Continuous water flow and static test apparatus were used in evaluating the influence of acclimation temperature, size, sex, season, life stage, current velocity, and testing method upon the upper-lethal temperature of the nymphal stage of the stonefly, Paragnetina media Walker.
Lethal temperature was raised by acclimation to higher temperatures throughout the thermal range examined. The rate of gain of heat resistance was approximately 5°C per day. Acclimation to colder temperatures was much slower.
Body size had no significant effect on upper-lethal temperature in summer or autumn, whereas in winter and spring larger nymphs were significantly less tolerant than smaller nymphs. This difference was attributed to sex, not body size per se.
Tolerance was significantly less in static waters than in flowing water, faster current significantly increasing heat resistance at all lethal temperatures examined.
Upper-lethal temperatures showed seasonal differences independent of acclimation, with nymphs least tolerant in spring, when temperature resistance was influenced by sex, molting condition, and proximity of emergence. The most temperature-sensitive stage was the period immediately following egg hatching.
The direct effect of upper-lethal temperature is apparently not a limiting factor for P. media in the environment in which it is found. Temperature increases do, however, effect final stages of nymphal development, emergence patterns, and survival after hatching.