Almost all insects undergo great changes, or metamorphoses, during their existence. Butterflies furnish no exception to this statement. They exist first as eggs; then they appear as caterpillars; the third stage is that of the chrysalis; the final stage is that of the imago, or perfectly developed insect.

THE EGGS OF BUTTERFLIES

The eggs of butterflies are beautiful objects when examined under a glass. They have various forms. Some are spheres or half spheres, some are conical, cylindrical or spindle-shaped, others are flat and resemble little cheeses, and still others have the form of turbans. There is endless variety of form displayed within certain limits. Their surfaces may be quite smooth or they may be adorned with raised ribs and sculpturings (see [Plate C], Figs. f and g) or marked with little pittings or depressions arranged in geometrical patterns. They vary in color. Some are white, some pale green, or blue-green; others are yellow, orange, red, or purple. They are often spotted and marbled like the eggs of some birds.

The eggs of butterflies are deposited by the female upon the plants which are appropriate to the development of the larvæ. Caterpillars are very rarely promiscuous feeders, and most species are restricted to certain species or genera of plants. Even when they feed upon different plants, observation shows that, having begun to feed upon a certain plant, they prefer this to all others, and do not willingly accept anything else. I have noticed frequently that larvæ which may for instance feed in nature upon the wild plum or the lilac, having begun to feed upon the one will steadily refuse the other if offered to them. On several occasions I have lost broods of caterpillars by attempting to change their diet, though knowing well that the species is found feeding in nature upon the plants which I have offered to them. Almost every plant has a butterfly or moth which is partial to it, and one of the most wonderful things in nature is the way in which the female butterfly, without having received a botanical education, is able to select the plant which will best meet the needs of her progeny, which she never lives to see.

The eggs are deposited sometimes singly, sometimes in small clusters, sometimes in a mass. Fertile eggs, soon after they have been laid, undergo a change in color, and it is then possible with a magnifying glass to see through the thin shell the form of the caterpillar which is being developed within.

When the development is completed the caterpillar emerges either from an opening at the side or at the top of the egg. Many species have eggs provided with a sort of lid, a portion of the shell being separated from the remainder by a thin section, which finally breaks under the pressure of the enlarging embryo within, this portion flying off, the rest adhering to the twig or leaf upon which it has been placed. Many larvæ have the habit, as soon as they have emerged from the egg, of making their first meal upon the shell from which they have just escaped.

CATERPILLARS

The second stage in which the insects we are studying exist is known as the larval stage. When it is reached the insect is spoken of as a larva, or caterpillar (see [Plate C], Fig. h). Caterpillars have long, worm-like bodies, which are often thickest about the middle, tapering before and behind, and more or less flattened on the under side. Sometimes caterpillars are oval or slug-shaped. Very frequently their bodies are adorned with hairs, spines, and tubercles of various forms. The body of the larva, like the body of the butterfly, consists normally of thirteen rings or segments, of which the three foremost, just behind the head, correspond to the prothorax, the mesothorax, and the metathorax of the perfect insect, while the remaining nine correspond to the abdomen of the imago. These three anterior segments bear legs, which correspond to the legs of the winged form in their location, and are known as the true legs of the larva. Besides these the caterpillar has about the middle of the body and at its posterior end paired pro-legs, as they are called, which are its principal organs of locomotion in this stage, but which do not reappear in the butterfly. The mouth parts of caterpillars are profoundly different from those of the butterfly. The imago lives, as we have seen, upon fluid nourishment, and therefore is provided with a sucking organ, the proboscis. The caterpillar, on the other hand, is armed with a pair of cutting mandibles, with which it shears off tiny strips of the leaves upon which it feeds. It holds the edge of the leaf in place with the three pairs of true legs, while it supports its body upon the pro-legs during the act of eating.

EXPLANATION OF PLATE C

Fig. a. Caterpillar of Anosia plexippus ready to change into a chrysalis.
Fig. b. Do. after having partly shed its skin.
Fig. c. Do. holding itself suspended in the air by grasping the shed skin between the edges of the third and fourth abdominal segments, and feeling about with the cremaster for the button of silk above.
Fig. d. Do. after having caught the button and assumed its final form as a chrysalis.
Fig. e. Chrysalis of Papilio philenor, held by button and girdle of silk.
Fig. f. Egg of Basilarchia disippus, greatly magnified.
Fig. g. Egg of Anosia plexippus, greatly magnified.
Fig. h. Caterpillar of Basilarchia disippus.
(All the figures are after Riley.)