Radius construction and structure in the orb-web of Zilla diodia (Araneidae).

In orb-webs, the tension of the sticky spiral produces a centripetal force on the radii, resulting in an increase in tension along each radius from the centre of the web to the periphery. Zilla diodia (Walckenaer, 1802) atypical of araneids, was found to adapt the structure of its radii to this tension gradient by building radii that are double stranded at the periphery of the web and single stranded near the centre. Furthermore, the proportion of each radius that is doubled was found to be larger in the upper part of the web - where the overall tensions in the radii are known to be higher than in the lower part of the web. suggesting that the spider adjusts the proportion of each radius that is doubled to the overall tension in the radius.