Metabolic conversions during pupation of the cecropia silkworm. 1. Deposition and utilization of nutrient reserves

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Abstract
Cecropia silkworms taken at intervals during the prepupal period were analysed quantitatively to secure evidence on metabolic conversions. The loss of total dry weight falls within the limits calculated from the rate of respiration, assuming a non-protein respiratory quotient corresponding to catab-olism of mixed carbohydrate and fat. Amounts of lipids decrease steadily during the prepupal period. Those of glycogen and free sugars decrease during spinning, then show an apparent increase about the time of the pupal moult. The amount of chitin, as well as total weight of cuticle, decreases sharply just before the molt. It is thus possible to account for the synthesis of glycogen and sugars by conversion of substances from the cuticle without invoking conversion of lipids. Larvae were injected with [1- 14C]glucose just before their 4th molt. By the end of instar V, the retained label was almost exclusively in the cuticle. Then, during the pupal molt, isotopic carbon was transferred to glycogen, sugars, and lipids. Chitin in the newly synthesized pupal cuticle has a high specific activity similar to that of the larval cuticle. Specific re-use of materials from the latter is suggested.