Carbohydrate and Fat Metabolism in Adult Lepidoptera

Abstract

The results of the present investigation can be summarised as follows :—1. Carbohydrate metabolism in females of all the species studied (except Operophthera brumata) shows a definite dependence on the period of maturation. The greatest quantity of sugar is consumed during the first 4–5 days after emergence. During the period of gonad maturation both the body weight and the oxygen consumption are increased. During the period of oviposition the oxygen consumption is increased once more, but the sugar consumption remains at a low level till death, while the body weight decreases.2. Sugar (glucose and saccharose) is completely digested when taken in solutions of 5–40 per cent. concentration ; the increase of the body weight during the period of maturation is due to fat synthesis for the formation of egg-yolk. Moths fed on concentrated sugar solutions (of about 20–40 per cent.) show an increased fat content not only before the period of oviposition but even after laying a large quantity of eggs (over 1,000 in Agrotis segetum). In starved moths a nearly complete disappearance of fat was observed at the end of life.3. The quantity of eggs deposited by starved Agrotis females is greatly reduced ; but such reduction is much less marked in some females of Loxostege sticticalis and almost negligible in Pyrausta nubilalis. Eggs deposited by starved Agrotis females do not complete their embryonic development, the embryos dying before hatching. Agrotis females fed on water only or with 5 per cent. glucose solution produce such “sterile” eggs only to some extent (about 40–50 per cent.). A sufficient food supply (20–40 per cent. solution of glucose) causes a complete absence of “sterile” eggs.4. The daily readings of the gaseous metabolism 8–10 hours after feeding, for the females fed on glucose, showed an inconsiderable difference as compared with those for starved moths. The respiratory quotient was variable and sometimes very low (0·50–0·60), but on the average somewhat higher than in starved females ; the oxygen consumption showed great variability also and increased in the average only.5. The readings of the gaseous metabolism directly after feeding showed definite changes. The respiratory quotient was highly increased by a great increase in carbon dioxide production ; it reached an average of 1·50–1·60 and a maximum of 2·09. Oxygen consumption was not considerably increased.6. Changes in the respiratory quotient show a peculiar metabolic phase after carbohydrate nutrition which is due to fat synthesis. The fat synthesis of carbohydrates results in a decrease in oxygen consumption, because the oxygen liberated by the fat synthesis reaction is used.7. Carbohydrate metabolism in adult Lepidoptera is highly specialised. The most common type in insects is represented by the combined direct use of carbohydrates and partly by the synthesis of fat ; usually fat synthesis causes no great change in the respiratory quotient. In adult bees the importance of fat synthesis is reduced and the metabolism is restricted to the direct use of carbohydrates ; in the adult Lepidoptera, on the contrary, the direct use of sugar is not developed.8. The biological importance of the specialisation of carbohydrate metabolism in adult Lepidoptera is evident. Fat is used partly to form the egg-yolk, and therefore carbohydrate nutrition is directly connected with fertility. On the other hand, by using a large quantity of sugar solution (sometimes more than 40 per cent. of the body weight) the live weight could be rapidly reduced owing to fat synthesis.