The increase in tolerance to topically applied chlorinated hydrocarbon insecticides shown by pregnant female tsetse flies was investigated in Glossina pallidipes. Solvent extracts of male and pregnant female flies, traeted with DDT and endosulfan labelled with carbon-14, were analysed by thin-layer chromatography and autoradiography of the developed chromatograms. No metabolites of these compounds were detected in the internal and faecal extracts. It was considered, therefore, that detoxication of absorbed insecticide was not the machanism responsible for the lower susceptibility to insecticide in the pregnant female fly. It was shown, however, by the above technique, that insecticide was absorbed by the in utero larva.
Measurement of labelled DDT absorbed by the larva was carried out by liquid scintillation counting. The results indicated that this insecticide was slowly taken up by the larva, in an amount increasing with time, and it was suggested that inert storage of toxicant in the larva is a pregnant female. This would be in addition to the effects of increase in weight and other possible physiological factors.