How do water striders, Aquarius paludum, react to brackish water simulated by NaCl solutions?

Several stages, from eggs to adults, of the water strider, Aquarius paludum (Fabricius), inhabiting fresh water are sometimes conveyed by heavy flow in the rainy or typhoon seasons in Japan to lotic brackish water in the mouth of rivers. The water striders might then respond to salinity either by remaining to wait for extensive rainfall to reduce osmotic pressure locally before reproducing (“breed here and later tactic”) or by flying away to reproduce in fresh waters elsewhere (“breed elsewhere and later tactic”). All first instars died before the first molt when they were exposed to 1.75 and 3.5% NaCl solutions in a laboratory experiment. Living on 0.5 and 0.9% solutions through larval and adult stages slowed down larval growth and suppressed female reproduction. When exposed to the 0.5 and 0.9% solutions, 90 and 92% of males, respectively, showed histolysis of their flight muscles. Therefore, in brackish natural habitats, larvae and adults seem to follow the strategy “breed here and later.” When water striders were exposed to 0.9% solution either just after emergence or 20 days later, females showed a higher flight propensity than those kept on fresh waters throughout, and they delayed the deposition of eggs. Therefore, when conveyed to brackish water after emergence by stream flow after heavy rain, adults seem to leave the area by flight, demonstrating the strategy “breed elsewhere and later” tactic. We conclude that water striders use alternative tactics for responding to salinity, depending on the stage of exposure.