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Distributions of oviposition times and intra-clutch oviposition intervals in continuous light (CL) and normal light (NL) environments were studied in 39 populations of 27 different genotypes. The populations showed a range of mean intervals from 23.1 to 27.6 h in CL, arising from the history of selection directly on interval in some populations. The data were analysed to infer empirical relations of descriptive statistics of the distribution to each other and to rate of lay to 301 d of age (PRL). In NL, as the mean intra-clutch interval decreased, more intervals tended to accumulate against the barrier of 24 h imposed by the light-dark cycle and the variability of intervals declined rapidly. Similar, but less striking, relations were found in CL above a mean interval of about 24 h. Under CL, the mean intra-clutch interval (CIM) decreased beneath 24 h and the variability of intervals tended to show a steep increase. The rate of lay to 301 d of age increased linearly at a rate of 6.3% for each hour by which CIM decreased to about 24 h, but no further below that level. Oviposition time in NL was advanced with a decrease in CIM, apparently in two linear phases of differing slopes, perhaps reflecting different physiological bases of the change. The degree of entrainment in NL, measured by the proportion of eggs laid in the model 8 h, increased with reduction of CIM. The distribution of oviposition times in CL showed a great deal of variation among the populations and departed significantly (P less than 0.05) from the uniform rectangular distribution, in all but three populations. The proportion of eggs laid in the modal 8 h of the day, suggested as an indicator of sensitivity to uncontrolled timing cues in CL, was positively correlated with CIM. Free-running periods were estimated for individual pullets from sequences of oviposition times in CL. The mean period for a population declined linearly with CIM, but only when CIM had decreased below about 25.75 h, suggesting that the period of endogenous circadian rhythm might have been altered in some populations with low CIM's. The proportion of eggs laid in the modal 8 h of the free-running period, perhaps an indicator for regularity of the circadian rhythm, was negatively correlated with CIM.

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The effects of variation in larval density and variation in proportions of the two species during development on the fitness components of fecundity, hatchability and adult viability were studied using the Oregon—R—C wild type strain of D. melanogaster and a vermilion mutant strain of D. simulans. In addition, the effects on these fitness components of a restricted period of starvation immediately following eclosion were studied. Thirty—two treatment combinations of species, species frequency, larval density and adult feeding were set up on each of two occasions, 14 days apart. Hatchability data were obtained only for the second time. This variable of time had significant effect on fecundity, which would contribute to the heterogeneity between generations in interspecific competition populations. Significant effects of larval density of fecundity were probably mediated through effects on adult body weight. D. melanogaster females raised in mixed species cultures were less fecund than those from pure cultures, while D. simulans showed the reverse effect. Therefore results in competition cultures cannot be predicted from those in pure species cultures. There were significant differences between the species for hatchability and viability, but these components of fitness showed relative few effects of the imposed treatments. The short—term adult starvation treatment was not as severe as that which occurs in at least some generations in interspecific competition populations. The effects of this and other environmental differences between this experiment and competition populations are discussed.