Chemical composition of the attachment pad secretion of the locust Locusta migratoria

Adhesive and frictional properties of tarsal attachment pads in two species of stick insects (Phasmatodea) with smooth and nubby euplantulae

In the present study, the tarsal attachment pads (euplantulae) of two stick insect species (Phasmatodea) were compared. While the euplantulae of Cuniculina impigra (syn. Medauroidea extradentata) are smooth, those of Carausius morosus bear small nubs on their surfaces. In order to characterize the adhesive and frictional properties of both types of euplantulae, adhesion and friction measurements on smooth (Ra=0.054 μm) and rough (Ra=1.399 μm) substrates were carried out. The smooth pads of C. impigra generated stronger adhesion on the smooth substrate than on the rough one. The adhesive forces of the structured pads of C. morosus did not differ between the two substrates. Friction experiments showed anisotropy for both species with higher values for proximal pulls than for distal pushes. In C. impigra, friction was stronger on the smooth than on the rough surface for both directions, whereas in C. morosus friction was stronger on the smooth surface only for pushes. This shows that smooth attachment pads are able to generate relatively stronger adhesion and friction on a flat smooth surface than on a rough one. In contrast, nubby pads have similar adhesion on both substrates, and also show no difference in friction in the pulling direction. This leads to the conclusion that smooth pads are specialized for rather smooth substrates, whereas nubby pads are better adapted to generate stronger forces on a broader range of surfaces.

Evaporation dynamics of tarsal liquid footprints in flies (Calliphora vicina)and beetles (Coccinella septempunctata)

Insect tarsal adhesive structures secrete a thin layer of fluid into the contact area. It was previously reported that the presence of this fluid significantly increases adhesion on various substrata. Previous data obtained from representatives of different insect groups suggest a difference not only in the chemical composition of the fluid, but also in its physical properties. In the present study, we have measured for the first time changes in the droplet geometry over time and the evaporation rate of the fluid in flies (Calliphora vicina) and beetles (Coccinella septempunctata) by the use of atomic force microscopy. Flattened droplets of the beetle had lower evaporation rates than hemispherical footprints of the fly. Within 1 h, the droplet volume reduced to 21% of the initial volume for the fly, and to 65% for the beetle, suggesting a larger fraction of volatile compounds in the fly fluid. It was revealed that drop geometry changes significantly during evaporation and shows pinning effects for the fly footprints due to an assumed self-organizing oil layer on top of the water fraction of the micro-emulsion. The data obtained suggest that the adhesion strength in capillarity-based switchable adhesive systems must be time-dependent because of the specific evaporation rate of the adhesive fluid. These results are important for our understanding of the functional mechanism of insect adhesive systems and also for biomimetics of artificial capillarity-based adhesive systems.

Functional morphology and adhesive performance of the stick-capture apparatus of the rove beetles Stenus spp. (Coleoptera, Staphylinidae)

The adhesive prey-capture apparatus of the representatives of the rove beetle genus Stenus (Coleoptera, Staphylinidae) is an outstanding example of biological adhesive systems. This unique prey-capture device is used for catching elusive prey by combining (i) hierarchically structured adhesive outgrowths, (ii) an adhesive secretion, and (iii) a network of cuticular fibres within the pad. The outgrowths arise from a pad-like cuticle and are completely immersed within the secretion. To date, the forces generated during the predatory strike of these beetles have only been estimated theoretically. In the present study, we used force transducers to measure both the compressive and adhesive forces during the predatory strike of two Stenus species. The experiments revealed that the compressive forces are low, ranging from 0.10 mN (Stenus bimaculatus) to 0.18 mN (Stenus juno), whereas the corresponding adhesive forces attain up to 1.0 mN in S. juno and 1.08 mN in S. bimaculatus. The tenacity or adhesive strength (adhesive force per apparent unit area) amounts to 51.9 kPa (S. bimaculatus) and 69.7 kPa (S. juno). S. juno beetles possess significantly smaller pad surface areas than S. bimaculatus but seem to compensate for this disadvantage by generating higher compressive forces. Consequently, S. juno beetles reach almost identical adhesive properties and an equal prey-capture success in attacks on larger prey. The possible functions of the various parts of the adhesive system during the adhesive prey-capture process are discussed in detail.

Attachment ability of sawfly larvae to smooth surfaces

Larvae of the sawfly Rhadinoceraea micans adhere properly to the anti-adhesive surface of their host plant Iris pseudacorus by using three pairs of thoracic legs, seven pairs of abdominal prolegs, and pygopodia, all provided with various smooth adhesive pads. Their attachment performance to smooth flat hydrophilic and hydrophobic glass and Plexiglas surfaces was studied in centrifugal force experiments. Obtained safety factors on Plexiglas were up to 25 in friction, and 8 in adhesion. Although larvae attached significantly stronger to the hydrophilic glass, they attached well also to the hydrophobic one. Pygopodia are suggested to dominate attachment force generation in the centrifugal force experiment. Transverse body position on the centrifuge drum was significantly advantageous for friction force generation than was longitudinal body position. Results are discussed in the context of the sawfly biology and provide a profound base for further detailed studies on biomechanics of sawfly larvae-plant interactions.

Effect of particulate contamination on adhesive ability and repellence in two species of ant (Hymenoptera; Formicidae)

Tarsal adhesive pads are crucial for the ability of insects to traverse their natural environment. Previous studies have demonstrated that for both hairy and smooth adhesive pads, significant reduction in adhesion can occur because of contamination of these pads by wax crystals present on plant surfaces or synthetic microspheres. In this paper, we focus on the smooth adhesive pads of ants and study systematically how particulate contamination and the subsequent loss of adhesion depends on particle size, particle surface energy, humidity and species size. To this end, workers of ant species Polyrhachis dives and Myrmica scabrinodis (Hymenoptera; Formicidae) were presented with loose synthetic powder barriers with a range of powder diameters (1–500 μm) and surface energies (PTFE or glass), which they would have to cross in order to escape the experimental arena. The barrier experiments were conducted for a range of humidities (10–70%). Experimental results and scanning electron microscopy confirm that particulate powders adversely affect the adhesive ability of both species of ant on smooth substrates via contamination of the arolia. Specifically, the loss of adhesion was found to depend strongly on particle diameter, but only weakly on particle type, with the greatest loss occurring for particle diameters smaller than the claw dimensions of each species, and no effect of humidity was found. We also observed that ants were repelled by the powder barriers which led to a decrease of adhesion prior to their eventual crossing, suggesting that insect antennae may play a role in probing the mechanical fragility of substrates before crossing them.

The influence of surface energy on the self-cleaning of insect adhesive devices

The ability of insects to adhere to surfaces is facilitated by the use of adhesive organs found on the terminal leg segments. These adhesive pads are inherently ‘tacky’ and are expected to be subject to contamination by particulates, leading to loss of function. Here, we investigated the self-cleaning of ants and beetles by comparing the abilities of both hairy and smooth pad forms to self-clean on both high and low energy surfaces after being fouled with microspheres of two sizes and surface energies. We focused on the time taken to regain adhesive potential in unrestrained Hymenopterans (Polyrhachis dives and Myrmica scabrinodis) and Coccinellids (Harmonia axyridis and Adalia bipunctata) fouled with microspheres. We found that the reattainment of adhesion is influenced by particle type and size in Hymenopterans, with an interaction between the surface energy of the contaminating particle and substrate. In Coccinellids, reattainment of adhesion was only influenced by particle size and substrate properties. The adhesive organs of Coccinellids appear to possess superior self-cleaning abilities compared with those of Hymenopterans, although Hymenopterans exhibit better adhesion to both surface types.

Walking on smooth and rough ground: activity and timing of the claw retractor muscle in the beetle Pachnoda marginata peregrina (Coleoptera, Scarabaeidae)

The activity pattern of the claw retractor muscle of Pachnoda marginata peregrina beetles was examined in this study. We found this muscle to be located in the tibia, without a femoral part, as is the case in other insects. Electromyograms (EMGs) of the muscle revealed a rather similar activity pattern during beetle locomotion on rough and smooth substrates. We recorded units with small and large amplitude, with the smaller one being active during almost the entire stance phase, the larger one roughly in the first half of stance. Small but significant differences were found in the precise onset and end of activity. Both small and large units began their activity earlier on the rough surface. While there was no difference at the end of activity in the small unit between both surfaces, the large unit ended its activity significantly earlier on the rough substrate. The spike frequencies on both surfaces were also significantly different for both small and large units. The small unit showed a higher spike frequency on the smooth surface, while the large unit had a higher spike frequency on the rough surface. From our experiments, we conclude that the muscle is controlled by the same basic activity pattern on different surfaces, with some adjustments due to sensory feedback. The adjustments cause differences in onset and end of activity, as well as in spike frequency of the involved muscle units.

Adhesive performance of the stick-capture apparatus of rove beetles of the genus Stenus (Coleoptera, Staphylinidae) toward various surfaces

Rove beetles of the genus Stenus possess a unique adhesive prey-capture apparatus that enables them to catch elusive prey such as springtails over a distance of several millimeters. The prey-capture device combines the hierarchically organized morphology of dry adhesive systems with the properties of wet ones, since an adhesive secretion is released into the contact zone. We hypothesize that this combination enables Stenus species successfully to capture prey possessing a wide range of surface structures and chemistries. We have investigated the influence of both surface energy and roughness of the substrate on the adhesive performance of the prey-capture apparatus in two Stenus species. Force transducers have been used to measure both the compressive and adhesive forces generated during the predatory strike of the beetles on (1) epoxy resin surfaces with defined roughness values (smooth versus rough with asperity diameters ranging from 0.3 to 12 μm) and (2) hydrophobic versus hydrophilic glass surfaces. Our experiments show that neither the surface roughness nor the surface energy significantly influences the attachment ability of the prey-capture apparatus. Thus, in contrast to the performance of locomotory adhesive systems in geckos, beetles, and flies, no critical surface roughness exists that might impede adhesion of the prey-capture apparatus of Stenus beetles. The prey-capture apparatus of Stenus beetles is therefore well adapted to adhere to the various unpredictable surfaces with diverse roughness and surface energy occurring in a wide range of potential prey.

Grip and detachment of locusts on inverted sandpaper substrates

Locusts (Locusta migratoria manilensis) are characterized by their strong flying and grasping ability. Research on the grasping mechanism and behaviour of locusts on sloping substrates plays an important role in elucidating the mechanics of hexapod locomotion. Data on the maximum angles of slope at which locusts can grasp stably (critical angles of detachment) were obtained from high-speed video recordings at 215 fps. The grasping forces were collected by using two sensors, in situations where all left legs were standing on one and the right legs on the other sensor plate. These data were used to illustrate the grasping ability of locusts on slopes with varying levels of roughness. The grasping morphologies of locusts' bodies and tarsi were observed, and the surface roughness as well as diameters of their claw tips was measured under a microscope to account for the grasping mechanism of these insects on the sloping substrate. The results showed that the claw tips and part of the pads were in contact with the inverted substrate when the mean particle diameter was in the range of 15.3-40.5 µm. The interaction between pads and substrates may improve the stability of contact, and claw tips may play a key role in keeping the attachment reliable. A model was developed to explain the significant effects of the relative size of claw tips and mean particle diameter on grasping ability as well as the observed increase in lateral force (2.09-4.05 times greater than the normal force during detachment) with increasing slope angle, which indicates that the lateral force may be extremely important in keeping the contact reliable. This research lays the groundwork for the probable design and development of biomimetic robotics.

Activity of the claw retractor muscle in stick insects in wall and ceiling situations

The activity of the middle part of the claw retractor muscle was examined in two species of stick insects (Carausius morosus and Cuniculina impigra). We performed electromyographic recordings while the animals were standing on a smooth or a rough surface of a platform in horizontal, vertical or inverted positions, as well as during rotations of the platform. We recorded tonic and phasic motor units. The tonic units were active all the time without significant differences in spike frequency, regardless of the position of the animals (although there was a tendency for higher discharge frequencies to occur during platform rotations). The phasic units were active almost exclusively during platform movement. In contrast to the tonic units, we detected significant differences in the activities of the phasic units; namely, higher spike frequencies during rotations compared with the stationary phases, especially for rotations into 'more awkward' positions. A comparison of the two species revealed no difference in muscle activity, despite differences in the animals' tarsal attachment structures. The same was true when comparing the muscle activity of the two species on both the smooth and the rough surfaces.

Arachnids secrete a fluid over their adhesive pads

Background

Many arachnids possess adhesive pads on their feet that help them climb smooth surfaces and capture prey. Spider and gecko adhesives have converged on a branched, hairy structure, which theoretically allows them to adhere solely by dry (solid-solid) intermolecular interactions. Indeed, the consensus in the literature is that spiders and their smooth-padded relatives, the solifugids, adhere without the aid of a secretion.

Methodology and Principal Findings

We investigated the adhesive contact zone of living spiders, solifugids and mites using interference reflection microscopy, which allows the detection of thin liquid films. Like insects, all the arachnids we studied left behind hydrophobic fluid footprints on glass (mean refractive index: 1.48–1.50; contact angle: 3.7–11.2°). Fluid was not always secreted continuously, suggesting that pads can function in both wet and dry modes. We measured the attachment forces of single adhesive setae from tarantulas (Grammostola rosea) by attaching them to a bending beam with a known spring constant and filming the resulting deflection. Individual spider setae showed a lower static friction at rest (26%±2.8 SE of the peak friction) than single gecko setae (Thecadactylus rapicauda; 96%±1.7 SE). This may be explained by the fact that spider setae continued to release fluid after isolation from the animal, lubricating the contact zone.

Significance

This finding implies that tarsal secretions occur within all major groups of terrestrial arthropods with adhesive pads. The presence of liquid in an adhesive contact zone has important consequences for attachment performance, improving adhesion to rough surfaces and introducing rate-dependent effects. Our results leave geckos and anoles as the only known representatives of truly dry adhesive pads in nature. Engineers seeking biological inspiration for synthetic adhesives should consider whether model species with fluid secretions are appropriate to their design goals.

Mechanisms of fluid production in smooth adhesive pads of insects

Insect adhesion is mediated by thin fluid films secreted into the contact zone. As the amount of fluid affects adhesive forces, a control of secretion appears probable. Here, we quantify for the first time the rate of fluid secretion in adhesive pads of cockroaches and stick insects. The volume of footprints deposited during consecutive press-downs decreased exponentially and approached a non-zero steady state, demonstrating the presence of a storage volume. We estimated its size and the influx rate into it from a simple compartmental model. Influx was independent of step frequency. Fluid-depleted pads recovered maximal footprint volumes within 15 min. Pads in stationary contact accumulated fluid along the perimeter of the contact zone. The initial fluid build-up slowed down, suggesting that flow is driven by negative Laplace pressure. Freely climbing stick insects left hardly any traceable footprints, suggesting that they save secretion by minimizing contact area or by recovering fluid during detachment. However, even the highest fluid production rates observed incur only small biosynthesis costs, representing less than 1 per cent of the resting metabolic rate. Our results show that fluid secretion in insect wet adhesive systems relies on simple physical principles, allowing for passive control of fluid volume within the contact zone.

Extensive collection of femtolitre pad secretion droplets in the beetle Leptinotarsa decemlineata allows nanolitre microrheology

Pads of beetles are covered with long, deformable setae, each ending in a micrometric terminal plate coated with secretory fluid. It was recently shown that the layer of the pad secretion covering the terminal plates is responsible for the generation of strong attractive forces. However, less is known about the fluid itself because it is produced in an extremely small quantity. We present here the first experimental investigation of the rheological properties of the pad secretion in the Colorado potato beetle Leptinotarsa decemlineata (Coleoptera, Chrysomelidae). Because the secretion is produced in an extremely small amount at the level of the terminal plate, we first developed a procedure based on capillary effects to collect the secretion for rheological experiments. In order to study the collected fluid (less than 1 nl) through passive microrheology, we managed to incorporate micrometric probes (melamine beads) that were initially in the form of a dry powder. Finally, the bead thermal motions were observed optically and recorded to determine the mechanical properties of the surrounding medium. We achieved this quantitative measurement with the collected volume, which is much smaller than the usual 1 µl sample volume required for this technique. Surprisingly, the beetle secretion was found to behave as a purely viscous liquid, of high viscosity (about 100 times that of water). This suggests that no specific complex fluid behaviour is needed by this adhesive system during beetle locomotion. We describe a scenario for the contact formation between the spatula at the setal tip and a smooth substrate, during the insect walk. We show that the attachment dynamics of the insect pad computed from the high measured viscosity is in good agreement with the observed insect pace. We finally discuss the consequences of the viscosity of the secretion on the insect adhesion.

Slippery pores: anti-adhesive effect of nanoporous substrates on the beetle attachment system

Traction experiments with adult seven-spotted ladybird beetles Coccinella septempunctata (L.) were carried out to study the influence of surface structure on insect attachment. Force measurements were performed with tethered walking insects, both males and females, on five different substrates: (i) smooth glass plate, (ii) smooth solid Al(2)O(3) (sapphire) disc, and (iii-v) porous Al(2)O(3) discs (anodisc membranes) with the same pore diameter but different porosity. The traction force of beetles ranged from 0.16 to 16.59 mN in males and from 0.32 to 8.99 mN in females. In both sexes, the highest force values were obtained on smooth solid surfaces, where males showed higher forces than females. On all three porous substrates, forces were significantly reduced in both males and females, and the only difference within these surfaces was obtained between membranes with the highest and lowest porosity. Males produced essentially lower forces than females on porous samples. The reduction in insect attachment on anodisc membranes may be explained by (i) possible absorption of the secretion fluid from insect adhesive pads by porous media and/or (ii) the effect of surface roughness. Differences in attachment between males and females were probably caused by the sexual dimorphism in the terminal structure of adhesive setae.

Impact of chemical manipulation of tarsal liquids on attachment in the Colorado potato beetle, Leptinotarsa decemlineata

Insect tarsal attachment forces are thought to be influenced by the viscosity and surface tension of a thin film of adhesive liquid (wet adhesion). In beetles, this fluid has been shown to be composed mainly of lipophilic substances that are similar to the cuticular lipids. In this study we investigate whether and how the chemical composition of footprint lipids affects attachment forces in the Colorado potato beetle, Leptinotarsa decemlineata. After application of standardised mixtures of synthetic n-alkanes or alkenes, or a concentrated hydrocarbon extract to the surface of the elytra, we tested the beetles' attachment performance using a beam force transducer. The results show that only the unsaturated components, but not the straight-chained alkanes reduced friction forces, confirming that attachment performance is influenced by the chemical composition of the adhesive secretion. We estimated the volume of footprint droplets and calculated a mean thickness of the liquid layer of 0.04 microm. The measured friction exceeded the viscous and capillary force expected for a film of this thickness. Therefore, alternative mechanisms (i.e. shear-thinning and solid-like behaviour) for the generation of attachment forces and their dependence on the chemical composition of the liquid are discussed.

Foot morphology and substrate adhesion in the Madagascan hissing cockroach, Gromphadorhina portentosa

Insects are successful terrestrial organisms able to locomote over a wide range of obstacles and substrates. This study investigated how foot morphology (tarsal structure) correlates with substrate adhesion and ecological niche in the Madagascan hissing cockroach, Gromphadorhina portentosa Schaum (Blattaria: Blaberidae). Using light and scanning electron microscopy, the morphology of the different structures of the tarsus of G. portentosa was analysed. Using an Instron universal testing machine, a series of peak force experiments were then conducted to record the force required to lift the cockroaches off different substrates. G. portentosa was pulled off 10 different substrates, which consisted of smooth Perspex; Perspex scored at 1cm intervals; Perspex hatched at 1 cm, 0.5 cm, and 1 mm intervals; Perspex abraded with fine grade sandpaper; Perspex abraded with coarse grade sandpaper; wood; glass; and Teflon. A clear relationship was seen where an increase in scoring on the Perspex caused a decrease in adhesive ability of G. portentosa. This may be due to there being adequate contact area for the attachment of the pads and to allow the claws to engage. The results obtained suggest that to achieve the greatest adhesion to substrates, G. portentosa uses a combined effect of both adhesive pads and pretarsal claws. Adhesion to a wide range of substrates appears to be an adaptation to life as a wingless forest floor dweller.

Tarsal morphology and attachment ability of the codling moth Cydia pomonella L. (Lepidoptera, Tortricidae) to smooth surfaces

Despite several studies on the attachment ability of different insect taxa, little is known about this phenomenon in adult Lepidoptera. In this study we combined morphological and experimental analyses of tarsal adhesive devices and the attachment ability of the codling moth Cydia pomonella (L.) (Lepidoptera, Tortricidae) to smooth surfaces. Pretarsi of C. pomonella attach to smooth substrates by means of their smooth, flexible and well developed arolia. Using the centrifugal force measurement technique, friction forces of males and females were assessed on hydrophobic and hydrophilic glass surfaces. Adults of both sexes generated similar forces in spite of the noticeable difference in their body masses. That is why males showed significantly higher safety factors (attachment force divided by body weight) compared to those of females. Hydrophobicity of the substrate had no considerable effect on friction forces. For females, friction forces (sliding parallel to the substrate plane) were compared with adhesive forces (pulling off perpendicularly from the substrate plane) measured on Plexiglas surfaces. It can be concluded that the attachment system of C. pomonella is rather robust against physico-chemical properties of the substrate and is able to achieve a very good attachment on vertical and horizontal substrata.

Insect tricks: two-phasic foot pad secretion prevents slipping

Many insects cling to vertical and inverted surfaces with pads that adhere by nanometre-thin films of liquid secretion. This fluid is an emulsion, consisting of watery droplets in an oily continuous phase. The detailed function of its two-phasic nature has remained unclear. Here we show that the pad emulsion provides a mechanism that prevents insects from slipping on smooth substrates. We discovered that it is possible to manipulate the adhesive secretion in vivo using smooth polyimide substrates that selectively absorb its watery component. While thick layers of polyimide spin-coated onto glass removed all visible hydrophilic droplets, thin coatings left the emulsion in its typical form. Force measurements of stick insect pads sliding on these substrates demonstrated that the reduction of the watery phase resulted in a significant decrease in friction forces. Artificial control pads made of polydimethylsiloxane showed no difference when tested on the same substrates, confirming that the effect is caused by the insects' fluid-based adhesive system. Our findings suggest that insect adhesive pads use emulsions with non-Newtonian properties, which may have been optimized by natural selection. Emulsions as adhesive secretions combine the benefits of 'wet' adhesion and resistance against shear forces.

Caterpillar footprints as host location kairomones for Cotesia marginiventris: persistence and chemical nature

Herbivores walking over the epicuticular wax layer of a plant may leave tracks that disclose their presence to hunting predators or parasitoids. The braconid wasp Cotesia marginiventris is a solitary parasitoid of young noctuid caterpillars. It can locate potential hosts from a distance by orienting toward the scent of herbivore-damaged plants. Upon landing on the caterpillars' food plant, the female parasitoid searches for further cues (kairomones) that confirm the presence of a suitable host. In a previous study, we showed that C. marginiventris recognizes the chemical footprints of absent Spodoptera frugiperda caterpillars on a leaf. Here, we report on the persistence and chemical nature of this host location kairomone. In a series of behavioral assays, we confirmed that caterpillars of S. frugiperda leave chemical tracks that elicit characteristic antennation behavior in C. marginiventris for up to 2 days. Both hexane extracts of caterpillar footprints and of the larvae's ventral cuticle induced antennation and contained almost identical long-chain hydrocarbons, thus suggesting the prolegs and claspers as the kairomones' main source. A series of linear C(21) to C(32) alkanes accounted for ca 90% of all identified compounds. Female wasps showed significant antennation responses on leaves treated with a reconstructed blend of these n-alkanes. However, wasp responses were relatively weak. Therefore, we presume that minor compounds, such as monomethyl-branched alkanes, which were also found, may contribute additionally to host recognition.

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.

Enhancement of capillary forces by multiple liquid bridges

Capillary forces can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. In this paper, we study numerically the effect of meniscus size on the force between two homogeneous flat plates for different contact angles. The force distance curves show excellent quantitative agreement with previous investigations. The results for n menisci of equal total liquid volume reveal interesting scaling properties and an unexpected maximum force for moderately hydrophilic surfaces (i.e., contact angles around 70 degrees ). Further, we calculate the minimum solid-liquid area for multiple bridges, the cohesive stress (i.e., force per area) between the plates, and the work required to separate them. The results are presented in two-dimensional maps, which may be useful in the understanding of biological attachment structures and in the design of artificial contact systems.

An insect trap as habitat: cohesion-failure mechanism prevents adhesion ofPameridea roridulaebugs to the sticky surface of the plantRoridula gorgonias

The glandular trichomes of the plant Roridula gorgonias release an extremely adhesive, visco-elastic, resinous secretion that traps a variety of insects, including those having a considerable body size. However, the specialized mutualistic mirid bug Pameridea roridulae lives and walks on this sticky plant surface without being trapped. We have sought to reveal the mechanism underlying the apparent non-sticky nature of the cuticle of this bug. In this study, we have visualized intact plant and insect surfaces using cryo-scanning electron microscopy and measured the adhesive properties of the plant secretion on different surfaces. We present a combination of structural and experimental results that suggest that a thick and cohesively weak film of an outermost, epicuticular greasy secretion acts as a `sloughing-off' layer,preventing the formation of contacts between the sticky plant secretion and the solid insect cuticle. In a comparative study of fresh cuticle fractures of flies representing a typical prey of R. gorgonias, a thin,fragmentary layer of epicuticular grease was revealed. These results indicate that, when trapping prey, the plant adhesive might form proper contact with solid islands of the insect cuticle that are free of epicuticular grease.

Biological attachment devices: exploring nature's diversity for biomimetics

Many species of animals and plants are supplied with diverse attachment devices, in which morphology depends on the species biology and the particular function in which the attachment device is involved. Many functional solutions have evolved independently in different lineages of animals and plants. Since the diversity of such biological structures is huge, there is a need for their classification. This paper, based on the original and literature data, proposes ordering of biological attachment systems according to several principles: (i) fundamental physical mechanism, according to which the system operates, (ii) biological function of the attachment device, and (iii) duration of the contact. Finally, we show a biomimetic potential of studies on biological attachment devices.

Adhesive properties of the arolium of a lantern-fly, Lycorma delicatula (Auchenorrhyncha, Fulgoridae)

The arolium in Lycorma delicatula is shaped as a truncated pyramid, tapering proximally. The base or the terminal area is corrugated, forming parasagittal wrinkles (period 1.5-5.0 microm), which are supported from inside by cuticular dendrites. Side faces of the arolium are made up of sclerotized dorsolateral plates. When claws slip on a smooth substrate and pronate, the dorsolateral plates diverge and expand the sticky terminal area. The real contact area with the glass plate was recognized by light reflection on its periphery. This area was measured and shown to be smaller when the leg was pressed perpendicularly to the substrate (0.02 mm(2)) than when it was sheared in a direction parallel to the substrate (0.05 mm(2)). Attachment forces were measured with the aid of dynamometric platforms during pulling of active insects from horizontal or vertical glass surfaces. Normal adhesive force (about 9-12 mN) was much less than friction force during sliding with velocity of 6-17 mm/s (50-100 mN); however, when expressed in tenacity per unit contact area the difference was less pronounced: 170 and 375-625 mN/mm(2), respectively. Sliding of the arolium during shear displacement was shown to be oscillatory in frame-by-frame video analysis. Relaxative oscillations consisted of periodical sticks-slips of the arolium along the glass surface.

The anatomy and ultrastructure of the suctorial organ of Solifugae (Arachnida)

Solifugae possess an evertable, adhesive pedipalpal organ (suctorial organ) at the tip of the distal tarsus of each pedipalp that is unique among arachnids. When inverted inside the pedipalp, the suctorial organ is covered with two cuticular lips, a dorsal upper lip and a ventral lower lip, but it can be protruded rapidly in order to facilitate grasping prey or climbing on bushes or even climbing on smooth surfaces due to its remarkable adhesive properties. In this study, the suctorial organs of different species from old world families Galeodidae and Karschiidae and new world families Ammotrechidae and Eremobatidae were investigated by means of light microscopy, scanning and transmission electron microscopy. In all representatives, the suctorial organ is formed by an evertable, cuticular pad with a complex internal stabilizing structure. The procuticle of this pad consists of a lattice-like basal plate and numerous stalked structures connected to this basal plate. The shafts of the stalked structures are regularly organized and ramify apically. The surface of the suctorial organ is constituted of a very thin epicuticle overlaying the ramifying apices forming ridges and furrows on the ventral side of the suctorial organ.

Foraging scent marks of bumblebees: footprint cues rather than pheromone signals

In their natural habitat foraging bumblebees refuse to land on and probe flowers that have been recently visited (and depleted) by themselves, conspecifics or other bees, which increases their overall rate of nectar intake. This avoidance is often based on recognition of scent marks deposited by previous visitors. While the term 'scent mark' implies active labelling, it is an open question whether the repellent chemicals are pheromones actively and specifically released during flower visits, or mere footprints deposited unspecifically wherever bees walk. To distinguish between the two possibilities, we presented worker bumblebees (Bombus terrestris) with three types of feeders in a laboratory experiment: unvisited control feeders, passive feeders with a corolla that the bee had walked over on its way from the nest (with unspecific footprints), and active feeders, which the bee had just visited and depleted, but which were immediately refilled with sugar-water (potentially with specific scent marks). Bumblebees rejected both active and passive feeders more frequently than unvisited controls. The rate of rejection of passive feeders was only slightly lower than that of active feeders, and this difference vanished completely when passive corollas were walked over repeatedly on the way from the nest. Thus, mere footprints were sufficient to emulate the repellent effect of an actual feeder visit. In confirmation, glass slides on which bumblebees had walked on near the nest entrance accumulated hydrocarbons (alkanes and alkenes, C23 to C31), which had previously been shown to elicit repellency in flower choice experiments. We conclude that repellent scent marks are mere footprints, which foraging bees avoid when they encounter them in a foraging context.

Physicochemical properties of functional surfaces in pitchers of the carnivorous plant Nepenthes alata Blanco (Nepenthaceae)

Pitchers of the carnivorous plant Nepenthes alata are highly specialized organs adapted to attract, capture, and digest animals, mostly insects. They consist of several well distinguishable zones, differing in macro-morphology, surface microstructure, and functions. Since physicochemical properties of these surfaces may influence insect adhesion, we measured contact angles of non-polar (diiodomethane) and polar liquids (water and ethylene glycol) and estimated the free surface energy of 1) the lid, 2) the peristome, 3) the waxy surface of the slippery zone, and 4) the glandular surface of the digestive zone in N. alata pitchers. As a control, the external surface of the pitcher, as well as abaxial and adaxial surfaces of the leaf blade, was measured. Both leaf surfaces, both lid surfaces, and the external pitcher surface showed similar contact angles and had rather high values of surface free energy with relatively high dispersion component. These surfaces are considered to support strong adhesion forces based on the capillary interaction, and by this, to promote successful attachment of insects. The waxy surface is almost unwettable, has extremely low surface energy, and therefore, must essentially decrease insect adhesion. Both the peristome and glandular surfaces are wetted readily with both non-polar and polar liquids and have very high surface energy with a predominating polar component. These properties result in the preclusion of insect adhesion due to the hydrophilic lubricating film covering the surfaces. The obtained results support field observations and laboratory experiments of previous authors that demonstrated the possible role of different pitcher surfaces in insect trapping and retention.

Material structure, stiffness, and adhesion: why attachment pads of the grasshopper (Tettigonia viridissima) adhere more strongly than those of the locust (Locusta migratoria) (Insecta: Orthoptera)

The morphology, ultrastructure, effective elastic modulus, and adhesive properties of two different smooth-type attachment pads were studied in two orthopteran species. Tettigonia viridissima (Ensifera) and Locusta migratoria (Caelifera) have a similar structural organization of their attachment pads. They both possess a flexible exocuticle, where the cuticular fibrils are fused into relatively large rods oriented at an angle to the surface. The compliant material of the pad contributes to the contact formation with the substrate. However, the pad material structure was found to be different in these two species. L. migratoria pads bear a thick sub-superficial layer, as well as a higher density of rods. The indentation experiments showed a higher effective elastic modulus and a lower work of adhesion for L. migratoria pads. When the indentations were made at different depths, a higher effective elastic modulus was revealed at lower indentation depths in both species. This effect is explained by the higher stiffness of the superficial pad layer. The obtained results demonstrate a clear correlation between density of the fibres, thickness of the superficial layer, compliance of the pad, and its adhesive properties. Such material structures and properties may be dependent on the preferred environment of each species.

Biomechanics of smooth adhesive pads in insects: influence of tarsal secretion on attachment performance

Many insects possess smooth adhesive pads on their legs, which adhere by thin films of a two-phasic secretion. To understand the function of such fluid-based adhesive systems, we simultaneously measured adhesion, friction and contact area in single pads of stick insects (Carausius morosus). Shear stress was largely independent of normal force and increased with velocity, seemingly consistent with the viscosity-effect of a continuous fluid film. However, measurements of the remaining force 2 min after a sliding movement show that adhesive pads can sustain considerable static friction. Repeated sliding movements and multiple consecutive pull-offs to deplete adhesive secretion showed that on a smooth surface, friction and adhesion strongly increased with decreasing amount of fluid. In contrast, pull-off forces significantly decreased on a rough substrate. Thus, the secretion does not generally increase attachment but does so only on rough substrates, where it helps to maximize contact area. When slides were repeated at one position so that secretion could accumulate, sliding shear stress decreased but static friction remained clearly present. This suggests that static friction which is biologically important to prevent sliding is based on non-Newtonian properties of the adhesive emulsion rather than on a direct contact between the cuticle and the substrate.

Scaling effects of wet adhesion in biological attachment systems

Insects have evolved fibrillar attachment devices based on wet adhesion to attach themselves to a variety of surfaces. This paper investigates the scaling effects of wet adhesion mediated by a liquid bridge between a fiber and a solid surface. The influences of liquid volume and contact angles are discussed via a scaling law indicating that the adhesive strength can be enhanced by contact size reduction. Due to the maximum negative pressure in the liquid bridge, there exists a critical length scale at which the system achieves the theoretical tensile strength of the liquid. We conclude that size reduction down to a critical scale results in optimization of the adhesive strength.

Adhesion design maps for bio-inspired attachment systems

Fibrous surface structures can improve the adhesion of objects to other surfaces. Animals, such as flies and geckos, take advantage of this principle by developing "hairy" contact structures which ensure controlled and repeatable adhesion and detachment. Mathematical models for fiber adhesion predict pronounced dependencies of contact performance on the geometry and the elastic properties of the fibers. In this paper the limits of such contacts imposed by fiber strength, fiber condensation, compliance, and ideal contact strength are modeled for spherical contact tips. Based on this, we introduce the concept of "adhesion design maps" which visualize the predicted mechanical behavior. The maps are useful for understanding biological systems and for guiding experimentation to achieve optimum artificial contacts.

Structure and properties of the glandular surface in the digestive zone of the pitcher in the carnivorous plantNepenthes ventrataand its role in insect trapping and retention

Carnivorous plants of the genus Nepenthes grow in nutrient-poor habitats and have evolved specialised trapping organs, known as pitchers. These are composed of different surface zones serving the functions of attraction, capture and digestion of insects, which represent a main source of nitrogen. To investigate the role of the glandular digestive zone in the trapping mechanism of the pitcher, structural, mechanical and physico-chemical studies were applied to N. ventrata and combined with insect behavioural experiments. It was found that the glandular surface is microscopically rough since it is regularly structured with multicellular glands situated in epidermal depressions. The presence of downward-directed`hoods' over the upper part of glands and sloped depressions in the proximal direction of the pitcher causes a marked anisotropy of the surface. The glandular zone surface is composed of relatively stiff material (Young's modulus, 637.19±213.44 kPa). It is not homogeneous, in terms of adhesive properties, and contains numerous areas without adhesion as well as adhesive areas differing greatly in tenacity values (range, 1.39-28.24 kPa). The surface is readily wettable with water (contact angle, 31.9-36.0°C)and has a high surface free energy (56.84-61.93 mN m-1) with a relatively high polar component (33.09-52.70 mN m-1). To examine the effect of the glandular secretion on attachment systems of insects having hairy and smooth adhesive pads, forces generated on different surfaces by Calliphora vicina flies and Pyrrhocoris apterus bugs,respectively, were measured. Flies attached equally well to both fresh and air-dried glandular surfaces whereas bugs generated a significantly lower force on the fresh glandular surface compared with the air-dried one. It is assumed that the contribution of the glandular surface to insect retention,due to its effect on insect attachment, differs depending on insect weight and the type of insect attachment system. Surface anisotropy does not facilitate effective claw interlocking so that insects possessing only claws are probably not able to cling to the glandular surface. However, stiffness of the pitcher wall material in the digestive zone can provide claw clinging viapunching of the pitcher wall by claws. Small insects lacking pads may use adhesive areas on the plant surface to attach themselves, but such solitary points with very strong adhesion possibly impede their overall locomotion and chance of escape. Pad-bearing insects are presumably able to attach to smooth parts of the glandular surface located between glands. High free surface energy of the plant substrate may promote adhesion. Gland secretion may decrease attachment ability in insects with smooth adhesive pads but not influence attachment of insects with hairy attachment systems.

Ontogenesis of the attachment ability in the bugCoreus marginatus(Heteroptera, Insecta)

Each tarsus of Coreus marginatus L. (Coreidae) bears a pair of smooth flexible pulvilli adapted for attachment to relatively smooth surfaces,such as their host plant Rumex crispus L. (Polygonaceae). This account quantifies insect attachment abilities on smooth surfaces at various stages of ontogenesis. Friction (shear) force (FF) of adults and juvenile insects was measured by the use of a computer controlled centrifugal force tester equipped with a fibre optical sensor. Pad area, body size and body mass were determined individually for each experimental insect. Light microscopy revealed no difference in pulvilli area between different leg pairs. Pulvilli area demonstrated a stronger increase with increasing linear dimensions, as predicted by scaling laws. Since friction coefficient (relationship between FF and body weight) (FC) was always higher than 1, it was concluded that adhesion has strongly contributed to the measured friction. The frictional properties of pulvilli do not change during ontogenesis. Thus, only the growth of pulvilli and, therefore, the increased contact area, contribute to the increasing attachment ability in insects at later larval stages. Due to different scaling of the body mass and area of attachment organs, smaller insects attach relatively more strongly. Both FF and FC were higher in experiments in which higher angular acceleration (AC) was applied. Lateral tenacity determined individually for experimental insects and pooled for all animals and accelerations is 0.097 N m-2. These data led us to suggest that viscosity of the pad secretion and/or visco-elastic properties of the foam-like material of pulvilli play an important role in the attachment ability of insects.

Biomechanics of ant adhesive pads: frictional forces are rate- and temperature-dependent

Tarsal adhesive pads enable insects to hold on to smooth plant surfaces. Using a centrifuge technique, we tested whether a 'wet adhesion' model of a thin film of liquid secreted between the pad and the surface can explain adhesive and frictional forces in Asian Weaver ants (Oecophylla smaragdina).

When forces are acting parallel to the surface, pads in contact with the surface can slide smoothly. Force per unit pad contact area was strongly dependent on sliding velocity and temperature. Seemingly consistent with the effect of a thin liquid film in the contact zone, (1) frictional force linearly increased with sliding velocity, (2) the increment was greater at lower temperatures and (3) no temperature dependence was detected for low-rate perpendicular detachment forces. However, we observed a strong,temperature-independent static friction that was inconsistent with a fully lubricated contact. Static friction was too large to be explained by the contribution of other (sclerotized) body parts. Moreover, the rate-specific increase of shear stress strongly exceeded predictions derived from estimates of the adhesive liquid film's thickness and viscosity.

Both lines of evidence indicate that the adhesive secretion alone is insufficient to explain the observed forces and that direct interaction of the soft pad cuticle with the surface ('rubber friction') is involved.

Locomotion and adhesion: dynamic control of adhesive surface contact in ants

Tarsal adhesive pads of insects are highly dynamic organs that play an important role in locomotion. Many insects combine fast running performance with strong resistance to detachment forces. This capacity requires an effective control of attachment forces at the tarsus and pretarsus. Here we investigate mechanisms of attachment control in Asian weaver ants (Oecophylla smaragdina) by measuring the dynamics of the adhesive contact area and the claws during locomotion. O. smaragdina ants walking upside down on a smooth substrate used only a fraction (approx. 14%) of their maximum possible contact area. When these ants were loaded with 30 mg weights (corresponding to approx. 6 times their own body weight), however, they employed much larger (but still submaximal; approx. 60%) contact areas. The increase of contact area was accompanied by a stronger flexion of the claws, which demonstrates the participation of the claw flexor muscle in the control of adhesive contact. However, only part of the contact area dynamics could be explained by the action of the claw flexor. During the stance phase, adhesive contact area changed while the claws remained motionless. Even when corrected for the effects of claw flexion, adhesive contact areas differed by a factor of 2.1 between loaded and unloaded ants. Our findings give evidence that running ants control their adhesive contact area by a combination of active movements of the claw flexor muscle and passive reactions of the mechanical system.

The role of adhesion in prey capture and predator defence in arthropods

Adhesive devices are used by arthropods not only in terrestrial locomotion but also in prey capture and predator defence. We argue that the physical mechanisms involved in both these contexts must mainly be capillarity and the viscosity of an adhesive secretion, whereas other mechanisms, such as friction or intermolecular forces, are of minor importance. Adhesive prey-capture devices might function as passive devices or might be actively extended toward the prey, sometimes in a very rapid manner. Adhesive mechanisms used for predator defence might involve firm adhesion to the substratum or the discharge of a sticky secretion to immobilize the appendages of the opponent. We review the occurrence of adhesive devices as employed in both functional contexts across the Arthropoda and argue that these mechanisms are of particular importance for slow-moving and relatively clumsy life forms. We discuss three case studies in more detail. (1) Loricera larvae (Carabidae) use galeae with an extremely flexible cuticle in combination with an adhesive secretion. (2) Adult Stenus species (Staphylinidae) employ two highly flexible paraglossae that are covered by an adhesive emulsion of lipid droplets dispersed in an aqueous proteinaceous liquid. (3) Springtails often adhere to the mouthparts, the antennae, the legs, or other parts of the integument of Stenus larvae before being captured with the mandibles.

The effects of plant epicuticular waxy blooms on attachment and effectiveness of predatory insects

Phytophagous insects and their natural enemies frequently must cling to plants in order to forage. Typically, this involves attachment to the layer of lipophilic materials, or 'epicuticular waxes' (EW) that covers all primary plant surfaces. EW occurring as crystalline waxy 'blooms' can provide a defense against herbivory by interfering with attachment to plants by phytophagous insects. On the other hand, EW blooms can reduce attachment by predators and parasitoids, potentially releasing populations of phytophagous insects from regulation by their natural enemies. The net effect of EW blooms on herbivory should therefore vary from system to system. When it has been measured, EW bloom typically reduces the attachment forces insects can generate on plants. Some herbivores and predators preferentially forage on plants with EW blooms. Among these, some appear to have physiological or behavioral adaptations that either improve attachment to EW blooms or cope with reduced attachment to plants surfaces bearing EW blooms. How EW blooms disrupt insect attachment and how insects potentially overcome this challenge has not been determined. Some hypotheses are offered in this review. Their elucidation may help understand the mechanics of attachment to plants, with applications including helping to improve biological control of some insect pests.