But there are two other ways in which larvæ prepare for the state of _pupa_, which are so curious, and prevail to such a large extent among insects, that we may be excused for following the example of all authors upon insect history, and treating our readers to the interesting facts which have been recorded by various observers upon this subject. One of these is the plan of hanging themselves up in the air, like sailors in their hammocks; and in other positions, this plan is commonly called the _suspension_ of the larva. The other is the formation of beautiful cases of various kinds, which are called _cocoons_, in which they wrap themselves up close during the period to be devoted to the pupa state. Let us speak of these each separately; and, first, of the case, or _cocoon_, as a place of concealment for the larva. It is right, however, to premise, that, while the greater number of insects are in a torpid or half-torpid state while in the pupa form, all are not so, as will be noticed in the next chapter; and those which are not do not of course require the protection of a cocoon, or the contrivance of a silken hammock. The material of which the case or cocoon is composed is a beautiful silken fibre, which is furnished by a spinning apparatus placed either at the extremity of the body, or more frequently at the mouth of the insect, whence it proceeds in a filament of the utmost delicacy and transparency, but at the same time of considerable tenacity. By its means alone cocoons, of a number of different kinds, some of extraordinary beauty and ingenuity, are constructed; but in other cases the insect has recourse to various materials to assist in giving strength to the walls of its cell. [Illustration: _Cocoon of Silk-worm._] The best example, and the most familiar one we can select of a purely silken cocoon, is that of those "----spinning worms That in their green shops weave the smooth-haired silk."