Micronanostructures of the scales on a mosquito's legs and their role in weight support

Harnessing Smectic Ordering for Electric-Field-Driven Guided-Growth of Surface Topography in a Liquid Crystal Polymer

The guided-growth strategy has been widely explored and proved its efficacy in fabricating surface micro/nanostructures in a variety of systems. However, soft materials like polymers are much less investigated partly due to the lack of strong internal driving mechanisms. Herein, the possibility of utilizing liquid crystal (LC) ordering of smectic liquid crystal polymers (LCPs) to induce guided growth of surface topography during the formation of electrohydrodynamic (EHD) patterns is demonstrated. In a two-stage growth, regular stripes are first found to selectively emerge from the homogeneously aligned region of an initially flat LCP film, and then extend neatly along the normal direction of the boundary line between homogeneous and homeotropic alignments. The stripes can maintain their directions for quite a distance before deviating. Coupled with the advanced tools for controlling LC alignment, intricate surface topographies can be produced in LCP films starting from relatively simple designs. The regularity of grown pattern is determined by the LC ordering of the polymer material, and influenced by conditions of EHD growth. The proposed approach provides new opportunities to employ LCPs in optical and electrical applications.

The origin of black and white coloration of the Asian tiger mosquito Aedes albopictus (Diptera: Culicidae)

Micro- and nanostructures of the white and black scales on the tarsi of the mosquito Aedes albopictus are analysed using scanning electron microscopy, transmission electron microscopy, and fluorescence microscopy. Reflectance spectra of the white areas are measured. No clear difference is present in the morphology of micro- and nanostructures of black and white scales in SEM and TEM, but black scales contain a dark pigment. The white colour of the scales has a structural origin. The structural white produced by the micro- and nanostructures of the scales on the tarsi of Ae. albopictus appears bright and is angle-dependent, since the reflected light changes according to the angle detection and according to the tarsus orientation. The optical appearance of the scale system of Ae. albopictus has a complex nature and can be explained by the combination of several effects. Among them, multiple refraction and reflection on the micro- and nanostructures of the scales are mainly responsible for the white appearance. The results suggest that mosquito scales, in addition to their superhydrophobic function, produce structural white. The biological role of white and black patches in mate recognition and defensive behaviour in the mosquitoes of the genus Aedes is hypothesized.

Complete Inhibition of Vertical Mosquito Landing through Topographical Surface Design

Mosquito-borne diseases such as dengue, chikungunya, and malaria have long been a health and economic burden in our society. Such illnesses develop after the pathogen, here arboviruses, are transmitted to humans by female mosquitoes during blood meals. In the case of dengue and chikungunya, such pathogens are transmitted to humans by infected Aedes aegypti females. Prior to feeding the insects rest on vertical surfaces. In this work, a surface roughness threshold was observed for live Aedes aegypti colonies, and below a root-mean-squared roughness of S_q < 0.124 μm the mosquitoes were physically incapable of gripping vertical substrates. This roughness threshold was unaffected by surface wettability or relative humidity. The importance of topographical feature height was understood using a claw-hooking model considering friction. Local defects above this threshold allowed claw hooking to take place, emphasizing the importance of surface uniformity. An antimosquito coating was developed that reduced surface roughness below this threshold when it was applied to realistic surfaces such as wood, brick, wall laminates, and tile. Lowering the surface roughness below the threshold reduced the number of mosquitoes capable of landing on the surfaces by 100%: i.e., no mosquitoes were able to adhere to the treated surfaces. The ability to completely inhibit Aedes aegypti females from landing on surfaces represents a new vector-borne disease control strategy that does not suffer from the nontarget toxicity, resistance, or ecosystem disruption associated with conventional chemical control strategies.

Scale-type-specific requirement for the mosquito Aedes aegypti Spindle-F homologue by regulating microtubule organization

Insect epithelial cells contain unique cellular extensions such as bristles, hairs, and scales. In contrast to bristle and hair, which are not divergent in their shape, scale morphology shows high diversity. In our attempt to characterize the role of the insect-specific gene, Spindle-F (spn-F), in mosquito development, we revealed a scale-type specific requirement for the mosquito Aedes aegypti spn-F homologue. Using CRISPR-Cas9, we generated Ae-spn-F mutants and found that Ae-spn-F is an essential gene, but we were able to recover a few adult escapers. These escapers could not fly nor move, and died after 3 to 4 days. We found that in Ae-spn-F mutants, only the tip part of the bristle was affected with bulbous with misoriented ribs. We also show that in Ae-spn-F mutants, only in falcate scales, which are curved with a sharp or narrowly rounded apex, and not in other scale types, the tip region is strongly affected. Our analysis also revealed that in contrast to Drosophila spn-F, which show strong defects in both the actin and microtubule (MT) network in the bristle, the Ae-spn-F gene is required only for MT organization in scales and bristles. In summary, our results reveal that Ae-spn-F is required for shaping tapered epithelial cellular extension structures, namely, the bristle and falcate scales by affecting MT organization.

A Three-Pronged Approach to Studying Sublethal Insecticide Doses: Characterising Mosquito Fitness, Mosquito Biting Behaviour, and Human/Environmental Health Risks

Worldwide, pyrethroids are one of the most widely used insecticide classes. In addition to serving as personal protection products, they are also a key line of defence in integrated vector management programmes. Many studies have assessed the effects of sublethal pyrethroid doses on mosquito fitness and behaviour. However, much remains unknown about the biological, physiological, demographic, and behavioural effects on individual mosquitoes or mosquito populations when exposure occurs via spatial treatments. Here, females and males of two laboratory-reared mosquito species, Culex pipiens and Aedes albopictus, were exposed to five different treatments: three doses of the pyrethroid prallethrin, as well as an untreated and a negative control. The effects of each treatment on mosquito species, sex, adult mortality, fertility, F1 population size, and biting behaviour were also evaluated. To compare knockdown and mortality among treatments, Mantel-Cox log-rank tests were used. The results showed that sublethal doses reduced mosquito survival, influencing population size in the next generation. They also provided 100% protection to human hosts and presented relatively low risks to human and environmental health. These findings emphasise the need for additional studies that assess the benefits of using sublethal doses as part of mosquito management strategies.

Actin bundles play a different role in shaping scales compared to bristles in the mosquito Aedes aegypti

Insect epithelial cells contain cellular extensions such as bristles, hairs, and scales. These cellular extensions are homologous structures that differ in morphology and function. They contain actin bundles that dictate their cellular morphology. While the organization, function, and identity of the major actin-bundling proteins in bristles and hairs are known, this information on scales is unknown. In this study, we characterized the development of scales and the role of actin bundles in the mosquito, Aedes aegypti. We show that scales undergo drastic morphological changes during development, from a cylindrical to flat shape with longer membrane invagination. Scale actin-bundle distribution changes from the symmetrical organization of actin bundles located throughout the bristle membrane to an asymmetrical organization. By chemically inhibiting actin polymerization and by knocking out the forked gene in the mosquito (Ae-Forked; a known actin-bundling protein) by CRISPR-Cas9 gene editing, we showed that actin bundles are required for shaping bristle, hair, and scale morphology. We demonstrated that actin bundles and Ae-Forked are required for bristle elongation, but not for that of scales. In scales, actin bundles are required for width formation. In summary, our results reveal, for the first time, the developmental process of mosquito scale formation and also the role of actin bundles and actin-bundle proteins in scale morphogenesis. Moreover, our results reveal that although scale and bristle are thought to be homologous structures, actin bundles have a differential requirement in shaping mosquito scales compared to bristles.

Mosquito repellence induced by tarsal contact with hydrophobic liquids

Mosquito legs have a unique highly water-repellent surface structure. While being beneficial to mosquitoes, the water-repellence of the tarsi enhances the wettability of hydrophobic substances such as oils. This high wettability induces strong attraction forces on a mosquito’s legs (up to 87% of the mosquito’s weight) towards the oil. We studied the landing behaviour of mosquitoes on oil-coated surfaces and observed that the mosquito contact time was reduced compared to that on hydrophilic-liquid-coated surfaces, suggesting that the oil coating induces an escape response. The observed escape behaviour occurred consistently with several hydrophobic liquids, including silicone oil, which is used globally in personal care products. As the repellent effect is similar to multiple hydrophobic substances, it is likely to be mechanically stimulated owing to the physical properties of the hydrophobic liquids and not due to chemical interactions. On human skin, the contact time was sufficiently short to prevent mosquitoes from starting to blood-feed. The secretion of Hippopotamus amphibius, which has physical properties similar to those of low-viscosity silicone oil, also triggered an escape response, suggesting that it acts as a natural mosquito repellent. Our results are beneficial to develop new, safe, and effective mosquito-repellent technologies.

Surface movement mechanism of abalone and underwater adsorbability of its abdominal foot

The abalone, a marine mollusk, inhabits fast-flowing rocky reefs. Its predation and crawling abilities strongly depend on the adhesion capacity of its abdominal foot. Here, the macroscopic and microscopic morphology of the abalone foot was observed. The foot is divided into four main regions, each having a large number of folds on the surface. The extensional ability of the folds is the source of the abalone’s locomotory power. A high-speed camera was used to photograph the movement of an abalone. It was found that two to three deep-yellow round fold areas appear and disappear periodically on the surface of the foot. The folds move forward with the movement of the abalone. This results in a crawling motion that ensures efficiency of movement and adaptability of the wide abdominal foot to adsorptive surfaces. According to underwater tensile testing of an abalone, the adhesion force of the foot is composed of suction force, capillary force, friction and an interlocking structure. Among them, suction force is the most important component of the adhesion force, and the other factors play an auxiliary and reinforcing role. The strong adhesivity and adhesion-based crawling motion of the abalone may inspire new design ideas and mechanisms for underwater suckers.

Mosquitoes cloak their legs to resist insecticides

Malaria incidence has halved since the year 2000, with 80% of the reduction attributable to the use of insecticides. However, insecticide resistance is now widespread, is rapidly increasing in spectrum and intensity across Africa, and may be contributing to the increase of malaria incidence in 2018. The role of detoxification enzymes and target site mutations has been documented in the major malaria vector Anopheles gambiae; however, the emergence of striking resistant phenotypes suggests the occurrence of additional mechanisms. By comparing legs, the most relevant insect tissue for insecticide uptake, we show that resistant mosquitoes largely remodel their leg cuticles via enhanced deposition of cuticular proteins and chitin, corroborating a leg-thickening phenotype. Moreover, we show that resistant female mosquitoes seal their leg cuticles with higher total and different relative amounts of cuticular hydrocarbons, compared with susceptible ones. The structural and functional alterations in Anopheles female mosquito legs are associated with a reduced uptake of insecticides, substantially contributing to the resistance phenotype.

Capillarity-driven migration of small objects: A critical review

The phenomena on the capillarity-driven migration of small objects are full of interest for both scientific and engineering communities, and a critical review is thereby presented. The small objects mentioned here deal with the non-deformable objects, such as particles, rods, disks and metal sheets; and besides them, the soft objects are considered, such as droplets and bubbles. Two types of interfaces are analyzed, i.e., the solid-fluid interface and the fluid-fluid interface. Due to the easily deformable properties of the soft objects and distorted interfacial shapes induced by small objects, a more convenient way to obtain the driving force is through the potential energy of the system. The asymmetric factors causing the object migration include the asymmetric configuration of the interface, and the difference between the interfacial tensions. Finally, a simple outlook on the potential applications of small object migration is made. These behaviors may cast new light on the design of microfluidics and new devices, environment cleaning, oil and gas displacement and mineral industries.

Mosquitoes modulate leg dynamics at takeoff to accommodate surface roughness

Insects perform takeoffs from a nearly unquantifiable number of surface permutations and many use their legs to initiate upward movement prior to the onset of wingbeats, including the mosquito. In this study we examine the unprovoked pre-takeoff mechanics of Aedes aegypti mosquitoes from two surfaces of contrasting roughness, one with roughness similar to polished glass and the other comparable to the human forearm. Using high-speed videography, we find mosquitos exhibit two distinct leg actions prior to takeoff, the widely observed push and a previously undocumented leg-strike, where one of the rearmost legs is raised and strikes the ground. Across 106 takeoff sequences we observe a greater incidence of leg-strikes from the smoother surface, and rationalize this observation by comparing the characteristic size of surface features on the mosquito tarsi and each test surface. Measurements of pre-takeoff kinematics reveal both strategies remain under the mechanosensory detection threshold of mammalian hair and produce nearly identical vertical body velocities. Lastly, we develop a model that explicates the measured leg velocity of striking legs utilized by mosquitoes, 0.59 m s^-1.

Hard to be killed: Load-bearing capacity of the leech Hirudo nipponia

With the evolution for several millions of years, leeches have developed a perfect capability to resist mechanical loads, which provides many inspirations to engineer new materials and new devices. To uncover the mechanism of its strong survival ability, several mechanical approaches, such as compression, tension, adhesion, impact and blood suction experiments were tried. Our experimental results show that a leech (Hirudo nipponia) can surprisingly withstand a compressive force of nearly 10⁶ times its body weight. In tension, this animal demonstrates large deformation and its strain can reach a value bigger than 3. To avoid being removed from the host skin, it produces an adhesion force superior to 118 times its body weight, and it can endure an impact force at least 1500 times its weight. Also the leech skin can bear an internal fluid pressure of around 6 times the atmospheric pressure. These data show that the leech cannot be killed easily through normal mechanical loading approaches. All these amazing performances lie in hierarchical structures and ductility of the skin with highly developed and compact annuluses, and this feature is beneficial to leech's survival.

A Triassic-Jurassic window into the evolution of Lepidoptera

On the basis of an assemblage of fossilized wing scales recovered from latest Triassic and earliest Jurassic sediments from northern Germany, we provide the earliest evidence for Lepidoptera (moths and butterflies). The diverse scales confirm a (Late) Triassic radiation of lepidopteran lineages, including the divergence of the Glossata, the clade that comprises the vast multitude of extant moths and butterflies that have a sucking proboscis. The microfossils extend the minimum calibrated age of glossatan moths by ca. 70 million years, refuting ancestral association of the group with flowering plants. Development of the proboscis may be regarded as an adaptive innovation to sucking free liquids for maintaining the insect's water balance under arid conditions. Pollination drops secreted by a variety of Mesozoic gymnosperms may have been non-mutualistically exploited as a high-energy liquid source. The early evolution of the Lepidoptera was probably not severely interrupted by the end-Triassic biotic crisis.

Chemical mediated elasto-capillarity of elastic sheets

Capillary forces can cause outstanding deformation of slender structures, and this behavior holds great potential in a plethora of areas. In this study, we propose a facile strategy to mediate the deformation of elastic structures via chemicals, which is named chemo-elasto-capillarity. The experiment shows that the added surfactant can significantly change the interfacial energy of the system, and then modulate the configuration of the adhered structures. The wetted length and deflection of each sheet can be predicted using the large and infinitesimal deformation theory of beams, and these theoretical values are in excellent agreement with the experimental results. Moreover, the proposed method can be successfully extended to unfold two adhered sheets, and one racket like sheet. The present scheme is accessible to accurately regulate elasto-capillarity, and provide some inspirations for engineering some chemical-sensitive devices and humidity-stimulated actuators.

A Rapid One-Step Process for Fabrication of Biomimetic Superhydrophobic Surfaces by Pulse Electrodeposition

Inspired by some typical plants such as lotus leaves, superhydrophobic surfaces are commonly prepared by a combination of low surface energy materials and hierarchical micro/nano structures. In this work, superhydrophobic surfaces on copper substrates were prepared by a rapid, facile one-step pulse electrodepositing process, with different duty ratios in an electrolyte containing lanthanum chloride (LaCl₃·6H₂O), myristic acid (CH₃(CH₂)₁₂COOH), and ethanol. The equivalent electrolytic time was only 10 min. The surface morphology, chemical composition and superhydrophobic property of the pulse electrodeposited surfaces were fully investigated with SEM, EDX, XRD, contact angle meter and time-lapse photographs of water droplets bouncing method. The results show that the as-prepared surfaces have micro/nano dual scale structures mainly consisting of La[CH₃(CH₂)₁₂COO]₃ crystals. The maximum water contact angle (WCA) is about 160.9°, and the corresponding sliding angle is about 5°. This method is time-saving and can be easily extended to other conductive materials, having a great potential for future applications.

Enhanced Adhesion of Mosquitoes to Rough Surfaces

Insects and small animals capable of adhering reversibly to a variety of surfaces employ the unique design of the distal part of their legs. In the case of mosquitoes, their feet are composed of thousands of micro- and nanoscale protruding structures, which impart superhydrophobic properties. Previous research has shown that the superhydrophobic nature of the feet allows mosquitoes to land on water, which is necessary for their reproduction cycle. Here, we show that van der Waals interactions are the main adhesion mechanism employed by mosquitoes to adhere to various surfaces. We further demonstrate that the judicious creation of surface roughness on an opposing surface can increase the adhesion strength because of the increased number of surface elements interacting with the setae through multiple contact points. Although van der Waals forces are shown to be the predominant mechanism by which mosquitoes adhere to surfaces, capillary forces can also contribute to the total adhesion force when the opposing surface is hydrophilic and under humid conditions. These fundamental properties can potentially be applied in the development of superior Long Lasting Insecticidal Nets (LLINs), which represent one of the most effective methods to mitigate mosquito-transmitted infectious diseases such as Malaria, Filaria, Zika, and Dengue.

Effective Elastic Modulus of Structured Adhesives: From Biology to Biomimetics

Micro- and nano-hierarchical structures (lamellae, setae, branches, and spatulae) on the toe pads of many animals play key roles for generating strong but reversible adhesion for locomotion. The hierarchical structure possesses significantly reduced, effective elastic modulus (Eeff), as compared to the inherent elastic modulus (Einh) of the corresponding biological material (and therefore contributes to a better compliance with the counterpart surface). Learning from nature, three types of hierarchical structures (namely self-similar pillar structure, lamella⁻pillar hybrid structure, and porous structure) have been developed and investigated.

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

The surface of the elytra in some species of aquatic beetles displays relatively low contact angles (CAs), even showing hydrophilic properties. In this study, we report on an observation that both sexes of Cybister chinensis beetle fresh elytral surface do not exhibit uniform CA, but rather a wettability gradient along the longitudinal axis in posterior direction. The wettability is very different between females and males due to the presence (female) or absence (male) of channels on the elytral surface. When a small drop of water touches the elytra surface, it tends to slide towards the anterior having a lower CA on the elytra. This gradient presumably supports a breathing-associated behavior of beetles in which they cause the tip of their abdomen to protrude into the surface of the water in order to collect an air bubble for oxygen uptake and, when floating on the surface, to keep the body inclined at a small angle to the water's surface with their heads immersed.

STATEMENT OF SIGNIFICANCE

Hydrophobicity on surfaces is a fundamental property which has attracted great interest across all scientific disciplines, here we have demonstrated that the gradually changing chemistry of the elytral surface facilitates the tilted beetle posture on the water's surface. The mechanism of water interacting with the elytra demonstrated the most energetically favorable posture in the diving beetles. Surfaces with directional wetting properties that promote droplet drainage are of significant practical importance in many fields. The anisotropic topography and wetting properties of the elytra may inspire microfluidic devices for medical and robotic applications.

Functional map of biological and biomimetic materials with hierarchical surface structures

The properties and functions of both biological and artificial materials with hierarchical surface structures are reviewed to establish the functional map of various hierarchical surface structures.

Bioinspired aquatic microrobot capable of walking on water surface like a water strider

Walking on the water surface is a dream of humans, but it is exactly the way of life for some aquatic insects. In this study, a bionic aquatic microrobot capable of walking on the water surface like a water strider was reported. The novel water strider-like robot consisted of ten superhydrophobic supporting legs, two miniature dc motors, and two actuating legs. The microrobot could not only stand effortlessly but also walk and turn freely on the water surface, exhibiting an interesting motion characteristic. A numerical model describing the interface between the partially submerged leg and the air-water surface was established to fully understand the mechanism for the large supporting force of the leg. It was revealed that the radius and water contact angle of the legs significantly affect the supporting force. Because of its high speed, agility, low cost, and easy fabrication, this microrobot might have a potential application in water quality surveillance, water pollution monitoring, and so on.

A new wetting mechanism based upon triple contact line pinning

The classical Wenzel and Cassie models fail to give a physical explanation of such phenomenon as the macroscopic contact angle actually being equal to the Young's contact angle if there is a spot (surface defect) inside the droplet. Here, we derive the expression of the macroscopic contact angle for this special substrate in use of the principle of least potential energy, and our analytical results are in good agreement with the experimental data. Our findings also suggest that it is the triple contact line (TCL) rather than the contact area that dominates the contact angle. Therefore a new model based upon the TCL pinning is developed to explain the different wetting properties of the Wenzel and Cassie models for hydrophilic and hydrophobic cases. Moreover, the new model predicts the macroscopic contact angle in a broader range accurately, which is consistent with the existing experimental findings. This study revisits the fundamentals of wetting on rough substrates. The new model derived will help to design better superhydrophobic materials and provide the prediction required to engineer novel microfluidic devices.

Mosquito proboscis: an elegant biomicroelectromechanical system

The mouthparts of female mosquitoes have evolved to form a special proboscis, a natural biomicroelectromechanical system (BMEMS), which is used for painlessly penetrating human skin and sucking blood. Scanning electron microscope observations show that the mosquito proboscis consists of a small bundle of long, tapering, and feeding stylets that are collectively called the fascicle, and a large scaly outer lower lip called the labium. During blood feeding, only the fascicle penetrates into the skin while the labium buckles back to remain on the surface of the skin. Here, we measured the dynamic force of penetration of the fascicle into human skin to reveal the mechanical principle underlying the painless process of penetration. High-speed video observations of movements associated with insertion of the fascicle indicate that the "smart" mosquito does not directly pierce its victim's skin with the fascicle. Instead, it uses the two maxillas as variable frequency microsaws with nanosharp teeth to advance into the skin tissue. This elegant BMEMS enables the mosquito to insert its feeding fascicle into human skin using an exceedingly small force (average of 16.5 μN).

Surface patterning of soft polymer film-coated cylinders via an electric field

Using the linear stability analysis method, we investigate the surface wrinkling of a thin polymer coating on a cylinder in an externally applied electric field. It is demonstrated that energy competition between surface energy, van der Waals interactive potential energy and electrostatic interaction energy may lead to ordered patterns on the film surface. The analytical solutions are derived for the critical conditions of both longitudinal and circumferential instabilities. The wavelengths of the generated surface patterns can be mediated by changing the magnitude of the electric field. Our analysis shows that the surface morphology is sensitive to the curvature radius of the fiber, especially in the micrometer and nanometer length scales. Furthermore, we suggest a potential approach for fabricating hierarchical patterns on curved surfaces.

Functional demands of dynamic biological adhesion: an integrative approach

Climbing organisms are constantly challenged to make their way rapidly and reliably across varied and often novel terrain. A diversity of morphologically and mechanically disparate attachment strategies have evolved across widely distributed phylogenetic groups to aid legged animals in scaling these surfaces, notable among them some very impressive adhesive pads. Despite the differences between, for example, the dry fibrillar pads of geckos and the smooth, secretion-aided pads of stick insects, I hypothesize that they face similar functional demands in their environment. I outline three broad criteria defining dynamic biological adhesion: reusability, reversibility, and substrate tolerance. Organismal adhesive pads must be able to attach repeatedly without significant decline in performance, detach easily at will, and adhere strongly to the broadest possible range of surfaces in their habitat. A survey of the literature suggests that evidence for these general principles can be found in existing research, but that many gaps remain to be filled. By taking a comparative, integrative approach to biological dynamic adhesion, rather than focusing on a few model organisms, investigators will continue to discover new and interesting attachment strategies in natural systems.

Floating objects with finite resistance to bending

We consider the equilibrium flotation of a thin, flexible cylinder at the interface between a liquid and a gas. In particular, we determine the maximum load that such a cylinder can support without sinking. We find that as the length of such a cylinder increases the maximum load at first increases. However, the maximum load reaches a plateau when the length of the cylinder is comparable to the elastocapillary length, which is determined by a balance between the bending of the cylinder and surface tension. We then consider the implications of our analysis for the walking on water of both arthropods and man-made robots. In particular, we show that the legs of water striders are typically slightly shorter than this 'optimal' length, suggesting that elastocapillary effects may act as a selection pressure.

Buoyant force and sinking conditions of a hydrophobic thin rod floating on water

Owing to the superhydrophobicity of their legs, such creatures as water striders and fisher spiders can stand effortlessly, walk and jump quickly on water. Directed toward understanding their superior repellency ability, we consider hydrophobic thin rods of several representative cross sections pressing a water surface. First, the shape function of the meniscus surrounding a circular rod is solved analytically, and thereby the maximal buoyant force is derived as a function of the Young's contact angle and the rod radius. Then we discuss the critical conditions for a rod to sink into water, including the maximal volume condition and the meniscus-contact condition. Furthermore, we study the sinking conditions and the maximal buoyant forces of hydrophobic long rods with elliptical, triangular, or hexagonal cross-section shapes. The theoretical solutions are quantitatively consistent with existing experimental and numerical results. Finally, the optimized structures of water strider legs are analyzed to elucidate why they can achieve a very big buoyant force on water.