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The present investigation was carried out to determine whether the susceptibility of the pregnant female tsetse fly to DDT and other chlorinated hydrocarbon insecticides could possibly be influenced by the presence in the uterus of the developing larva. Burnett (1962) has shown that pregnant females of Olossina morsitans Westw. exhibit an appreciable increase in tolerance to topically applied DDT compared with males and virgin females, and has suggested the possibility of detoxication of absorbed insecticide by the developing larva (Burnett, 1963). The increased tolerance that he found was greater than that which could be accounted for solely by increase in weight.Owing to the difficulty of obtaining sufficient numbers of laboratory-reared tsetse flies during this work, it was decided to use wild-caught G. pallidipes Aust., since large numbers of this species, containing a proportion of pregnant females, can usually be obtained by netting around a vehicle moving through its habitat. Materials and methodsTest insects. Wild-caught G. pallidipes from the Lake Manyara area of northern Tanganyika, were used for this work, and catching was carried out over a short period of time to minimise the possibility of fluctuations in susceptibility due to seasonal changes. All flies were given a blood meal on return to the laboratory and were used for experiment 24 hours after feeding. Only those females containing a large third-instar larva were used throughout the investigation. The presence of a third-instar larva was revealed by the dark appearance of the two larval respiratory lobes, visible beneath the integument of the female fly, on the ventral surface and close to the anus.Toxicity tests. Dosage/mortality determinations were carried out on 79 males and 93 females by topical application of unlabelled p-p' DDT in acetone solution from a 1 p.1 capacity Drummond " Microcap " pipette. The insects were anaesthetized with CO 2 for treatment and held for 24 hours before mortalities were recorded. The temperature was maintained at 26°C. during treatment and the holding period.Metabolism of DDT and Endosulfan. It was decided to determine first of all whether the increased tolerance of the pregnant females to DDT could be due to the metabolism of the latter to a less toxic substance. Batches of 50 to 100 male and pregnant female flies were treated with 0-2 fig. per fly of dichlorodi(phenyl-C14(U)) trichloroethane * in benzene solution. The labelled compound had a specific activity of 4 mc/mM. At the same time, a similar quantity of insects were treated with -01 ^g. per fly of labelled endosulfan (1,2,3,4,7,7,-hexachlorobicyclo 2,2,1,2-heptene 5,6-bis(hydroxymethylene-C14) sulphite of specific activity of 0-32 mc/mM. The insects were held individually * U-denotes statistically uniform labelling in a carbon-14 compound.

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