The life cycle of a southern hemisphere cricket Bobilla poene (Orthoptera: Gryllidae) and variation in its size, sex ratio and egg production

Abstract

AbstractLife cycles of southern hemisphere crickets are poorly studied compared with their northern hemisphere counterparts. The life cycle and associated traits of Bobilla poene Otte & Alexander (1983) (Nemobiinae) were investigated based on pitfall trap samples collected monthly for 12 months at lowland (320 m) and montane (800 m) locations in Tasmania, the southernmost state of Australia (42–43°S). Unlike most other Gryllidae, B. poene has a semivoltine life cycle overwintering as an egg during its first winter followed by nymphs and then adults in subsequent winters. There was some evidence to suggest that a small proportion of the populations completed development from egg to adult within 1 year (i.e. univoltine). The proportion of adult females with eggs present was high (>70%) for all months except December when there was a greater proportion of adult females with no eggs present. Mean number of eggs per female was largest from mid‐summer to early autumn. Spermatheca diameter increased in size from December to May. Aspects of B. poene life cycle differed between locations. At the lowland location, most crickets (92%) overwintered as A3 instars and adults, whereas at the montane location, most crickets (87%) overwintered as A3 and A4 instars. Head width, femur length, body length and spermatheca diameter were larger at the lowland location than at the montane location, but forewing and ovipositor lengths did not differ in size between locations. A small proportion (1.2%) of montane adult crickets was long winged, and none were recorded at the lowland location. Sex ratios were generally similar at both locations and varied throughout the year. For most of the year, the sex ratios for the A2, A1 and adult stages were female biased. During spring and early summer, sex ratios were male biased or close to parity. This variation in sex ratios is consistent with the concepts of protandry and adaptive growth, with males maturing earlier than females and at a smaller size. The range of climates and altitudes available in Tasmania, where B. poene occurs widely, offers opportunities for further investigation of life cycle variation within this species.