Web Orientation in the Spider Micrathena Gracilis (Araneae: Araneidae)

Web orientation (direction that spiders faced) in a forest—dwelling orb web—building spider, Micrathena gracilis (Araneidae), was related to the microhabitat where webs are constructed. Individuals in shaded (closed) microhabitats faced predominantly north/south, while those in well—lighted microhabitats (open and patchy) faced east/west. Several variables were examined in an effort to explain the patterns that emerged. Wind speeds were found to be so low within the interior of the forest that available instrumentation could not measure them. Nethertheless, webs were not aligned with prevailing winds. There were no significant differences in web site parameters (height, bridge thread distance) between microhabitats. Artificial web studies indicate that prey abundance is greater in open microhabitats and at lower heights, though prey movement through all microhabitats is nondirectional. Spider body temperature, however, was positively correlated with the amount of solar radiation striking the animal, which varied with both microhabitat and direction of orientation. The compass orientation of a spider's web influenced the amount of radiation to which the spider was exposed, and thus affected its body temperature. A model is proposed that relates the time an individual spider can spend on its web to the amount of heat accrued by the spider. Both too little or excessive body heat might affect activity. Since time on the web is positively correlated with total prey captured, more time spent on the web effectively increases the availability of prey. Web orientation is suggested as a behavioral thermoregulatory mechanism which allows Micrathena gracilis enough flexibility to exploit a variety of microhabitats and to maximize its time on the web, thus optimizing its prey take in each habitat.