Local polymorphisms in spider web orientation: evaluating the effectiveness of vertical and horizontal webs.

Abstract

In the area surrounding Douglas Lake, horizontal webs, built by Pityohyphantes costatus (Hentz), and vertical webs, built by Tetrignatha straminea (Emerton), coexist. The objectives of this study are to determine if 1) there is a correlation between orientation of webs and the amount of prey captured at given heights and locations, 2) there is a difference between the kinds of prey caught by the two kinds of webs, and 3) there is a relation between the performance of each web type and their prevalence at certain sites. We answered these questions by comparing the amount and kind of prey captured by vertical and horizontal webs at two sites, a lake shore and a hillside. We compared these results to those of artificial webs which allowed us to isolate the variable of orientation. Our results showed that at the lake shore site, vertical webs caught more prey, with a greater proportion of small sized prey, but that these differences were due to a factor other than orientation. At the hillside site, we found no significant difference in the amount or size frequency distribution of prey caught by the two web types. The lake shore site results suggest that coexistence of the two webs is possible because selection is favoring resource partitioning. The hillside site results suggest that coexistence is possible because food is not the limiting resource.