To understand this process, it suffices to know that after having lengthened its body to a certain point, this caterpillar can turn back its head on to its back, and reach to the fifth ring, having its three pairs of true legs in the air. But without putting the caterpillar into such an unnatural position, let us take it in a position in which it is simply bent sideways in such a manner that its head, with the thread-spinning apparatus, which is below, can be applied opposite and pretty near to one of the legs belonging to the first pair of membranous legs. Our caterpillar begins by fixing on this point a thread, which is the first of those that are intended to tie it up securely. "This thread," says the illustrious author of the "Mémoires pour servir à l'Histoire des Insectes," "must pass over the caterpillar's body, and be attached by its other end near the leg corresponding to that near which the first end was fastened. To spin the thread the proper length, and at the same time to fix it in its proper place, the caterpillar has only to bring round its head to the fifth segment. The thread will be drawn from the spinning apparatus as the head advances over half the circumference of the circle which it has to describe; and when it has described this, there will only remain for it to secure the second end of the thread against the support. Thus the head, which was at first placed against one of the legs, advances little by little on the outline of the fifth ring as far as to its middle (Fig. 108). It is the facility the caterpillar has of reversing its body that enables it to make its head perform this journey, in proportion as it moves it over the circumference of the ring, it twists its body. And at last, when it has brought it over the top of the segment, its body is exactly folded in two; it draws it little by little from this situation by bending towards the other side, and by causing its head to pass gently over the last quarter of the circle. At last the caterpillar finds itself bound on the second side; the head rests on the thread-covered plane, and the insect fixes the second end of the thread." [Illustration: Fig. 108.--Caterpillars of the Cabbage Butterfly (_Pieris brassicæ_).] It has only to repeat the same manoeuvre as many times as there are threads wanted to make a strong band. But each thread embraces the head, or rather the lower part of the head, for it knows how to make each thread it spins glide into the bend or crease of its neck by a little movement of its head. It must disengage its head from under the band, not a difficult operation. It causes it to slide along the threads near one of the places where they are fixed, and it is then in the position indicated by the foregoing engraving (Fig. 109). [Illustration: Fig. 109.--Caterpillar of the Pieris brassicæ.]