DESIGN VARIABILITY IN WEB GEOMETRY OF AN ORB-WEAVING SPIDER

We studied the effect of several variables (environmental and physiolo gical) on web geometry in the garden cross spider Araneus diadematus. Variables were: web support, wind, temperature, humidity, and silk sup ply. All had an effect. The spiders generally attempted to fit their w ebs to the shape of the supporting frame (standard, small, vertical, o r horizontal). Windy conditions (0.5 m s(-1)) during web construction caused spiders to build smaller and rounder webs, laying down fewer ca pture spirals while increasing the distances between capture-spiral me shes. Decreasing temperature from 24 degrees to 12 degrees C caused th e capture spiral to have fewer and wider spaced meshes, which did not change overall capture area but reduced the length of capture-spiral t hreads laid down. Subsequent increase of temperature to 24 degrees C r estored the number of meshes laid down, but the wider mesh was retaine d, causing the capture area to be increased over initial control value s. Decreased humidity (from 70 to 20% rH) had the effect of reducing w eb and capture-spiral size, the latter by reducing mesh number while k eeping mesh spacing constant. Subsequent increase of humidity to contr ol level (70%) restored web and capture area. However, this was achiev ed by laying down capture meshes at larger distances, rather than retu rning to initial mesh numbers. Silk supply also had a strong effect. W ebs built in unnaturally rapid succession by the same spider (4 in 24 h when 1 is the norm) became sequentially smaller, had fewer radii, sh orter capture spirals, and were wider meshed. (C) 1997 Elsevier Scienc e Inc.