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Cocoon Construction By the Cecropia Silkworm I. the Role of the External Environment

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The Cecropia silkworm spins a stereotyped cocoon consisting of an outer and an inner envelope, separated by a spongy intermediate layer. Each envelope has a localized weakness at its upper end which serves as an escape valve for the emergence of the adult moth. The valves are oriented by gravitational and tactile stimuli. Late in the last larval instar the silkworm ceases to feed, empties its gut, and then wanders for several hours before spinning begins. During this period of wandering the caterpillar will pass by locations suitable for cocoon construction. The caterpillar may be forced to spin in a variety of unlikely situations where the construction of a normal cocoon is impossible. Two mutually exclusive movement patterns, the stretch-bend and the swing-swing, are responsible for the spinning of the envelopes. Periodically, one of these patterns stops, the animal turns around, and the second pattern begins. If the animal is prevented from turning around, only the stretch-bend movement pattern, which is responsible for spinning the valve end of the cocoon, is observed. In abnormal environments the prevention of turning may force the silkworm to spin a cocoon shaped like a cylinder rather than a cone, or a cocoon from which the animal excludes itself. Double cocoons, which are occasionally found in nature, are shown to be expressions of the normal movement patterns under the abnormal condition of crowding. It is concluded that the behavior patterns are compounded of reflex-like responses to external stimuli and of "emitted" behavior independent of external stimulation.

Affiliations: 1: The Biological Laboratories, Harvard University, Cambridge, Massachusetts


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