[a]Fig. 9.]—Upper end of egg of Pieris oleracea, greatly magnified, showing the micropyle.
[a]Fig. 10.]—Egg of Grapta comma, laid in string-like clusters on the under side of leaf. (Magnified.)
The eggs are deposited sometimes singly, sometimes in small clusters, sometimes in a mass. Fertile eggs, a few days after they have been deposited, frequently undergo a change of color, and it is often possible with a magnifying-glass to see through the thin shell the form of the minute caterpillar which is being developed within the egg. Unfruitful eggs generally shrivel and dry up after the lapse of a short time.
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.
[a]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.