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350 Biomimetics: Biologically Inspired Technologies

the size of its image as it closes in on the object). The observer of the object thus sees no movement
away from the direction of the fixed point. The fixed point could be a part of the background against
which the dragonfly is camouflaged, or the initial position of the dragonfly, in which case the
dragonfly appears not to have moved from its starting point (Anderson and McOwan, 2003).

13.3.8 False Target Generation

A device that creates and presents an image of a target that causes a weapon to aim at a false
target. Used as a countermeasure to precision guided weapons (Alexander et al., 1996).
This is a common ploy in insects; for instance, butterflies have eye spots on the trailing edge of
the hind wing. Predating birds tend to aim for the eyes rather than the body of the insect, and so the
insect escapes with relatively slight damage to the hind wing. Similarly fish can have an eyespot on
the tail fin with the true eye concealed in a dark marking across the head. A number of moth larvae
have a false ‘‘head’’ at the tail end which can simply be eye spots or an image of the head of another
animal such as a snake. The advantage then is not just that the attack will be at the ‘‘wrong’’ end of
the animal, thus protecting the nervous system, but that the animal will apparently move backwards
in order to escape.
A more sophisticated false target is generated by autotomy of part of the animal. A well-known
example is the salamander which leaves the end of its tail behind. A more sophisticated example
is provided by certain opilionids (harvestmen), which can autotomize a leg which will continue to
move and thus confuse and divert the predator whilst the putative prey makes its escape (Gnaspini
and Cavalheiro, 1998). Since the opilionid has eight legs (at least at the start of the chase) it can
employ this subterfuge a number of times. However, studies on wolf spiders (which play a similar
trick) show that the loss of a leg slows them down (Amaya et al., 1998).

13.4 BARRIERS

13.4.1 Slick Coating

Teflon lubricants that create a slippery surface because of their chemical properties. These
chemical agents reduce friction with the intent to inhibit the free movement of the target. In the
1960s Riotril (‘‘Instant Banana Peel’’) was applied as an ostensibly inert white powder to a hard
surface and wetted down. It then became like an ice slick. It is virtually impossible for an individual
to move or stand up on a hard surface so treated; tyres skid. Riotril, if allowed to dry, can easily be
peeled away or, because it’s water-soluble, can be washed away (Alexander et al., 1996).
A similar phenomenon is found in the carnivorous pitcher plants (Figure 13.3). Several
mechanisms have been proposed for the way they capture insects, mostly slippery surface wax
crystals. But the important capture mechanism is due to the surface properties of the rim of the
pitcher, which has smooth radial ridges. This surface is completely wettable by nectar secreted by
the rim, and by rain water, so that a film of liquid covers the surface when the weather is humid. The
rim is then slippery both for soft adhesive pads (the liquid sees to that) and for the claws, due to the
surface topography. This dual system starts sliding ants down the slippery slope (Bohn and Federle,
2004).
13.4.2 Sticky Coating

Polymer adhesives used to bond down equipment and human targets. Also known as stick’ems’ and
superadhesives (Alexander et al., 1996).
The best known biological adhesives are those occurring in spiders’ webs and those on the
leaves of the sundew, Drosera. Neither adhesive has yet been characterized.