Fig. 109.—Caterpillar of the Pieris brassicæ.
It has only to repeat the same manœuvre 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]).
Fig. 110.—Pupæ of Pieris brassicæ.
About thirty hours after the caterpillars have succeeded in making themselves fast, they have completed their transformation into chrysalides ([Fig. 110]), where the chrysalis of the above-mentioned caterpillar is seen in two different positions, and held by the same band which first supported the caterpillar.
Those caterpillars which construct cocoons make them of silk and other substances. These cocoons are, for the most part, oval or elliptical, sometimes boat-shaped, and ordinarily white, yellow, or brown in colour. The threads may very slightly adhere together, or be closely united by a gummy substance with which the caterpillar lines the interior of the cocoon, and which it expels from the anus. Some cocoons are composed of a double envelope, others are of a uniform texture. Some are of a tissue so close that they entirely hide the chrysalis contained within; others form a very light covering, through which the chrysalis can be easily perceived ([Fig. 111]).
Fig. 111.—Cocoon, after Réaumur.