The Utilization of Reserve Substances in Drosophila During Flight

Abstract

The chief reserve substance in Drosophila is glycogen. This forms dense deposits in the cells of the fat body and the haltere knobs. It is distributed throughout the indirect flight muscles as minute granules in the meshwork of sarcoplasm that fills the space between the fibrils and the sarcosomes. Somewhat larger masses lie along the surface of the fibre bundles and around their insertions. There is a large deposit in the proventriculus and small amounts in the cells of the mid-gut. The visible deposits of fat are much smaller and are confined to the fat body and the mid-gut cells. During starvation, glycogen and fat are consumed concurrently. At the time of death (2-3 days) glycogen has disappeared completely, save in the thoracic muscles; only minute droplets of fat remain in the fat body. In the insect which has flown to exhaustion (4-5 hr. in the mature fly) there is no apparent reduction in the stored fat. Glycogen is greatly reduced in all the deposits, but has disappeared completely only in the flight muscles and the proventriculus. ‘Exhaustion’ of the flying insect supervenes when the glycogen can no longer be mobilized rapidly enough to meet the metabolic demands of the flight muscles. Flight can be resumed for a brief period after the exhausted fly has rested; and the duration of flight increases with the duration of rest. By observing the duration of flight after giving a measured quantity of sugar to the exhausted insect it is shown that 1 µg. of glucose will maintain D. melanogaster in flight for an average of 6.3 min. The efficiency of substances as sources of energy for flight has been compared by giving them to the exhausted fly. Glucose will restore the capacity for continuous flight within 30-45 sec. of the commencement of feeding. Fructose, maltose, sucrose, etc., require a little longer. Galactose, xylose, etc., will allow repeated brief flights but will not support uninterrupted flight. Mannitol, glycerol, etc., merely increase the duration of flight after a standard period of rest. Lactose, sorbose, etc., have no effect. Glycine and alanine actually diminish the capacity for flight. It is suggested that the apparent failure of fats, etc., to support flight in Drosophila is due to the comparatively slow rate of their metabolism. It is suggested that the deposits of glycogen in the haltere knob may serve to increase the inertia of the haltere and so its efficiency as a gyroscopic sense organ.