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Cocoons
Nature Bulletin No. 468   October 27, 1956
Forest Preserve District of Cook County
Daniel Ryan, President
Roberts Mann, Conservation Editor
David H. Thompson, Senior Naturalist

COCOONS
From now until early spring is the time to hunt for cocoons. There are many kinds to be found in various places: attached to branches or twigs, under loose bark of dead trees, among fallen loaves, beneath stones and rotting logs, on the bottom edges of window sills and overlapping siding on houses, or inside outbuildings such as garages and sheds. A board lying on the ground is likely to hide a cocoon or the papery cocoon-like egg sacs of spiders. Cocoons should be kept in a cool moist place. It will be interesting to see the kinds of adult insects that emerge from them in spring or early summer.

Adult female insects lay eggs. Wormlike larvae hatch from the eggs of most kinds of land insects and these, when full-grown, become helpless pupae. By one of the most mysterious and wonderful processes in nature, a pupa is transformed into au adult. This four-stage life cycle is called complete metamorphosis.

The larvae of many species merely retreat to some secluded place, or bury themselves in the ground, where they become pupae. With the exception of the Skipper butterflies, all butterfly pupae -- often called chrysalides -- are not enclosed in a cocoon but are attached to a leaf or twig by a silken disk or a band of silk. The larvae of the well-known Monarch butterfly feed on milkweeds and you may find one of the exquisitely beautiful pupae dangling, "like a green house with golden nails", from a milkweed leaf. Such chrysalides, too, may be collected and kept until the adults emerge.

The larvae of most moths and many other kinds of insects, including ants and fleas, build cocoons around themselves and it is in those chambers that the changes, from larva to pupa and from pupa to adult, take place. Cocoons are usually made of silk spun from two glands filled with a thick glue-like material. This is pressed out in two slender threads that stick together as they emerge from an opening or "spinneret" on the larva's lower lip. In the air they harden into a tough silk fiber. It is fun to watch a larva spin its cocoon, weaving its head back and forth in a comical figure-eight looping motion.

Hairy caterpillars, such as the familiar Woolly Bear which is a larva of the Isabella tiger moth, merely use silk as a woof to hold together a warp of hairs from their bodies. Some caterpillars roll up in a leaf and fasten the edges together with silk. Many wood-boring larvae make cocoons largely of chips, and those which undergo transformation in the ground may include earth in the walls of their cocoons. The ant-lion larva uses grains of sand. The shells of many cocoons are fibrous and dense; others, like those of saw-flies, are like parchment or even tissue paper; and a few kinds are lacelike in texture.

The conspicuous cocoon of the Cecropia, largest of our giant silkworm moths, is shaped like a hammock and fastened, lengthwise, close to the underside of a branch or twig. It has two walls of silk with an insulating mat of loose silk between them. Youngsters frequently find that one and also the long slim cocoon, wrapped in a leaf, of the Promethea -- most common of the giant silkworm moths. The leaf stem is fastened to a branch by a band of silk and this cocoon is commonly mistaken for a dead leaf.

Adult insects escape from cocoons in various ways. Those with biting mouthparts can gnaw their way out. Others must soften one end with "spit" and push through the fibers. But the Cecropia and Promethea larvae provide an "escape hatch" -- a conical valve-like arrangement at one end of the cocoon.

That instinct is almost as miraculous as the change from pupa into adult.


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