SummaryStrains of the housefly MUBOO domeatica L., derived by selection from the Canberra laboratory colony established in 1939, were examined genetically and cytologically to determine their composition, in respect to resistance to DDT, and the modes of sex determination and inheritance of this resistance.DDT resistance was found to be determined by an incompletely dominant allele of a gene in chromosome II that confers the ability to metabolize DDT to DDE. In flies of the normal karyotype, 2n = 12: XX females and XY males, resistance is inherited independently of sex, but in atypical XX males, lacking the Y -chromosome, the male determinant is linked with presumably the same resistance allele in one of the chromosome II homologues. The resistance gene in this homologue is thus obligatory for, and confined to, atypical males, but on rare occasions is inherited by a female, indicating that the linkage with the male determinant is not completely stable.Atypical males were not detected in the unselected colony. They were brought into prominence by selection either for early adult emergence or for DDT resistance. Under DDT selection pressure, the progression of one strain towards homozygosity for the resistance allele was retarded slightly by the complete replacement of XY males by the atypical XX type, and the strain was still heterogeneous after 200 generations of selection.Each of 10 lines propagated from single homozygous resistant pairs became heterogeneous when mass-reared in the absence of DDT. Instability of the resistance gene may therefore have been a factor opposing selection in the strain under DDT pressure.Rare XXY males and XXX females among flies of the normal karyotype, and rarer XO and XXX males among the XX males of atypical strains, were considered to be products of non-disjunctions. No XO females were found.

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Selection for early maturation applied to a laboratory colony of Musca domestica L. eliminated autosomally controlled DDT�resistance from both sexes, but a proportion of the males exhibited a genetically new type of resistance which was shown to be not transmitted through the females but to involve the y-chromo� some. By a single selection with DDT, applied to males only, the early-maturing strain was separated into two true-breeding strains omogeneous in both sexes with respect to DDT-tolerances, the one susceptible to DDT in both males and females, the other susceptible in females but showing at least an eightfold resistance to DDT in all its males.

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Potentiation in joint action was demonstrated between solutions of L-leucine and sodium phosphate buffer (pH 6.3) as feeding stimulants for protein-deprived females of the house fly, Musca domesticu L. Both components alone elicited feeding. In two-choice feeding tests, mixtures consisting of equi-stimulating concentrations of the two components were taken in greater quantities than either component alone at twice the concentration in the mixture.The presence of 1x10-' M phosphate buffer markedly lowered the threshold for detection of L-leucine. The presence of phosphate buffer strengthened the preferences shown by flies given choices of concentrations of L-leucine differing by a factor of 2 and enabled them to display preferences at lower concentrations. M L-leucine increased, somewhat, the ability of flies to detect low concentrations of phosphate buffer. Its presence had relatively little effect on the strength of preference shown between two-fold differences in concentration of phosphate buffer when the higher concentration was 6 . 3~ M or less, but markedly strengthened the preferences when the higher concentration was 2.5X lo-' M or greater. Leucine increased the optimal concentration of phosphate buffer by a factor of more than 2 and converted 2 X lo-' M phosphate buffer from a mild feeding deterrent to a powerful feeding stimulant.The presence of 1 X