CONSPECIFIC AND HETEROSPECIFIC ATTRACTION - A MECHANISM OF WEB-SITE SELECTION LEADING TO AGGREGATION FORMATION BY WEB-BUILDING SPIDERS

This study investigated whether conspecific and/or heterospecific attr action to silk is a mechanism of web-site selection leading to aggrega tion formation by two species of web-building spiders, Hypochilus,thor elli Marx: (Araneae: Hypochilidae) and Achaearanea tepidariorum (C.L. Koch) (Araneae: Theridiidae). We determined that the spatial distribut ion of these two spiders was clumped and that H. thorelli had a greate r tendency to aggregate than did A. tepidariorum,. To determine the me chanism responsible for this spatial pattern, we conducted three field experiments. We examined web-site selection by H. thorelli in three c ontexts: no spiders or webs present (cue removed), vacant webs present , and occupied webs present. In the case where no webs were present, t here was no tendency for spiders to choose previously occupied sites a s web sites. When vacant webs were present, spiders chose to occupy th e vacant webs. When occupied webs were present, spiders either invaded webs and evicted the owners, or settled adjacent to and attached thei r webs to those of residents. Various microhabitat variables (height, angle, temperature, humidity, and substrate character of the rock surf ace) of randomly selected unoccupied sites and the web sites chosen by spiders were compared. There were no detectable differences between w eb sites and unoccupied sites with respect to any of the variables mea sured. This leads us to conclude that web-site choice by immigrating s piders was based on the presence of silk rather than other features of the site.