Behavioral thermoregulation: Solar orientation in Frontinella communis (Linyphiidae), a 6-mg spider

Summary

• 1. The Linyphiid spider Frontinella communis changes its orientation from random with respect to the sun (Fig. 1A) to aligned with the rays of the sun (Fig. 1B) on days when both insolation and ambient temperature are high (Fig. 2). The orientation is not mediated by visual photoreception. Because the orientation behavior is not elicited when insolation is high and ambient temperature is low, an antipredator or antiprey behavioral role is rejected and a thermoregulatory function proposed.

• 2. Body temperature measurements of a spider at different orientations to the sun reveal that alignment of the spider's oral-anal axis with the sun's rays results in at least a 0.5°C decrease in T_B relative to T_B when the spider's axis is perpendicular to the sun.

• 3. F. communis has a LT₅₀ of 41.8°C, a temperature that the spider probably never approaches over much of its geographic range. Therefore, it is unlikely that the orientation behavior functions to keep the spider within a nonlethal temperature range. Instead, the behavior probably functions in depressing this poikilotherm's metabolic rate over a broad temperature range, and thus permits reallocation of nutrient resources from maintenance to reproduction.