VI.—HOW INSECTS WALK.
Grown-up insects seem to be very short of legs compared with many of their distant relatives. Thus, while no member of the insect tribe—when grown up—has more than six legs, the Centipede or the Millipede may, as their names imply, possess a far greater number—as many, indeed, as two hundred and forty-two! But there is one curious likeness between the legs of the insects and those of their relatives—the number of pairs of legs is always odd. The insect has three pairs; the centipede and millipede have a very variable number, ranging from fifteen to one hundred and twenty-one pairs!
We have seen how wonderful the foot of the fly is, with its two sticky plates for smooth surfaces, and its two claws for rough ones. The Honey-bee has very similar feet, but the two plates are joined to form one! As in the fly, when climbing rough surfaces the flat plates are raised up, and the claws used instead; but when a smooth or slippery place has to be crossed, the claws are pulled backwards and the plates are brought down.
The legs of insects vary much, according to the purpose for which they are used. Thus, the Gnats, which spend the greater part of their time on the wing, have long slender legs, suitable for breaking the shock of alighting. Whilst in other insects the legs are used for all kinds of work, such as seizing prey, carrying it, climbing, digging, and so on. When this is the case the legs are provided with spines, or bristles.
In the Mole Cricket (fig. [1]) the fore-legs are very strong, being short and broad, and ending in a broad comb-like plate, which is used for digging. They are very like the great digging paws of the mole.
The exact way in which insects walk is not easy to describe, and much study has been given to this most puzzling subject. Many devices have been adopted to make the insect draw a map of its course. In one instance the legs of a slow-walking beetle were painted, and the insect was then made to walk upon a clean sheet of paper; the track made by each leg being distinguished by the use of a different colour.
From this and other experiments it appears that there are always three legs in motion at the same time, or nearly so; meanwhile the remaining three legs support the body. First (as in fig. [2]) the left fore-leg steps out, then the right middle-leg and the left hind-leg. Then the movement is taken up by the legs of the opposite side of the body, and so on.
If the movement of the legs in the six-legged insects is difficult to find out, what shall we say when the centipede (fig. [3]) and millipede come to be examined? These, though not insects, are nearly related to the insects, and since they are common in our gardens, must be referred to here.
