Cobwebs are an annoyance that most of us only face when tracking something down in the loft or basement. We rarely pay any attention to their sophisticated architecture and silk thread structure as we brush them aside. But scientists have long been intrigued by the enigmatic material's plastic structure, which is dependent on both intrinsic and extrinsic factors. Previous work has indicated that cobweb weaving spiders can control the mechanical performance and material properties of their silk, but whether they can tune structural or material properties of silk threads to the prey they intend to capture remains unknown. To address the question of whether common house spiders, Achaearanea tepidariorum, respond to different prey by altering their cobweb silk, Cecilia Boutry and Todd Blackledge from the University of Akron in Ohio tested whether a spider's diet affected the style and structure of the web it built.
First of all, Boutry and Blackledge assigned 29 wild house spiders to two groups and kept them on different diets for 1 week before analysing their cobweb silk both structurally and mechanically. One group was fed crickets; large, fast prey items, which would require significant restraint by a web. The other group was kept on pillbugs, which are small and slow and would need much less restraint. The duo suspected that the cobweb's architecture, as well as the silk's composition, thread diameter and mechanical properties could be affected by the spider's diet.
The team observed that spiders fed on a cricket diet spun threads that had up to a 20% larger diameter than the pillbug-fed spiders. These differences were equally pronounced in silk threads from two different functional regions of the cobweb; the sticky gumfooted threads that adhere directly to prey and anchor the web to solid structures, and supporting threads that maintain the web's integrity. Boutry and Blackledge also tested the silk's mechanical properties by exerting loads on the silk until it broke and found out that cricket-fed spiders produced a silk which broke at higher loads and was over 40% tougher than the pillbug's silk. Next, the researchers observed that the spiders that had been fed on heavy crickets were heavier and also had a better body condition than spiders supplied with pillbugs, even though the two groups were offered the same amount of food. When comparing the amount of ingestible biomass it turned out that pillbugs contained 16% less digestible energy and, thus, the spiders on the pillbug diet may not be able to afford the energy required to produce high quality silk.
Based on their findings, Boutry and Blackledge offer two alternative, non-exclusive, hypotheses for the way that spiders may tune their silk to different prey types. House spiders may either spin silk threads in response to the physical deformation inflicted on their cobwebs by their prey or they may adjust silk composition and structure in response to their own body mass and body condition. For the spiders in our attics these new findings indicate that their cobweb quality depends on their diet, so if we starve them their webs may be weak and easier to brush aside. However, if you wish to encourage your local spider population you should probably consider feeding them properly.
- © The Company of Biologists Limited 2008