The author gives the results of a survey made in about two years from late July, 1952 through late September, 1954 on the diurnal and seasonal succession of the natural population of Drosophila inhabiting a latifoliate forest in Anjo City, Aichi Prefecture. The collections were made with traps baited with fermenting fruits. In the collections made at regular intervals, the author obtained 17266 specimens belonging to 21 species(Table 1). The following four types of diurnal activity(Fig. 1)were discriminated : A : With a high peak in the morning and a low peak in the evening. This is the commonest type of diurnal activity. B : With two peaks at sunrise and sunset. D. auraria. C : With a peak in the evening. D. coracina. D : With a peak in the midday. D. subtilis. The following four types of seasonal activity (Fig. 2) were recognized : Spring-autumn type : D. lutea, D. auraria, D. rufa, D. transversa(D. nigromaculata), D. sp. 1 of robusta group, D. sutlilis and D. immigrans. Summer type : D. coracina. Autumn type : D. suzukii(D. pulchrella), D. melanogaster and D. bizonata. Winter type : D. sp. 1 of immigrans group. The seasonal change in the composition of the natural population was investigated, and distinguished according to the following criteria (Fig. 3) : total number of the collected specimens, number of specimens of the dominant species, number of species, percentage of specimens of the dominant species, variance of percentages of the individual species, constants "a" and "b" of the formula of geometrical series, PARETO constant (α) and correlation coefficient of population between the successive periods. The following phases were recognized : 1. The hibernating phase(early November through early May) : Population of low density monopolised by D. sp. 1 of immigrans group. 2. The first active phase(early May through early August) : Population of high density dominated by D. lutea and D. immigrans with mixture of D. auraria. 3. The summer resting phase(early August through late August) : Population of low density dominated by D. auralia in the morning and evening, and by D. corarina in the daytime. 4. The second active phase(late August through early November) : Population of high density consisting of many species, the dominant species being D. melanogaster, D. lutea, D. auraria and D. bizonata. From these findings the author considers that in the animal population composed of closely related species, if they are highly motile and broadly adaptable, co-existence of them in the same environment as well as habitat-segregation, is very common.
