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Warming-Induced Changes in Life-History Traits of Water Striders
Reference
Harada, T., Takenaka, S., Maihara, S., Ito, K. and Tamura, T. 2011. Changes in life-history traits of the water strider Aquarious paludum in accordance with global warming. Physiological Entomology 36: 309-316.

Background
The authors write that "in temperate areas, insects are adapted physiologically or behaviourally to either [1] tolerate seasonally fluctuating changes in temperature through diapause or to [2] avoid adverse conditions through migration," with the most predictable time cue for seasonal adaptation by insects being photoperiod (Tauber et al., 1986). Thus, they state "it is assumed that the critical photoperiod of insects is becoming gradually shortened as a result of global warming," and that if populations are adapting to the consequent longer growing seasons and later onsets of winter, it would be expected that the number of yearly broods produced "should increase, that photoperiodic responses of diapause induction and wing-form determination will continue to diminish ... and that overwintering adults will cease to migrate between water courses and overwintering land sites far from water, and will begin overwintering nearer summer habitats."

What was done
In an effort to determine if such adjustments have been occurring in the life cycle of nymphs of the water strider Aquarius paludum in the Kochi prefecture of Japan over the past two decades, Harada et al. measured a number of pertinent parameters that had been assessed by Harada and Numata (1993) two decades earlier, over the period 1989-1991.

What was learned
The five researchers report that A. paludum nymphs were trivoltine (i.e., produced three broods yearly) in 1991, but that more recently the generation number appears to have increased to four or more. They also found that overwintering adults of both sexes had no mature flight muscles in October and November of 2008, which "contrasts with previous observations," and they observed that "the 2008 population also shows a low flight propensity in response to shorter day lengths."

What it means
The Japanese scientists, all from Kochi University, opine that "the absence of mature flight muscles in the autumn, and the lower flight propensity under shorter days, may comprise evidence of a cessation of dispersal between the freshwater summer habitats and overwintering sites on land far from the bodies of water." And they add that "the increase in daily-minimum temperature during the winter in the Kochi-Nankoku area over the last 15 years may allow adults of A. paludum to overwinter without dispersal nearer to their summer habitat," which could lead to the proportion of adults overwintering close to the water bodies increasing from the current estimate of 60-70% to between 90 and 100% within ten years. Thus, they conclude that A. paludum populations in the Kochi-Nankoku region "are continuing to show adaptive change, apparently in relation to global warming."

References
Harada, T. and Numata, H. 1993. Two critical daylengths for the determination of wing forms and the induction of adult diapause in the water strider, Aquarius paludum. Die Naturwissenschaften 80: 430-432.

Tauber, M.J., Tauber, C.A. and Masaki, S. 1986. Seasonal Adaptations of Insects. Oxford University Press, New York, New York, USA.

Reviewed 7 March 2012