Butterflies Worth Knowing

by Clarence Moores Weed

Egg of Baltimore Butterfly, much magnified.
(From Holland)

Caterpillars are like snakes in at least one respect: in order to provide for their increase in size they shed their skins. When a caterpillar hatches from the egg it is a tiny creature with a soft skin over most of its body but with rather a firm covering for its head. While we might fancy that there could be a considerable increase in size provided for by the stretching of the soft skin it is easy to see that the hard covering of the head will not admit of this. So the story of the growth of a caterpillar may be told in this way:

A butterfly lays an egg upon a leaf. Some days later the egg hatches into a larva, which is the technical name for the second stage of an insect's life. In the case of the butterfly we call this larva a caterpillar. The little caterpillar is likely to take its first meal by eating the empty egg shell. This is a curious habit, and a really satisfactory explanation of it seems not to have been made. Its next meal is likely to be taken from the green tissues of the leaf, commonly the green outer surface only being eaten at this time. The future meals are also taken from the leaf, more and more being eaten as the larva gets older.

After a few days of this feeding upon the leaf tissues the little caterpillar becomes so crowded within the skin with which it was born that it is necessary to have a larger one. So a new skin begins to form beneath the first one. Consequently the latter splits open in a straight line part way down the middle of the back just behind the head. Then the new head covering is withdrawn from the old one and the caterpillar wriggles its way out of the split skin and finds itself clothed in a new one. At first all of the tissues of the new skin are soft and pliable and they easily take on a larger size as the body of the caterpillar expands. A little later these tissues become hardened and no further expansion is possible.

This process of skin-shedding is called moulting. The cast skin is often called the exuviae. The period of the caterpillar's life between the hatching from the egg and this moult is often called a stage or instar—that is, the caterpillar up to the time of this moult is living in the first caterpillar stage or instar.

During the actual moulting the caterpillar is quite active in freeing itself from the exuviae. But as soon as it is free it is likely to rest quietly for some hours while the tissues of the new skin are hardening. Then it begins feeding upon the leaf again and continues taking its meals at more or less regular intervals for several days. By that time it will again have reached its limit of growth within this second skin and the process of moulting must be repeated. It takes place in the same way as before and the caterpillar enters upon the third instar of its larval life.

This process of feeding and moulting is continued for several weeks, the number of moults being usually four. During the later stages the increase in size is more marked each time the skin is shed, until the caterpillar finally reaches its full growth as a larva and is ready for the wonderful change to the quiet chrysalis in which all its caterpillar organs are to be transformed into the very different organs of the butterfly.

In the case of butterfly larvae one of the most interesting features of the growth of the caterpillar is that of the remarkable changes in colors and patterns of marking which the caterpillar undergoes. One who had not followed these changes would often be at a loss to recognize caterpillars of slightly differing sizes as belonging to the same species. These changes commonly show a remarkable adaptation to the conditions of life, and generally tend to the concealment of the caterpillar upon its food plant. The stages of growth of the green-clouded swallowtail caterpillar are illustrated on plate opposite [page 80].

Before each moult the caterpillar is likely to spin a silken web upon the leaf surface. It then entangles its claws in the web to hold itself in place while the skin is cast. (See plate, [page 17].)