EXPERIMENTS ON THE EFFECTS OF FOOD AND DENSITY ON VOLTINISM IN A STREAM-DWELLING WATER STRIDER (AQUARIUS-REMIGIS)

1. The stream water strider Aquarius remigis shows a latitudinal. patt ern of variation in voltinism. In general, populations with shorter gr owing seasons (e.g. in eastern Canada) tend to be univoltine (animals that reach adulthood in the summer overwinter before reproducing in th e following spring), whereas populations with somewhat longer growing seasons (e.g, in the north-eastern United States) tend to be bivoltine . 2. This pattern was broken at our study site in the south-eastern Un ited States (Kentucky) where A. remigis had a long growing season, but was almost always univoltine. In summer 1993, however, adult A. remig is in central Kentucky displayed a bivoltine reproductive cycle; i.e, individuals in some pools began breeding shortly after maturing to the adult stage. 3. A field survey documented a negative relationship bet ween local water strider density and reproductive activity in prediapa use adults. A laboratory experiment manipulating food availability and density, revealed that animals held at low density with high food lev els displayed greater mating activity and egg production than did thei r counterparts at higher density or lower food levels.4. A laboratory experiment also showed that high water strider density resulted in a g reater frequency of very short pair durations (< 10 min). 5. Although the observed effects of density and food availability on mating activi ty of prediapause adults seem intuitively reasonable, they differ from the patterns observed in overwintered adults. The difference in repro duction patterns might be due to differences in selective pressures on prediapause vs. post-diapause adults.