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

We used laboratory experiments to study the effects of rearing density on n ymphal duration, dispersal dimorphism, and reproduction of two wing-dimorph ic 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 permanen t habitats. First-instar nymphs from field-collected or laboratory-cultured parents were reared with superabundant food at high (2900 or 3100 first-in star individuals/m(2)) or low (145 or 155 first-instar individuals/m(2)) st arting density under a photoperiod of 19L:5D and at 20 +/- 2 degrees C. Ave rage nymphal period of both species under high density was significantly sh orter by 4-5 d than under low density. There were no significant difference s in percent survival between densities in either species. In G. pingreensi s all new adults were apterous and entered reproductive diapause. In contra st, both wing morphs were expressed in G. buenoi; however, a lower proporti on of macropters emerged under high density than under low density. All fem ales of G. buenoi tested were reproductively active, regardless of wing mor ph. There was no effect of density on preoviposition period for G. buenoi o f 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 un der crowding may be less subject to cannibalism and intraguild predation.