By using these and several of the pro-legs immediately before them, the caterpillar can cling and erect the front part of the body so that it can strike from side to side when disturbed.
And while the gurnard has no less than six of these pro-legs, the American land fish has only a single pair with which to accomplish his arduous journeys.
This is the reverse of recapitulation: in the case of legless maggots, and caterpillars with pro-legs, the adult is more similar to the ancestor than the larva. But the same principle holds, that where functions and habits are different, there organs are different.
The thorax is covered by the first three rings behind the head, and on it are six legs, two on each segment. The remainder of the caterpillar is abdominal and carries small pro-legs with which to help it cling to twigs and leaves, and the heavy anal props that support the vent.
They are little conical or cylindrical papillæ, closely resembling the pro-legs of caterpillars, and are composed of two or three joints, the terminal one of which is pierced with a greater or less number of minute holes, the sides of these, in some, if not all, cases, being prolonged into tubes.
The only rational conclusion is that legless larvae have lost the disuse, since those larvae which are destitute of legs do not go in search of food but either live in the midst of it or are fed by others, and that the pro-legs of the caterpillar have been developed by the muscular action of the insect in clinging to leaves.