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For one stock ofD. simulans and two stocks ofD. melanogaster the percentage of females that produced viable progeny (i.e. percent fertility) was measured at two female ages (50 and 72 hrs), for all combinations of six densities (20, 40, 80, 160, 320, 640 individuals) and three species frequencies (20, 80, 100%). Significant differences were recorded between species and betweenD. melanogaster stocks. Increasing density significantly reduced fertility forD. simulans but not for either of theD. melanogaster stocks. Although species frequency had no effect, the associated stock ofD. melanogaster influencedD. simulans fertility to a high degree-theOr-R-C stock reducingD. simulans fertility because of physical interference, this interference being particularly evident at low densities.

Survival (viability) of newly eclosed adults ofD. simulans st and D. melanogaster Or R-C, which had excess quantities of dead yeast available throughout life, was measured daily for 7 days in an experiment where adult density (6 levels), species frequency (6 levels) and a2p tissue content (2 levels) were varied factorially. A separate experiment comparing viability in different types of experimental unit also was done. Similar experiments were done for D. simulansst competing against D. melanogastery w. The entire data were subjected to a least-squares (unbalanced, missing-plot) analysis of variance.D. melanogaster Or R-C had a higher average viability than D. melanogastery w or D. simulans st, which were equal. The competing strain of D. melanogaster influenced D. simulans st viability -viability being higher when y w was the competitor. Viability decreased over the seven day period but at different rates for the three strains. Increasing density reduced viability for all three strains, but species frequency effects, although significant, were generally not consistent. Females had higher viability than males in both D. mdanogaster strains, but the reverse was true in D. simulans st. 32p .lowered viability and experimental unit type altered viability. Numerous inter. actions were significant.Adult density was shown to have a delayed effect on viability -the delay before the appearance of the effect (an increase in death rate) being decreased as density rose. The term 'variably delayed density dependent' has been adopted to describe the fitness component, adult viability. Some high density populations showed a readjustment (a decrease) in their death rate as a reaction to the effect of reduced density caused by high early mortality.

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The number of eggs laid in aDrosophila culture and the survival of these eggs may depend on the number of larvae that are still inhabiting or that have already used the culture medium.A known average number ofD. simulans st larvae (designated original inhabitants) were introduced into culture vials by allowing adults to lay for 24 h (low density) or 48 h (high density). On each day for 14 days, adults of three competitor strains (D. melanogaster Or-R-C, D. melanogaster yw andD. simulans st) were added to different samples of these vials and allowed to lay for 24 h. The numbers of "effective eggs" (eggs expected to be laid less those withheld, cannibalised or buried) produced by competitor strains were estimated from adult emergences. Survival of original inhabitant larvae to the adult stage also was measured.At the lower density of original inhabitants, the mean proportions of effective eggs (number of effective eggs/number of eggs expected in an uninhabited culture) were not significantly different for the three competitor strains. The mean proportions were lower at the higher density (significantly so for the twoD. melanogaster competitors), and at this density, the mean proportion forD. melanogaster yw was significantly less than that forD. simulans st. These results are consistent with a simple egg destruction hypothesis, but suggest thatyw females were retaining more eggs at the higher density.Original inhabitants showed higher survival when at the higher density. Each of the three competitors caused a significantly different reduction in original inhabitant survival, which was directly related to competitor larval activity. Increasing larval activity probably reduced survival of original inhabitants by increasing pupal mortality due to drowning in the medium.Day of introduction of competitors influenced survival of original inhabitants and also the proportion of effective eggs from each competitor. The proportion of effective eggs decreased to the day 5 introduction. From day 5 to day 8, the proportion increased because the original inhabitants were pupating. After day 8, effective egg proportions again decreased, possibly due to inhibition of egg-laying or reduced survival of immature stages.