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The two families of dipteran glutathione S-transferases (GST-1 and GST-2) were located separately by immunohistology on sections of adult houseflies. GST-1 was distributed in haemolymph cells, whereas GST-2 was found in the indirect flight muscles of the thorax and in the central nervous system. In the muscles, the distribution of GST-2 seemed to be uniform in cells, whereas in the brain and the thoracic ganglia GST-2 was found mainly in the cortical areas which are made up by cell bodies. Comparison of the GSTs' location between an insecticide susceptible strain of housefly and resistant ones indicated no variation due to resistance. Enzyme-linked immunosorbent assay tests were used to dose GST-2. In houseflies, there were 60 pmol of GST-2/fly, 80-90% being found in the thorax, about 10% in the head and the remainder in the abdomen. Furthermore, the roles of these GSTs are discussed in relation to their location and our knowledge on their catalytic activities or their transport ability in invertebrates and mammals.

The metabolism and rate of penetration of leptophos (O-methyl O-4-bromo-2,5-dichlorophenyl phenylphosphonothioate) was determined in a susceptible strain and a strain of housefies which was 50-fold resistant to leptophos. Penetration of leptophos into resistant flies was substantially slower than into susceptible flies but large differences in metabolism, both quantitatively and qualitatively, were not observed. No difference was observed in the sensitivity of flyhead and thorax acetylcholinesterase to leptophos-oxon in vitro, and tolerance to leptophos by the resistant strain is explained in terms of decreased rates or penetration and minor differences in metabolism.