Critical Thermal Maxima of Nymphs of Three Plecoptera Species from an Ozark Foothill Stream

Prostoia completa, Isoperla namata and Phasganophora capitata nymphs captured from a second order stream in northeastern Oklahoma were exposed to a steady rise in temperature of 0.34°C/min to determine their critical thermal maxima (CTM). At a collection temperature of 6.5°C, the univoltine, winter emerging P. completa had a significantly lower mean CTM (31.5°C) than the univoltine, spring emerging I. namata and semivoltine, summer emerging P. capitata which had mean CTMs of 33.2 and 33.4°C, respectively. A one month age difference in I. namata (February to March) had no significant effect on mean CTM, and neither head capsule width nor sex of nymphs had a significant effect on the CTM within any of the three species. Phasganophora capitata nymphs collected at 6.5°C and acclimated to 17.0°C had a significantly higher mean CTM (36.3°C) than those maintained at the 6.5°C collection temperature (33.4°C). Isoperla namata treated similarly did not show a significant increase in temperature tolerance. Therefore, it appears that adaptation to summer emergence or the evolution of a semivoltine life cycle by P. capitata includes a greater ability for tolerance acclimation than found in I. namata.