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

In biochemical studies of insect metamorphosis, much attention has been given to the deposition and utilization of nutrient reserves, and it has been inferred that insects may carry out net conversion of fat into carbohydrate (cf. Deuel & Morehouse, 1946; Buck, 1953; Chauvin, 1956). Since vertebrate animals are not at present known to possess any pathway for the net conversion of fatty acids into carbohydrates, the existence of such a pathway in a major invertebrate group would be of great interest. In micro-organisms and higher plants, the utilization of fat and metabolically related substances such as acetate for carbohydrate synthesis is well established (Kornberg & Krebs, 1957). The occurrence of such conversion in the nematode, Ascaris, has also been indicated though the mechanism has not been elucidated (Passey & Fairbairn, 1957). In insects, however, none of the evidence adduced to support net conversion of fat into carbohydrate is unequivocal. Values of respiratory quotients below 0-7, suggesting oxidation of substrates to products other than carbon dioxide, have frequently been recorded for insects (see Buck, 1953). The measurement of respiratory quotients in insects, however, is complicated by several factors, e.g. the release of carbon dioxide in periodical bursts (Schneiderman & Williams, 1953) and the excretion of bicarbonate (Brown, 1938). The earliest assertion of the conversion of fat into carbohydrate in an insect was based on the observation in silkworms (Bombyx mori) that, after the cessation of food intake, glycogen increased concomitantly with decrease in lipids (Couvreur, 1895). The alleged increase in glycogen during pupation of Bombyx is supported by Vaney & Maignon (1905), Bialascewicz (1937) and Zaluska (1959), though not by others (Kotake & Sera, 1909; Yonezawa & Yamafuji, 1935). Increases of carbohydrate during the prepupal or pupal periods, i.e. non-feeding stages, have been reported also for other insect species: an unspecified blowfly (Frew, 1929), the blowfly, Phormia regina (Haub &