalophora cecropia, reflect the existence of two distinct architectural syndromes

The architectural design of animal structures forms part of an individual's extended phenotype that can be subjected to strong selection pressures. We examined cocoon architectural dimorphism in robin moths (Hyalophora cecropia), which construct multilayered silk-woven cocoons that possess either a 'baggy' or 'compact' morphology. These dimorphic cocoons reflect extended phenotypes that can enable survival during a critical developmental period (pupal stage to adult emergence), with cocoons occurring either sympatrically or as monomorphic groups across different climatic regions in North America. We hypothesized that cocoon dimorphism is related to the cocoon's role as a mediating barrier for moisture. We predicted that the macro- and micro-architectural differences between the cocoon morphs would be consistent with this function. We compared the cocoon morphs in terms of their orientation when spun under natural field conditions, examined how these orientations affected cocoon water absorption under simulated rain trials, and performed material surface tests to compare the hydrophobicity of cocoons. We found that compact cocoons had traits that increased water resistance, as they had significantly greater hydrophobicity than baggy cocoons, because they absorbed less water and released water vapor faster. In contrast, the increased water absorptiveness of baggy cocoons can allow for greater moisture retention, a function related to the prevention of desiccation. Our study suggests that cocoon dimorphism in robin moths reflects distinct architectural syndromes, in which cocoons are spun to optimize either water resistance or retention. These different functions are consistent with strategies that act to respond to uncertain external environmental conditions that an individual might encounter during development.

Wing wettability gradient in a damselfly Lestes sponsa (Odonata: Lestidae) reflects the submergence behaviour during underwater oviposition

The phenomenon of hydrophobicity of insect cuticles has received great attention from technical fields due to its wide applicability to industry or medicine. However, in an ecological/evolutionary context such studies remain scarce. We measured spatial differences in wing wettability in Lestes sponsa (Odonata: Lestidae), a damselfly species that can submerge during oviposition, and discussed the possible functional significance. Using dynamic contact angle (CA) measurements together with scanning electron microscopy (SEM), we investigated differences in wettability among distal, middle and proximal wing regions, and in surface nanostructures potentially responsible for observed differences. As we moved from distal towards more proximal parts, mean values of advancing and receding CAs gradually increased from 104° to 149°, and from 67° to 123°, respectively, indicating that wing tips were significantly less hydrophobic than more proximal parts. Moreover, values of CA hysteresis for the respective wing parts decreased from 38° to 26°, suggesting greater instability of the structure of the wing tips. Accordingly, compared with more proximal parts, SEM revealed higher damage of the wax nanostructures at the distal region. The observed wettability gradient is well explained by the submergence behaviour of L. sponsa during underwater oviposition. Our study thus proposed the existence of species-dependent hydrophobicity gradient on odonate wings caused by different ovipositional strategies.

A laboratory-based study examining the properties of silk fabric to evaluate its potential as a protective barrier for personal protective equipment and as a functional material for face coverings during the COVID-19 pandemic

The worldwide shortage of single-use N95 respirators and surgical masks due to the COVID-19 pandemic has forced many health care personnel to use their existing equipment for as long as possible. In many cases, workers cover respirators with available masks in an attempt to extend their effectiveness against the virus. Due to low mask supplies, many people instead are using face coverings improvised from common fabrics. Our goal was to determine what fabrics would be most effective in both practices. Under laboratory conditions, we examined the hydrophobicity of fabrics (cotton, polyester, silk), as measured by their resistance to the penetration of small and aerosolized water droplets, an important transmission avenue for the virus causing COVID-19. We also examined the breathability of these fabrics and their ability to maintain hydrophobicity despite undergoing repeated cleaning. Laboratory-based tests were conducted when fabrics were fashioned as an overlaying barrier for respirators and when constructed as face coverings. When used as material in these two situations, silk was more effective at impeding the penetration and absorption of droplets due to its greater hydrophobicity relative to other tested fabrics. We found that silk face coverings repelled droplets in spray tests as well as disposable single-use surgical masks, and silk face coverings have the added advantage over masks such that they can be sterilized for immediate reuse. We show that silk is a hydrophobic barrier to droplets, can be more breathable than other fabrics that trap humidity, and are re-useable via cleaning. We suggest that silk can serve as an effective material for making hydrophobic barriers that protect respirators, and silk can now be tested under clinical conditions to verify its efficacy for this function. Although respirators are still the most appropriate form of protection, silk face coverings possess properties that make them capable of repelling droplets.

Circumventing surface tension: tadpoles suck bubbles to breathe air

The surface tension of water provides a thin, elastic membrane upon which many tiny animals are adapted to live and move. We show that it may be equally important to the minute animals living beneath it by examining air-breathing mechanics in five species (three families) of anuran (frog) tadpoles. Air-breathing is essential for survival and development in most tadpoles, yet we found that all tadpoles at small body sizes were unable to break through the water's surface to access air. Nevertheless, by 3 days post-hatch and only 3 mm body length, all began to breathe air and fill the lungs. High-speed macrovideography revealed that surface tension was circumvented by a novel behaviour we call 'bubble-sucking': mouth attachment to the water's undersurface, the surface drawn into the mouth by suction, a bubble 'pinched off' within the mouth, then compressed and forced into the lungs. Growing tadpoles transitioned to air-breathing via typical surface breaching. Salamander larvae and pulmonate snails were also discovered to 'bubble-suck', and two insects used other means of circumvention, suggesting that surface tension may have a broader impact on animal phenotypes than hitherto appreciated.

Tunable Spreading and Shrinking on Photocontrolled Liquid Substrate

Droplets of n-hexadecane were observed to shrink under ultraviolet (365 nm) and spread under blue light (475 nm) irradiation on an aqueous solution of photosensitive surfactant AzoTAB. We demonstrate that the change of wettability of n-hexadecane droplet on the solution depends on the change of oil-water interface tension. According to the addition of ethanol into the substrate, the change of relative diameter ΔD/D exceeds 20%, much larger than the system without ethanol. With light-emitting diode (LED) light as a sole power source, without any other triggers, we provide a contactless and isothermal method to realize photocontrolled alternative spreading and shrinking of a droplet on a liquid surface, which provides a basis for a chromocapillary-based optical zoom liquid lens.

Micro- and nanoscopic observations of sexual dimorphisms in Mecynorhina polyphemus confluens (Kraatz, 1890) (Coleoptera, Cetoniidae, Goliathini) and consequences for surface wettability

In the animal kingdom, macroscopic variations in size, color, and even hairiness are frequently observed between male and female, making the sex of various species easy to discern. In the case of insects, similar variances also exist. While direct observation is a quick and efficient way to differentiate between sexes, there are also variations which are unseen to the naked eye and occur on a micro- or nanoscopic scale. Sometimes, these micro/nanoscopic variations can lead to significant variations in surface properties as a function of sex. Such is the case for the Mecynorhina polyphemus confluens (Kraatz, 1890). In this work, we characterize these micro- and nanoscale differences, and describe their impact on the surface properties (e.g. wettability). It is found that water interacts quite differently with the surface of the cuticle of Mecynorhina polyphenus confluens, depending on the specimen sex. On a female, water spreads readily across the elytra indicating hydrophilic behavior. However, on the surface of the male elytra, strong hydrophobicity is observed. Microscopic observations reveal differences in microscale surface morphology across the male and female cuticle. These observations contribute to a better, global understanding of the wettability behavior observed on M. polyphemus confluens.

Variation of Goliathus orientalis (Moser, 1909) Elytra Nanostructurations and Their Impact on Wettability

Among the different species of flower beetles, there is one of particular notoriety: the Goliath beetle. This large insect can grow up to 11 cm long and is well-known for its distinctive black and white shield. In this paper, we focus on a particular Goliathus species: G. orientalis (Moser, 1909). We investigated the variations in properties of both the black and white parts of the upper face of G. orientalis; more precisely, the variation in surface properties with respect to the wettability of these two parts. This work reveals that the white parts of the shield have a higher hydrophobic character when compared to the black regions. While the black parts are slightly hydrophobic (θ = 91 ± 5°) and relatively smooth, the white parts are highly hydrophobic (θ = 130 ± 3°) with strong water adhesion (parahydrophobic); similar to the behavior observed for rose petals. Roughness and morphology analyses revealed significant differences between the two parts, and, hence, may explain the change in wettability. The white surfaces are covered with horizontally aligned nanohairs. Interestingly, vertically aligned microhairs are also present on the white surface. Furthermore, the surfaces of the microhairs are not smooth, they contain nanogrooves that are qualitatively similar to those observed in cactus spines. The nanogrooves may have an extremely important function regarding water harvesting, as they preferentially direct the migration of water droplets; this process could be mimicked in the future to capture and guide a large volume of water.

The influence of the topography and physico-chemical properties of the cuticle surface on the wettability and adhesive properties of the elytra of the dung beetle Geotrupes stercorarius (Coleoptera, Scarabaeidae)

The elytral surface of dung beetles is generally accepted to be self-cleaning due to its anti-adhesive properties. In this article, the wettability and adhesive properties of elytral surface (intact and treated with Acetone and Ethanol) of the beetle Geotrupes stercorarius were characterized. Since these properties are influenced by the surface structure and chemistry, the micro- and nanostructure of the elytra were observed using scanning electron microscopy and the surface roughness were estimated with white light interferometery, whereas the water contact angle and adhesion force of the elytra were evaluated using contact angle measurement device and force transducer, respectively. The results show that the microstructure of the elytra changed greatly after different chemical treatments and the changes in wettability were mainly related to the changes of elytra roughness after treatments. The differences in adhesion force between differently treated elytra and wet sand indicate that the molecular polarity of the elytral surface is decisive for its anti-adhesive (self-cleaning) property.