Life histories of stoneflies (Plecoptera) in the Rio Conejos of southern Colorado

Thirty-one stonefly species representing eight famihes were collected during the March 1987 to May 1990 study period.Genera represented by more than one species included Capnia.Utacapnia, Taenionema.Siiuallia, Triznaka, Isogenoides, and Isoperla.Peak species richness was recorded on or near the summer solstice in 1988 and 1989.Climatic differences between years were reflected in nymphal development and emergence phenology of most species.New or important corroborative life histoiy data are presented for 1 1 stonefly species of this assemblage.The hyporheic nymphal development of most chloroperlid species limited the number of early instars sampled and our capacity' to inteipret voltinism.Limited nymphal data suggested a univoltine-slow cycle for Phimiperia diversa (Prison).Adults of Suwallia pallidula (Banks) and S\ uardi (Banks) were present for an extended summer period, but the bulk of their respective emergence times was temporally separated.Isogenoides zionensis Hanson, Pteronarcella hadia (Hagen), and Pteronarcijs califomica Newport were all showTi for the first time to have a 9-10-mo egg diapause, and all three species have a semivoltine life cycle.Skwcda americana (Klapalek) and Isoperla ftdva Claassen were further confimied to have univoltine-slow cycles.Univoltine-fast and univoltine-slow life cycles are reported for the first time in /. phalerata and /. qiiinquepunctata, respectively.Regression analysis revealed that si.x of the eight abimdant species had extended emergence patterns (slopes of <5%/d), while only two had synchronous patterns.Warmer spring and summer temperatures in 1989 increased the slopes for five of the eight species studied, but did not change their synchrony designation.Nine of 11 abundant species advanced their median emergence date in 1989 over 1988.This and the higher slope values are consistent with a hurried nymphal development and narrower emergence period due to the warmer thermal regime of 1989.