Food utilization and metabolic efficiency in larval and adult house crickets

Abstract

The most rapid growth of the fat body, ovaries and residual body occurred only at the time of maximal feeding, which was the time of highest metabolic rate. Larvae ceased feeding before the next moult and lost weight. Adult females did not cease feeding and never lost weight. Mated females maintained a high feeding rate that corresponded to a constant oviposition rate with no gonotrophic cycles. Virgin females maintained a lower feeding rate and maintained fully mature eggs in a viable state for life. Growth during the last larval stadium was entirely somatic and characterized by a great accumulation of a lipid rich (65%) fat body. Net growth during the first 10 days of adult life was entirely gonodal, because the small amount of residual body growth was matched by the amount of fat body loss. However, since the residual body was 60% protein and the fat body was 60% lipid, some of the somatic protein growth had to come from the diet and some of the fat body lipid went for ovarial growth. The 20% dietary protein was more than sufficient for all somatic and ovarial growth, and some protein was catabolized for energy. The amount of uric acid in larval faeces accounted for almost all protein catabolism, but in adult females half of the catabolized protein nitrogen did not appear in the faeces. With only 5% dietary lipid both larvae and adults had to synthesize additional lipids from carbohydrates to supplement absorbed lipids for growth demands. Almost all absorbed and synthesized lipids were used for somatic growth in the larvae and for ovarial growth in the adults, because the per cent of food calories used for growth greatly exceeded the per cent dry weight of food used for growth. Based on the calories and dry weight of available fuel, almost all energy production was based on carbohydrate oxidation. Some carbohydrate was used for lipid synthesis but very little was used for growth. Probably the absorption efficiency (72%) and growth efficiency (28%) was the same for both the last instar larvae and virgin females because they ate exactly the same food. However, the larval metabolic efficiency (42%) was higher than that of females (37%), which indicated that more of the absorbed food was converted to tissue during larval growth (somatic) than during adult growth (ovarial). Of the dry weight of food eaten during the last larval stadium, 30% was egested, 32% was oxidized, 28% appeared as growth, and 10% was condensed to lipid and became part of the 28% growth. Of the dry weight of food eaten during the first 10 days of adult life, 27% was egested, 41% was oxidized, 25% appeared as growth, and 6% was condensed to lipid.