Changes in life‐history traits of the water strider Aquarius paludum in accordance with global warming

Seasonal records of nymphs of the water strider Aquarius paludum Fabricius (Heteroptera: Gerridae) at a waterway in Kochi prefecture, Japan (33°00′N to 33°30′N), in 2007, show that populations of new generation adults occur four or more times during the year, in mid and late May; late July to early August; September; and October to November. In laboratory experiments, more than 50% of adults are reproductive, irrespective of photoperiod, and this ratio increases to 80% even under a short‐day LD 11.5 : 13.5 h photocycle, which corresponds to the winter season at Kochi (33°N). There is a diminishing photoperiodic response in relation to the induction of diapause compared with measurements made in previous years (1995–1997), suggesting that reproduction may be possible over a much longer period in the near future (and even in winter) if local climate warming is sustained. A high proportion of males sampled in 2008 have small testes and a high proportion of females do not have mature oöcytes even in summer. This contrasts with earlier data showing that the testis volume index for the same period is greater in 1995–1997, and that 75% of females have mature oöcytes. These changes suggest that the Kochi‐Nankoku population of A. paludum is beginning to show partial aestivation. Overwintering adults of both sexes are found to have no mature flight muscles in October/November 2008. This is in marked contrast with earlier data from 1995 and 2004. Adults that are reared in the laboratory also show a lower flight propensity in response to shorter day lengths. The absence of mature flight muscles in the autumn and lower flight propensity under shorter days may indicate a cessation of dispersal between the summer habitats and overwintering sites on land far from the waterways. Taken together, these data suggest that populations of A. paludum in the Kochi‐Nankoku region are continuing to show adaptive changes, apparently in relation to global warming.