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. [Illustration: Fig. 110.--Pupæ of Pieris brassicæ.] 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). Among caterpillars that make a very slight cocoon, some, as the _Catocalas_, gather together two or three leaves into a ball, to protect them. Others strengthen their cocoons, and render them opaque by adding earth or other substances, often obtained from their own bodies. Some, after having spun their cocoon, cast forth through the anus three or four masses of a matter resembling paste, which they apply with their head to the inside of the cocoon, and which, drying quickly, becomes pulverulent. Others employ for the same purpose the hairs with which their bodies are covered. [Illustration: Fig. 111.--Cocoon, after Réaumur.] [Illustration: Fig. 112.--Larvæ of Catocala fraxini.] [Illustration: Fig. 113.--Larva of Acronycta aceris.] The larva of _Acronycta aceris_ (Fig. 113) is covered with tufts of yellow hair. Réaumur made these caterpillars work under his own eye in glass vases. They make the layer which is to form the exterior surface of their shell, or cocoon, of pure silk, and when it is thick enough, tear out their hair, now from one place, now from another. But we will leave the illustrious observer to relate this operation himself, which must without doubt be painful to the poor animal:--