Climatic adaptation and species status in the lawn ground cricket : II. Body size.

The cricket tentatively identified as Pteronemobius taprobanensis shows a saw-toothed pattern of variation in adult size along the latitudinal gradient of the Japanese Islands. A slight but abrupt increase in adult size at about 28° N indicates the replacement of the subtropical form by the temperate one. The body size of the latter slightly decreases north to about 33° N, then conspicuously increases to about 39° N and again decreases to the northern extreme. This pattern of variation seems to be related to the local adjustment of nymphal development by means of the photoperiodic response and genetic variation, since the adult size varies as a function of the duration of nymphal development. Multiple regression analysis of the variance between local populations reared at various photoperiods suggests, however, that the decrease in body size due to selection for shorter development in cooler climates is to a certain extent counteracted by selection for a higher rate of growth. The latter component of climatic selection is possibly due to greater egg production by larger-sized females within the shorter reproductive season. This hypothesis may also account for the divergence in size between the two climatic forms of this nominal species.