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Insect adhesion
Arthropods make use of smooth adhesive pads as well as hairy pads for climbing and locomotion along non-horizontal surfaces. Both types of pads in insects make use of liquid secretions and are considered 'wet'.〔 Dry adhesive mechanisms primarily rely on van der Waals’ forces and are used by organisms other than insects. The fluid provides capillary and viscous adhesion and appears to be present in all insect adhesive pads. Little is known about the chemical properties of the adhesive fluids and the ultrastructure of the fluid producing cells is currently not extensively studied.〔 Additionally, both hairy and smooth types of adhesion have evolved separately numerous times in insects.〔 Few comparative studies between the two types of adhesion mechanisms have been done and there is a lack of information regarding the forces that can be supported by these systems in insects.〔 Additionally, tree frogs and some mammals such as the arboreal possum and bats also make use of smooth adhesive pads.〔〔 The use of adhesive pads for locomotion across non-horizontal surfaces is a trait that evolved separately in different species, making it an example of convergent evolution. The power of adhesion allows these organisms to be able to climb on almost any substance.〔 The exact mechanisms of arthropod adhesion are still unknown for some species but this topic is of great importance to biologists, physicists and engineers.〔〔〔 These highly specialized structures are not restricted to one particular area of the leg. They may be located on different parts, such as claws, derivatives of the pretarsus, tarsal apex, tarsomeres or tibia.〔 From the scaling analysis, it has been suggested that animal lineages relying on the dry adhesion, such as lizards and spiders have a higher density of terminal contact elements compared to systems that use wet adhesive mechanisms such as insects.〔 Since these effects are based on fundamental physical principles and highly related to the shape of the structure, they are also the same for artificial surfaces with similar geometry.〔 Adhesion and friction forces per-unit-pad area were very similar in smooth and hairy systems when tested.〔 Strong adhesion may be beneficial in many situations but it also can create difficulties in locomotion.〔 Direction-dependence is an important and fundamental property of adhesive structures that are able to rapidly and controllably adhere during locomotion.〔 Researchers are unsure whether direction-dependence is achieved through changes in contact area or through a change in shear stress.〔 Friction and adhesion forces in most animal attachment organs are higher when they are pulled towards the body than when they push away from it.〔 This has been observed in geckos and spiders but also in the smooth adhesive pads of ants, bush-crickets and cockroaches.〔 Adhesive hairs of geckos are non-symmetrical and feature distally pointing setae and spatulae that are able to generate increased friction and adhesion when aligned with a proximal pull.〔 The adhesive hairs of some beetles behave similarly to those of geckos.〔 While directional-dependence is present in other animals, it has yet to be confirmed in insects with hairy adhesive pads.〔
It has been observed that a surface micro-roughness asperity size of less than five micrometres can strongly reduce insect attachment and climbing ability and this adhesion reducing effect has been put to use in a variety of plant species that create wax crystals.〔
Adhesive chemical secretions are also used for predation defence, mating, holding substrates, anchor eggs, building retreats, prey capture and self-grooming.〔
==Smooth adhesive pads for locomotion==
Smooth adhesion has evolved in many families of organisms independently, which creates structures that appear unrelated to each other but generate the same function.〔 Phylogenetic analyses indicate that adhesive structures of arthropods evolved several times.〔 Organisms such as ants, bees, cockroaches and grasshoppers use smooth adhesive pads.〔 There are different types of smooth adhesive pads in these organisms such as the arolia, pulvilli, and euplantulae, all of which have a cuticle that is extremely soft and deformable.〔〔 The arolia of some ant species has been observed to be fluid-filled and is extended and contracted to provide adhesive force.〔 The euplantulae in crickets have a hexagonal microstructure which is similar to toe pads in tree frogs.〔 Generally, insects are able to adhere to surfaces through the contact between the insect adhesive organs and substrates that are mediated by nanometre-thin films of adhesive fluid.〔 Some functional principles of smooth pads (adaptability, viscoelasticity, pressure sensitivity) are similar to those known from industrial pressure-sensitive adhesion.〔 Smooth adhesive organs are ‘‘pillowlike,’’ which refers to the soft and fluid-filled, cuticular sac, that moulds to the surface increasing the contact area on rough surfaces.〔 It appears that the fluid in smooth adhesive systems mainly serves to maximize contact on rough substrates.〔 The internal fibrous structure of smooth pads might be vital to their ability to deform or for shear-induced lateral increase in contact area or for efficient transfer of tensile forces, yet at this point its specific function is unknown.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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