Defense and Attack Strategies and Mechanisms in Biology - Biomimetics: Biologically Inspired Technologies - page 356

Nature commonly uses bistable mechanisms. This is intimately associated with the separation
of the assumption of the loaded configuration from the storage of strain energy. The mechanism
is drawn over center by the main spring, and then the spring is loaded. The main spring has a low
mechanical advantage and can store a large amount of strain energy, generating high forces.
When the system is ''fired'' the trigger, which can generate only a low force but has a high
mechanical advantage, allows the mechanism to move back over center and the energy from the
main spring is fed into the system (Bennet-Clark and Lucey, 1967). This has the advantage that
there are no firing pins or hooks to jam or break. Thus, control is smoother and reliability
improved. In the snap-jaw ant, the mandibles are clicked against each other, rather like snapping
finger and thumb over each other. The ant can then move comparatively massive objects. The
mandibles are first held with the tips just touching, then loaded. Large muscles contract against
the closed mandibles that are thus bent and store some elastic energy. However, most of the
muscular energy is transformed and elastically stored within the apodeme and its cuticular
threads, within the muscle fibres and probably also within the entire head capsule. Slight rotation
of one of the mandibles then causes its lower edge to bend slightly inwards and lets the other
mandible slide above it, powered by the strain energy stored within the contracted muscles and
the mandible shaft (Gronenberg et al., 1998). Immediately afterwards the unstimulated mandible
hits the object and bounces it away. The stored energy thus is spent and the mandibles are
decelerated during the second half of their trajectory and come to a hold before they could bump
into the front of the head.
The Venus fly trap ( Dionaea muscipula ) preys on insects and other small animals that venture
onto its trap leaves and trigger their closure by disturbing certain sensitive hairs. The leaves
routinely shut in 1/25 s. Such speed of movement is uncommon amongst plants and so has attracted
attention and theories for many years. The mechanism is based on a turgor-driven elastic instability
of the leaf, which is in effect a prestressed mechanical bistable structure (Forterre et al., 2005;
Thom, 1975). A better understanding of this mechanism and the way in which it is designed and
actuated would not only solve a long-standing conundrum, but could also give rise to a series of
novel hydraulic actuators and switches.
Nature does use explosives, in the sense that an explosive chemical reaction proceeds at very
high speed, is exothermic, and produces large amounts of hot gas that do the damage. The insect
in question is the bombardier beetle, of which there are many species, for example Brachinus
explodens , which produces a jet of steam and hydroquinone at a temperature probably in excess of
100 8 C. The propellant is oxygen produced from the breakdown of hydrogen peroxide. The jet is
pulsed (at about 500 Hz) and can, depending on the species of beetle, be aimed very accurately
(Dean et al., 1990).

13.7 ELECTRICAL

13.7.1 Stun Gun

A small, two-pronged, hand held electrical discharge weapon. Effective range is less than an arm
length. It works by affecting the muscle signal paths, disturbing the nervous system (Alexander
et al., 1996).
The electric eel is different from other electric fish in its ability to generate a stunning or even a
killing electrical discharge. The electric eel can produce up to 600 V in a single discharge. The
electric organ, which consists of a series of modified tail muscles, is similar to a row of batteries
connected in a series. It is subdivided into three sections: two small and one large. One small battery
is used for navigational signals. The large battery and the other small one are used to generate the
stunning discharge. After delivering a strong shock, the electric eel must then allow the electric
organ to recharge (Heiligenberg, 1977).