About ten days after the egg is laid it hatches into a small black caterpillar marked in a characteristic fashion with a blotch of white in the middle of the body which is suggestive of a saddle. The caterpillar immediately begins to feed upon the green substance of the leaf, continuing thus about a week before the first moult. At this time it does not change much in appearance, still being a spiny creature blackish in color and marked by the curious white saddle. A little later it moults again, retaining its original coloring. At each moult, of course, it gets larger and feeds more freely upon the celery or other plant on which it may happen to be.
When the caterpillar becomes about half grown it takes on a very different appearance from that of its early life. The skin is smooth rather than spiny, and the general colors are green, black, and yellow. The ground color of the skin is green, which is marked with black cross-bands along the middle of each body ring. On these bands there are many large dots of orange yellow, the whole coloring giving the insect a very striking appearance, especially when it is placed by itself against a plain background. When they finally become full grown in this larva state, these caterpillars are almost two inches long.
The larvae of the Black Swallowtail have certain characteristics in which they differ from many other caterpillars. After each moult they do not devour their cast skins, which happens in the case of many of their relatives. When feeding, as well as when resting, they remain exposed upon the leaf and seem never to attempt to conceal themselves, as is the habit with a large proportion of caterpillars. It is probable that this instinct for remaining exposed to view bears some relation to the curious means of protection possessed by this as well as other Swallowtail caterpillars. When disturbed one of these larvae will push out from just back of the head the strange-looking, orange-yellow Y-shaped organ which gives off a very disagreeable odor. These osmateria organs are generally believed to be defensive against the attack of birds and various other enemies, although they seem not to be effective against insect parasites.
The full-grown caterpillars are likely to leave their food plants when ready to change to the chrysalis state. They wander in various directions until suitable shelter is found. A piece of board, a fence post, or possibly the bark of a tree will answer for this purpose. Here the caterpillar spins a mat of silk in which to entangle its hind legs and a short distance away near the front end of the body it spins a loop of silk attaching the ends to the support. These serve to hold the chrysalis in place during this helpless period. After the loop is made the caterpillar keeps its head through it so that the loop holds the insect in position a short distance back of the head. It is now ready to moult its last caterpillar skin and become a chrysalis.
One who has watched hundreds of these caterpillars go through this change, Miss Mary C. Dickerson, describes the process in these words: "In this final moult the chrysalis has to work very hard. The bulk of the body is again slipped forward in the loosened caterpillar skin, so that this becomes tensely stretched over the anterior end, and very much wrinkled at the posterior end. The skin splits back of the head and is forced back by its own taut condition and by the efforts of the chrysalis, until only the extreme posterior end of the chrysalis is within it. Then the chrysalis withdraws this posterior end with its many very tiny hooks, from the skin on the dorsal side, and, reaching around, securely fastens the hooks into the button of silk. Then the old skin is removed both from its fastening to the chrysalis and from its attachment in the button of silk."
A short time after the caterpillar's skin has thus been cast off the chrysalis takes on a brownish color which as is so often the case is likely to vary somewhat according to the tint of the surrounding surfaces. This is doubtless a protective device and helps the insect to escape attack by birds during the long period of exposure. For this butterfly passes through the winter only in the chrysalis condition, and the larva which went into the chrysalis in September does not come out as a butterfly until the following May or June. There are, however, two broods of the butterflies in the North and at least three in the South. As the adults live for about two months and there is considerable variation in the periods of their development it happens that one can find these Black Swallowtail butterflies upon the wing almost any time in warm weather, either North or South.
The Giant Swallowtail
Papilio thoas
The largest of our North American butterflies is a magnificent insect with a wing expanse of some four inches and with a rich coloring of black and yellow more or less suffused with greenish or bluish iridescence that gives it a striking beauty as it flies leisurely about from flower to flower or stops to lay an egg upon some bush or tree. The tails are long and expanded toward the tip, their prevailing color being black with a broad splash of yellow near the end. In a general way we may say that the upper wing surface is black marked with two bands of orange-yellow, while the under surface is yellow marked with two bands of black. (See plate, [page 64].)
The Giant Swallowtail is a tropical species which is abundant throughout the Southern states and during recent years seems to have been gradually extending its northern range. It is now commonly found as far north as forty-two degrees latitude, from Nebraska eastward. In New England it is occasionally taken in Connecticut, Massachusetts, and even in Maine, but its appearance in this region is exceptional.
In the orange-growing regions of the Southern states the caterpillars of this butterfly feed freely upon the leaves of citrus fruits and they are often called "orange puppies" or "orange dogs." Probably their curious appearance and their habit of resting for long periods upon leaf or twig gave rise to this name. In the region indicated the life-history of the insect may be summarized thus: