Walking in Insects

Insects are primarily terrestrial, though some have secondarily taken to life in water. The aquatic insects have adopted various methods of swimming or skating on water surface, but vast majority of insects walk on a solid surface. An insect has three pairs of legs; hence the class of insects is also referred to as Hexapoda.

How does an insect walk with six legs? At any moment during walking the insect rests on the substratum with minimum of three legs, a middle leg on one side and anterior and posterior legs on the opposite side. These three legs support the weight of the body like a tripod. The other three legs make the insect move; among them the front leg produces traction for the body or pulls it forward, the hind leg propels the body forward, and the middle leg mainly supports the weight of the body on its side. Cinematographic records reveal that the three legs, providing tripod like support at one moment, are lifted from the substratum at the next moment in the following order: first the front leg, next the middle leg of the other side, followed by the hind leg. Thus at any moment 3, 4, or 5 legs are on the ground. In fast moving there may be a change in the sequence of leg movements.

Some terrestrial insects show significant deviations from the general way of walking, described above. Nymphs of bugs, called cicadas, lead a subterranean life, sucking juices from rootlets of various trees. They make burrows in soil. Greatly enlarged tibia and femora of their fore leg form a sort of shovel for digging soil. Traction for movement in a burrow is provided by long middle legs, while long hind legs provide a crutch like

— Fig. 41.1. Nymph of a Cicada. Note its shovel-like front legs.

support and propel the body forward. Urine of the nymph cements soil particles and prevents caving in.

Preying mantis, with its front legs modified into raptorial organs for catching and holding preys, also moves on middle and hind pairs of legs. Insects jump (alticines, fleas, Orthoptera, etc.) with the help of enlarged hind legs. They even negotiate obstacles, walk up a vertical surface and even some, like flies, can walk on the ceilings without any problem (thanks to a combination of claws and pads, covered with oily secretions, as in coackroaches and leaf-beetles). Mole crickets (Gryllotalpa) dig deep in the soil using their huge and dentate forelegs.

In 1972, Bernadette Delage-Darchen discovered a new and interesting species of ants, Melissotarsus weissi (=titubans) in Ivory Coast. These tiny ants make and live in burrows in wood of trunks of various trees, and feed on cochineals (Diaspididae), which live and grow inside the burrows. Locomotion in these ants in the burrows is due to anterior and posterior pairs of legs, while the middle pair of legs is modified for palpating the ceiling of the burrow. The middle legs, besides being sensory, are provided with glands secreting a pheromone for marking the burrow, through which the ant moves. When kept on an open surface, with no ceiling above for palpating, the ants are unable to make normal and proper locomotion.

References

Delage-Darchen, B. 1972. Une fourmi de Cote d'Ivoire : Melissotarsus titubans Del., n. sp. Ins. Sociaux 19 (3) : 13-34.

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