Geographical variation of life cycle in crickets (Ensifera: Grylloidea)

Crickets are convenient systems for studying life-cycle evolution. They show a considerable diversity in life-cycle types, being homodynamic in some parts of the tropics and heterodynamic with various kinds of diapause and other regulatory responses in the temperate region. Crickets are relatively free from constraints by food supply, because they are omnivorous. Therefore, their geographical variation may clearly reflect climatic selection as exemplified by latitudinal dines in adult size, egg size and ovipositor length. These morphometric dines are closely related to development time, egg diapause and photoperiodic responses, indicating that crickets are highly variable within the framework of their species-specific patterns of life cycle. More fundamental variation is divergence in life-cycle pattern that may be associated with speciation, because closely related species are often different in their life-cycle patterns (e.g., homodynamic versus heterodynamic, or egg overwintering versus larval overwintering). Evolutionary flexibility of life-cycle traits may be assessed by studying cricket populations under special natural or artificial conditions. In Japan, a population of the nemobiine cricket (Dianemobius mikado) introduced from a temperate to a subtropical island has almost lost egg diapause presumably in the last 130 years. At volcanic geothermal spots in the northern island (Hokkaido), presumptive relict populations of the nemobiine species Dianemobius nigrofasciatus occur and sing in mid-winter, but this cricket retains the photoperiodic response of a southern bivoltine type. In the subtropical nemobiine D. fascipes, artificial selection has been effective in changing the incidence and depth of diapause, and strains comparable to the tropical and temperate forms were established. The responsiveness to photoperiod in wing-form determination was also remarkably changed by selection, but this change was not associated with a change in critical photoperiod. Alternated selection in the opposite directions at photoperiods above and below the critical value to be selected was necessary to shift the critical photoperiod