1. Fat contents (ether extracts) of unflown 24 hr. old alatae of Aphis fabae Scop., from different host plants, range from 3-12% of the live weight and 9-33% of the dry weight. Glycogen contents of alatae reared in culture range from 0.5-1% of the live weight and 1.7-3.4% of the dry weight.

2. Both fat and glycogen are consumed during tethered flight. Glycogen is used during early flight and fat is the principal fuel after the first hour, when it is consumed at a mean rate of 0.005 mg./aphid/hr. and provides about 90% of the energy for a 6 hr. flight. The amounts of glycogen in laboratory-reared aphids alone could not maintain flight for more than ¾ hr.

3. Metabolic rates during tethered flight range from 52 to 66 cal./g. live wt./hr. or 11-14 ml. O2/g./hr. The flight muscles constitute about 13% of the live body weight of 24 hr. old aphids, and, attributing most of the metabolism during flight to these muscles, their metabolic rates range from 400 to 500 cal./g./hr.

4. Flight capacity of 24 hr. old aphids at 25-26° C. is directly related to initial fat content and varies between 3 and 8 hr. in aphids from culture (mean of 4% fat by live weight) and between 7 and 12 hr. in aphids from the field (10% fat). Flight fatigue occurs before all the fat reserves are used; possibly fat stored in parts of the body remote from the flight muscles cannot be mobilized rapidly enough to support continuous flight.

5. Fat and glycogen reserves occur mainly in the fat-body cells of the thorax and abdomen; fat also occurs between the fibrils of the indirect flight muscles and glycogen along the surface of the fibres. Flight-exhausted insects have little or no fat in the thorax, but small deposits remain in the abdomen; little glycogen can be detected in culture aphids flown to exhaustion.

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