In insect attachment systems, the main force contributing to overall attachment is surface tension in a thin layer of epidermal secretion. Surface tension requires the presence of secretion in the contact area (figure: SEM picture of fly terminal elements attaching to a glass surface)
If sufficient contact between the substrate and adhesive interface is reached, forces will be established between molecules of both contacting materials, and they will adhere. Van der Waals forces are the most common component of such forces together with secondary forces, such as hydrogen bonds (figure: SEM picture of gecko setae)
Microscopical studies have revealed secretory substances in attachment systems based on two complementary surfaces, and in systems adapted for a variety of surfaces. It is well known that insects, walking on smooth surfaces, leave microscopically small fluid prints (figure: TEM replica of a fly's frozen footprints)
Knowledge of anti-adhesive mechanisms may aid in understanding the principles of how other biological adhesive systems work. The best example is the competition between anti-wetting mechanisms of plants and attachment devices of insects walking on them (figure: SEM picture of the crystallites on the plant surface)
Materials preventing separation of two surfaces may be defined as adhesives. There are at least three reasons for using adhesive systems: (1) they join dissimilar materials; (2) they show improved stress distribution in the joint; and (3) they increase design flexibility. These reasons are relevant to the evolution of natural attachment systems and man-made joining materials.
In biological systems, adhesive organs used for attachment to substrates as well as those involved in catching prey, demonstrate a huge diversity among living creatures due to their structural and chemical properties. However, for attachment during locomotion, animals have developed only two distinctly different mechanisms: smooth pads or setose (hairy) surfaces. Due to the flexibility of the attachment structure materials, both mechanisms can maximise the possible contact area with the substrate, regardless of its microsculpture. It is remarkable that these highly-specialised structures are not restricted to one particular area of the leg. For example, in insects, they may be located on different parts, such as claws, derivatives of the pretarsus, tarsal apex, tarsomeres, or tibia. Therefore, it appears obvious that these structures, adapted for attachment, have evolved several times, independently.
