Spinners of Silk - 11 October 2006

Richard Jones, professional arachnologist and author of the first ever Field Guide to Spiders, drove all the way from Portsmouth to give this fascinating talk.  What follows is just a brief taste of the host of information he imparted, which included an overview of the arachnid group, and details of the anatomy and life history of the various groups of spiders, of which there are some 640 species in Britain.

The arachnids, which include not only spiders but also scorpions, pseudoscorpions, mites, ticks and harvestmen, are among the earliest land animals, with fossil scorpions dating back to 500 million years ago, and the earliest recorded spider fossil over 400 million years old. True spiders are characterised by having a body with a head region (formed from the fused head and thorax) and an abdomen, with four pairs of legs covered with sensory hairs, and a set of spinners - the source of their silk - at the end of the abdomen.  Most spiders have eight eyes, though this varies among species from 0 to 12, and these are located at the front of the head along with the jaws/fangs (called chelicerae) and sensory palps.

In the male these palps are particularly large, and are used to deposit sperm into the female spider's reproductive organ, the epigyne, the male having first spun a small web onto which he deposits semen, and dips the palps into it.  Then comes the dangerous part - getting the female to accept it without eating him, a real danger among many species.  In wolf spiders, for example, the male signals to the female to find out if she is willing to mate: if she is not interested she simply ignores him, and if he is too persistent will give him a thump on the front legs as a warning!  Another example of spider courtship is the habitat of the male crab spider of wrapping the female in a "bridal veil" of silk, although Dick assured us that she was not actually tied up so presumably the aspiring groom is still at some risk. Some males bring the females courtship gifts of food.

Spider females tend to be larger but are often more drab than their male counterparts, perhaps because they have to remain stationary when guarding their eggs and therefore need to be camouflaged against predators.  Examples shown in some of Dick's stunning slides were the aforementioned crab spider, in which the female is pink and the male bright orange, and another species where the female is pale green but the male has a splendidly red and yellow striped abdomen. This is not always the rule though: for example, the female wasp spider, Argiope bruennichi, has a black and yellow striped abdomen.  However, this species does not guard the egg sac, which is particularly tough, being constructed by weaving silk around it then part-digesting the fibres so that they fuse into a papery cup.

As intimated above, spiders are very caring mothers when it comes to their eggs.  Wolf spiders even lay their eggs in a bowl of silk, then add more silk on top and carry the whole thing around attached to their spinners until hatching.  So assiduous are they that they will pick up anything of a similar shape and size - Dick described finding one on Box Hill carrying around a (presumably either dead or very bewildered) woodlouse!  After two weeks the spiderlings hatch and initially remain attached to the mother's body hairs, so that she looks as though she is wearing a fur coat. Other species with long legs carry their egg sac beneath their abdomen, eventually depositing it on a plant where they guard it.  The ultimate in maternal care, though, must be the social spiders that stay with the eggs until they die, their body providing food for their offspring.

Once the spiderlings hatch, however, they are on their own, and in most species they disperse rapidly into the surrounding environment.  One strategy for doing this is "ballooning", whereby the spiderling climbs up a grass stalk, produces a strand of silk and is carried away on it by the wind; when conditions are right, during the autumn, hundreds of millions of animals may take off in this way simultaneously.

The amazing variety of webs produced by spiders was beautifully illustrated, from the orb webs of genera such as Argiope, to the funnel webs, all masterpieces of engineering designed to maximise prey capture.

Our thanks again to Dick for a great talk: I am sure we will all treat that spider in the bath with greater respect in future.

LYNN WHITFIELD