THREAD BIOMECHANICS IN THE 2 ORB-WEAVING SPIDERS ARANEUS-DIADEMATUS (ARANEAE, ARANEIDAE) AND ULOBORUS-WALCKENAERIUS (ARANEAE, ULOBORIDAE)

The absorption of high kinetic energy by a small amount of material depends not only on the quality of the material but also on the structural design of the elements involved. Using a rapid response microbalance, we measured the tensions of radial threads in webs of the garden cross spider, Araneus diadematus. We also measured the stress-strain characteristics of dry radius and wet spiral threads laid down by A, diadematus, as well as of the very different and dry spiral threads laid down by the hackled-band weaver, Uloborus walckenaerius. The radius threads of A. diadematus showed good extensibility (e = 39.4%), high tensile strength (s = 1153.8 MPa) and large hysteresis (56%) which indicates that they can function as shock absorbers and structural elements. Although fewer radii were built in the upper than in the lower half of the Araneus web, our method found no systematic difference between the average pretensions of individual radius threads in these two halves. However, pretension in the upper half of the web showed greater variation. Orb weavers employ two different mechanisms to increase the energy-absorbing capacity of their respective capture spirals. The sticky spiral of Araneus diadematus absorbed energy by large extensibility (about 475%) of the wetted thread which developed substantial force only after 100-200% extension, and the entire thread failed suddenly. The hackled band of Uloborus walckenaerius had shorter extensibility (about 125%), and it absorbed energy by friction of the fine hackled fibres, many of which needed to break in succession before a thread failed. (C) 1995 Wiley-Liss, Inc.