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Dana Michaud, age 12, of St. Catharines, Ont., Canada, for her question:

How long does a caterpillar stay in its cocoon?

Some of the best little spinners of fine silk are the spiders, but spiders are not insects. Many members of the insect world are silk spinners, and some of them use their silken threads for cocoons in which to wrap themselves. Some insects encase their eggs in woven silk, and some caterpillars build tents of silken threads.

A large number of insects sleep through the pupa stage: wrapped in cozy blankets of woven silk. Each variety has its own pattern for making the cocoon, its own method of hiding the cocoon and its own time for emerging as an adult insect. The peach tree borer pupates in its cocoon for about a month in early spring. The beautiful polyphemus moth usually stays cuddled in his cozy cocoon all through the winter.

The cocoon building caterpillars go through a life cycle of four distinct stages. They begin as eggs, which may hatch after a few warm summer days or remain unhatched through the winter. From the eggs come hungry caterpillars, and these grubby larvae must devour enough food to last them almost all their lives. When the caterpillars have eaten their fill they become pupae. This is the stage at which the cocoon building insects weave their silken blankets.

Inside their protective cocoons the caterpillars are changing into adult winged insects. Hatching time is very important, for the weather must be warm and sunny, so most cocoons hatch in early spring. Many insects lay two or more broods of eggs during the summer season and go through three or four of their life stages in a month or two. In some cases, the last batch of eggs sleeps through the winter, and in some cases, the insects spend the winter as cocoons.

Timing is very important in the changing from one stage of the life cycle to the next. Caterpillars must hatch when and where there is plenty of their proper food. The woolly bear caterpillar feeds on plantain and grasses that can be found through the winter. The fuzzy fellow weaves a fat fuzzy cocoon in early spring and emerges as a pretty Isabella moth after about one month. The fat green polyphemus caterpillar cannot find his favorite food in the winter. He weaves his little barrel cocoon in fall and does not emerge as a beautiful brown moth until spring.

Adult insects with large wings tend to be quite delicate, and only a few of them can survive the winter. Caterpillars are sturdier, so if there is plenty of food to be had many of them dine happily through the cold season. A silken cocoon provides protection from the cold and from prowling enemies. As a rule, the warm blanket is disguised to look like its surroundings, and hungry birds fail to notice it. The insect may stay in this protected stage for perhaps a week, a month or through four or five bleak months of winter.


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