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Cocoon Construction By the Cecropia Silkworm Ii. the Role of the Internal Environment

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The Cecropia cocoon consists of outer and inner envelopes separated by a loose intermediate layer. Each envelope is invariably spun and, once spun, cannot be repeated. Silk that is spun as a flat sheet is equivalent to silk actually invested in envelope formation. The spinning of the inner envelope begins when 60 to 70 per cent of the silk has been spun. Silkworms with their silk glands removed performed figure-8 movements of the head, resembling the corresponding type of movements during spinning and the "visual" movements of non-spinning larvae. A characteristic feature of the normal behavior is the animal's periodic turning around of the entire body during spinning. The time interval between turns appears to be a function of the amount of silk which has passed out of the spinneret, after approximately 60 per cent of the silk has passed out of the animal the time between turns increases. Although silkworms continue this turning after the spinneret has been blocked, removal of the silk glands eliminates it. Therefore, it appears that the presence of the silk glands provides the stimulus for the initiation of spinning movements. The silk glands grow at a constant rate throughout the final instar and silk synthesis continues during spinning. We regard the intermediate layer of the cocoon as a structure corresponding to the strands attaching the outer envelope to the spinning site, rather than as an independent and separately determined layer. We conclude that the internal environment plays an important role in both the genesis and temporal regulation of the behavior pattern during the construction of the cocoon.

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


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