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Caterpillars of Eastern Forests

Life Cycle

Butterflies and moths pass through four developmental stages: egg, larva, pupa, and adult. The larval or caterpillar stage has many vernacular names: inchworm, looper, cutworm, fruitworm, hornworm, silkworm, and even woolly bear. It is the growth stage for Lepidoptera, and thus the daily agenda is simple: eat, eat, avoid being eaten, and eat! The monarch increases its mass by 30,000 times as a caterpillar. In order to accommodate this growth, the larva must molt, or shed, its skin multiple times. The larval stages between molts are called instars. The first instar develops within the egg and lasts until the first molt. In some families there are as few as 3 larval molts, in others more than 15. Most of the species appearing here have 5 larval instars, although in some species (for example, the gypsy moth) it is common for the female caterpillars to go through an extra instar. Presumably this enables females to put on more mass for reproduction and ensures that males will be on the wing before females emerge.

Significant differences in larval behavior, morphology, and coloration may occur between successive instars. Young gypsy moth larvae remain in the canopy both day and night. Beginning in the fourth instar, the caterpillars descend to the boles of trees by day and move back into the canopy at night to feed. Yet even this seemingly stereotyped behavior is modified under high densities, so that late instar larvae may remain in the canopy foliage and feed both night and day. Some dagger moths undergo striking color changes. The cherry dagger is green and red with a reddish head capsule in the fourth instar, but then molts into a stunning black and scarlet caterpillar in its last larval instar. Early instars of spicebush swallowtail, mottled with black and white, closely resemble a bird dropping. Radical changes accompany the molt to the fourth instar when the species takes on a snakelike appearance. Unlike the early instars that lay exposed on leaves, these older larvae use silk to fashion a leaf shelter in which they rest. In general, we have tried to illustrate common color forms of late instar larvae.

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