It has been shown that adults of Glossina morsitans Westw. that have fed from a bait ox in their natural environment digest their blood meals more rapidly than others that have emerged and been fed in the laboratory, even when both are maintained under identical environmental conditions after feeding.

In further experiments with G. morsitans in Rhodesia, flies caught in the field and fed in the laboratory were found to lose their ability to digest their meals rapidly. Measurements, made throughout three hunger cycles, of the rate of digestion, as reflected in the rate of excretion, of blood meals by field-caught flies fed on guineapigs in the laboratory showed that this was not significantly different from that of the normal, flied-fed flies during the first two hunger cycles but that during the third it fell to a level comparable to that of flise that emerged and were fed in the laboratory.

It is concluded that whatever may be the events that condition the field flies to digest their meals rapidly in the natural environment, these are repeated with the ingestion of each meal, and that laboratory conditions cause a rapid loss of this greater digestive capability.