The period of time requisite for the development of the embryo in the egg varies. Many butterflies are single-brooded; others produce two or three generations during the summer in temperate climates, and even more generations in subtropical or tropical climates. In such cases an interval of only a few days, or weeks at the most, separates the time when the egg was deposited and the time when the larva is hatched. When the period of hatching, or emergence, has arrived, the little caterpillar cuts its way forth from the egg through an opening made either at the side or on the top. Many species have eggs which appear to be provided with a lid, a portion of the shell being separated from the remainder by a thin section, which, when the caterpillar has reached the full limit allowed by the egg, breaks under the pressure of the enlarging embryo within, one portion of the egg flying off, the remainder adhering to the leaf or twig upon which it has been deposited. [Illustration FIG. 11.--Eggs of _Vanessa antiopa_, laid in a mass on a twig.] CATERPILLARS _Structure, Form, Color, etc._--The second stage in which the insects we are studying exist is known as the larval stage. The insect is known as a larva, or a caterpillar. In general caterpillars have long, worm-like bodies. Frequently they are thickest about the middle, tapering before and behind, flattened on the under side. While the cylindrical shape is most common, there are some families in which the larvæ are short, oval, or slug-shaped, sometimes curiously modified by ridges and prominences. The body of the larvæ of lepidoptera consists normally of thirteen rings, or segments, the first constituting the head. [Illustration FIG. 12.--Caterpillar of _Papilio philenor_ (Riley).] [Illustration FIG. 13.--Head of caterpillar of _Papilio asterias_, front view, enlarged.] [Illustration FIG. 14.--Head of caterpillar of _Anosia plexippus_, lower side, magnified 10 diameters: _lb_, labrum, or upper lip; _md_, mandibles; _mx_, maxilla, with two palpi; _lm_, labium, or lower lip, with one pair of palpi; _s_, spinneret; _a_, antenna; _o_, ocelli. (After Burgess.)]