Thermal tolerances and preferences of the crab spiders Misumenops asperatus and Misumenoides formosipes (Araneae, Thomisidae)

Misumenops asperatus (Hentz 1847) and Misumenoides formosipes (Walckenaer 1 837) are diurnally-active, flower-dwelling crab spiders (Thomisidae) common ly inhabiting open fields. In laboratory experiments, both species remained active over a temperature range of approximately 46 degrees C. The spring- maturing M. asperatus tolerated significantly lower temperatures than the s ummer-maturing M. formosipes (CTmin = -1.4 degrees C and 2.2 degrees C, res pectively), while M. formosipes tolerated significantly higher temperatures than M. asperatus (CTmax = 48.2 degrees C and 45.1 degrees C, respectively ). Misumenops asperatus displayed thermal discomfort over a broader range o f temperatures (36-44 degrees C) than did M. formosipes (41-46 degrees C). In a laboratory thermal gradient apparatus M. asperatus tended to prefer co oler temperatures than M. formosipes (14.4 degrees C and 18.4 degrees C, re spectively). Regression analysis of literature data for 21 species of spide rs showed a significant positive relationship between the thermal preferenc e of a species and its CTmax. For their CTmax's, which were high compared t o most other spider species, M. asperatus and M. formosipes preferred low t emperatures. The coupling of low thermal preferences and high thermal toler ances displayed by M. asperatus and M. formosipes is unusual for ectothermi c organisms.