The band-legged ground cricket group comprises a pair of very closely related species; Dianemobius fascipes occurs widely over tropical and subtropical regions of Southeast Asia, while D. nigrofasciatus is restricted to temperate regions. A similar situation is found in the lawn ground cricket group; the tropical/subtropical species D. taprobanensis is replaced by D. mikado in temperate regions. In both southern species, tropical strains were almost homodynamic, but they produced a few delayed eggs independently of photoperiod and a variable proportion of macropters in response to photoperiod; subtropical strains showed clearer responses. Both northern species controlled egg diapause, nymphal development and wing form by various photoperiodic responses. The two species groups showed parallel geographical profiles of size variation, and the adults and eggs were smaller in the southern than in the northern species. In the absence of strong climatic selection for larger growth rate, size increase in tropical regions would be counteracted by predation pressure. The optimal adult size thus seems to be smaller in tropical than in temperate regions, being determined by the geographically variable balance between counteracting selecting forces.