The pupa has no orifices to the alimentary canal or sexual organs, but the respiratory openings are pervious. It has no means of locomotion, but it can move a certain number of the posterior segments (the number variable according to kind). In some cases it is provided with spines, "adminicula," by means of which, aided by the wriggling movements of the abdominal segments, considerable changes of position can be effected. The pupae of the genus _Micropteryx_ apparently use the legs for locomotion, as do the pupae of Trichoptera. The study of the pupa of Lepidoptera is less advanced than that of the imago and larva, between which it is, in many points of structure, intermediate.[191] The interior of the pupa contains a {328}quantity of cream-like matter, including the results of histolysis—but this, as well as the condition of the internal organs, differs much according to whether the change from the caterpillar to the moth is much or little advanced. Many pupae are protected by cocoons. These are masses of silk—very various in form—disposed by the caterpillar around itself during the last stage of its existence. Some of these cocoons are so perfect that the moth has considerable difficulty in escaping when the metamorphosis is complete. Various devices are used for the purpose of emergence; the Puss-moth excretes a corrosive fluid, containing potassium hydroxide, and then protects itself from this by retaining on the head while passing through it a shield formed of a portion of the pupa-skin.[192] Lepidopterous pupae usually have the body terminated by a projection of very various and peculiar form called "cremaster." In certain cases these projections are used for the suspension of the pupa, and are then frequently provided with hooks (Fig. 177, C, D). In other cases the cremaster is frequently called the anal armature (Fig. 205, B). [Illustration: Fig. 168—Wing-rudiments of _Pieris brassicae_. A, Rudiments of a wing before the first moult of the caterpillar: _ce_, embryonic cells; _ch_, external cuticle; _h_, hypodermis; _o_, opening of the invagination; _tr_, trachea. B, posterior wing-rudiment of full-grown caterpillar; _b_, semicircular pad; _c_, a bundle of the rolled tracheae; _e_, envelope; _i_, pedicel; _tr_, trachea. (After Gonin.)]