Sunday, 17 October 2010

Simple skills of a silken kind

It is on these damp autumnal mornings that the architectural skills of our many and varied spider species can be best appreciated. While bits of wall display simple webs, the gaps between plants support the more-skilled constructions of orb-web spiders belonging to the genus Araneus. Undoubtedly the most familiar of these is the Garden Orb-Web Spider Araneus diadematus, delicately-marked with greys and browns and with a white cross on her back. It was this cross that saw her venerated widely during the Middle Ages and which provides a useful means of identification.

Dew-covered webs provide an opportunity for you to view the web’s construction, revealing the fine threads typically formed by the spider during the previous night. The Garden Orb-web produces a sizeable web, with a defined central hub of meshed thread. Outside of this there is a narrow spiral, known as the strengthening spiral, which circles six or seven times around the hub. Radiating out from the centre are two or three dozen threads which stretch out to the stout outer frame. It is to these radiating threads that the main spiral of threads is attached, the spiral beginning a little way out from the central hub. This leaves a gap, known as the free zone, between the strengthening spiral and the main body of the web itself. The main spiral is the key to securing a meal, since the spiralling threads are studded with blobs of glue. Flying insects unfortunate enough to encounter the web are held fast, affording the spider the opportunity to seize the prey and deliver a venomous bite. Some prey are deemed too large or too dangerous to be tackled, for example wasps, and the spider simply cuts them free rather than risk injury.

Sometimes the spider will sit motionless, head down in the centre of her web, her eight legs alert to the vibrations caused by an insect caught in the web. At other times she will tuck herself away on the edge of the web and use a signal thread, which runs from her hiding place to the centre of the web, to detect prey. That the spider can move across the web with impunity, not becoming caught in her own sticky trap, comes down to her use of the non-sticky radial threads when moving about the web. She also makes use of special oily secretions which cover her legs and reduce the chances of her becoming stuck.

These webs are particularly important to the female orb-web spiders, as they need to secure food in order to complete their series of moults and to produce the hundreds of eggs which will be deposited nearby at the appropriate time.

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