Friday, 18 September 2009

Orb webs adorn the garden

This is the time of the year when orb web spiders are at their most obvious. Several have cast their beautiful spiral webs across the corners of our windows, with others strung between the branches of garden shrubs or across the door of the greenhouse. In the centre of each web sits a female spider, Araneus diadematus, the familiar Garden Orb Web Spider, head down and motionless. On the rare occasions when the web appears empty, the telltale signal thread running from the centre of the web to a suitable nook in the brickwork soon reveals her location. Should an insect become caught in her web she will rush down the signal thread, take up a position in the centre of the web to determine where the victim is caught and then dash along the appropriate radial thread to deliver a fatal bite.

The patterning on the back of the female resembles a white cross and this may explain why this particular spider was the object of veneration during the Middle Ages. By now, in early September, she will have completed eight moults, shedding her exoskeleton to grow in size and reaching just over a centimetre in body length. The smaller male, who will be travelling around in search of a mate, undergoes just six moults. Understandably, the smaller males make a careful approach to any suitable female, the risk of being eaten increasing as the season progresses. The male very gradually moves forward onto the female’s web, attaching a safety line that will allow him to drop rapidly from the web should the female lunge at him. Each time the female attacks, so the male has to begin all over again, the process tedious to all but the most patient observer.

The male makes his courtship all the more complicated by the fact that he seeks to lure the female onto a special mating thread. If he succeeds in getting her onto the thread then she will signal her compliance by hanging motionless, head down. This affords the male the opportunity to insert one of his palps into her reproductive organ; the other palp, however, requires yet more of the gradual and patient courtship. After mating, at some time during September or October, the female will leave the web to lay her eggs somewhere close by. The eggs, which are held together in a tight solid mass, may number up to eight hundred and are collectively wrapped in rough silk, sometimes camouflaged with small bits of debris. By now the female will be close to death; emaciated she takes up a position next to her egg sac, where she will remain over the last few weeks of her life.

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