The changes in carbohydrate content, fatty acid content, and nitrogen distribution are described in detail during the metamorphosis from larva to adult in the sheep blow-fly, Lucilia sericata Meigan. Carbohydrate decreases rapidly until the thirteenth day, after which it remains constant. Glycogen is present. The fatty acid content decreases rapidly until the ninth day. A synthesis of fatty acid now occurs, reaching a maximum on the fourteenth day, after which a decrease sets in until emergence

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... of the adult. The amount of saturated fatty acid present remains constant, the unsaturated fatty acids only being utilised. The course of histolysis and histogenesis is reflected in the nitrogen distribution curves. The initial decrease in insoluble protein correlated with a rise in peptone is associated with the formation of the hard chitinous puparium. Continued decrease of the insoluble protein accompanied by a fall in peptone and an increase of soluble protein continues until histogenesis of the imaginal thoracic muscles commences. The insoluble protein now abruptly rises and the soluble protein shows a corresponding decrease. Formation of excretory-N occurs mainly during histolysis.