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Thermal Tolerances of Dragonfly Nymphs. I. Sources of Variation in Estimating Critical Thermal Maximum

Thermal tolerance was determined for two species of dragonfly nymphs, Libellula auripennis from a thermal effluent stream, and Macromia illinoiensis from a thermal recovery stream. Tolerances were measured at acclimation temperatures of 16, 24, and 32 C under different laboratory conditions. The major source of variation in thermal tolerance was acclimation temperature, which accounted for approximately one-third of the overall variation. Body size, investigator, rate of heating, species, time of day, and number of individuals per test flask had significant effects on thermal tolerance. Aeration of the test water was not a significant variable. Increasing acclimation temperature increased thermal tolerance in both species. Libellula nymphs acclimated at 16 and 24 C exhibited a daily cycle in thermal tolerance with a peak during midafternoon (1400-1600 h). Libellula nymphs acclimated at 32 C, and Macromia nymphs at all acclimation temperatures exhibited no daily cycle in thermal tolerance.