Nymphal density and life histories of two water striders (Hemiptera: Gerridae)

We used laboratory experiments to study the effects of rearing density on nymphal duration, dispersal dimorphism, and reproduction of two wing-dimorphic water strider species, Gerris buenoi Kirkaldy (wing morphs macropterous and micropterous) and Gerris pingreensis Drake and Hottes (mainly apterous amd macropterous), that specialize, respectively, in temporary and permanent habitats. First-instar nymphs from field-collected or laboratory-cultured parents were reared with superabundant food at high (2900 or 3100 first-instar individuals/m(2)) or low (145 or 155 first-instar individuals/m(2)) starting density under a photoperiod of 19L:5D and at 20 +/- 2 degrees C. Average nymphal period of both species under high density was significantly shorter by 4-5 d than under low density. There were no significant differences in percent survival between densities in either species. In G. pingreensis all new adults were apterous and entered reproductive diapause. In contrast, both wing morphs were expressed in G. buenoi; however, a lower proportion of macropters emerged under high density than under low density. All females of G. buenoi tested were reproductively active, regardless of wing morph. There was no effect of density on preoviposition period for G. buenoi of either wing morph, although preoviposition period was about twice as long in long-winged as in short-winged females. In G. buenoi, nymphal crowding promotes development of nonflying reproductives, which allocate nutritional resources to rapid reproduction rather than dispersal, at least when food conditions are sufficient. In G. pingreensis, nymphs that develop faster under crowding may be less subject to cannibalism and intraguild predation.