抄録
The seasonal variation in the population density of earthworms in alluvial soil grassland was studied, in Sapporo, by twice-a-month sampling for two consecutive years. Six species, Allolobophora japonica, A. caliginosa, Pheretima hupeiensis, P. hilgendorfi, P. agrestis and P. sp. were obtained. Among them A. japonica was dominant, accounting for 62.5% of the total number, and P. sp. subdominant, these two species making up more than 99% of the total number. The variation in the population density of the total number in the first one-year period, from May, 1965 to April, 1966, appeared more or less similar to that in the second one-year, from May, 1966 to April, 1967, but it was somewhat greater in the latter. The density was high in autumn, especially in October, and low in winter, especially in January and February. No regular variation in the vertical distribution was noticeable, but the number of earthworm in 0-10 cm below the ground surface was larger in summer than in winter. In August, 1966, two individual of A. japonica, probably diapausing with empty alimentary tract, were found in 20-30 cm below the ground surface, coiling themselves in their own capsules.
