Differences in the Critical Thermal Maximum Between Two Size Classes of Stenonema femoratum (Ephemeroptera: Heptageniidae)

Temperature is an important variable affecting the behavior and survival of aquatic organisms; however, little is known about the effects of size and corresponding developmental differences on aquatic insect temperature tolerance. We tested the critical thermal maximum (CTM) of large (head capsule width mean = 3.5 mm) and small (1.9 mm) specimens of Stenonema femoratum (Say) by raising the experimental temperature by 0.1, 0.3, and 0.5ºC per minute during laboratory CTM trials. Size class and temperature increase rate combinations were randomized, and each combination was tested over four trials, for a total of 24 trials of four specimens each. Two-way Analysis of Variance determined CTM was significantly higher for small specimens (35.2°C) compared to large specimens (33.0°C) regardless of temperature increase rate. There was no significant difference in CTM based on temperature increase rate, or interactions between size class and temperature increase rate. This result demonstrates that the highest thermal tolerance of a species is not necessarily it its largest specimens, and that determining a single CTM for a species based on multiple size classes would likely lead to erroneous results