Wednesday 31 October 2012, 15:09
We’re delighted to introduce Geoff Oxford, of British Arachnological Society, to bring a new insight to the life story of large house spiders.
Autumn is the time when you might see a large, brown, hairy spider scuttle across the carpet or find one trapped in a bath or sink. What species is it likely to be, why has it suddenly emerged and where does it lurk in other seasons?
Depending on where you live in the British Isles, your spider is probably one of four species of large house spiders. Three of the species look identical (see photos) whereas the fourth, the Cardinal spider, is more strikingly marked and with extremely long, banded legs. The spider you see is most likely to be a male – look out for short, leg-like palps on the head, their ends swollen like boxing gloves. He is looking for a mate.
At this stage, the male with have abandoned his web but the female will still be in hers, hidden under furniture, festooning neglected corners of sheds and garages or, in the wider countryside, utilising crevices in rocks and trees. Male and female webs are there all year; it’s just that we tend not to notice their occupants until the mating season. Indeed, the web of a large house spider can be extremely long-lived. In a protected environment like a garage it can last for several years and, over its lifetime, can have a number of different individuals occupying it, like an ancestral home. It is unlikely that the large specimens we see in our houses in autumn have come in from outside, as is commonly assumed; they will have spent their entire lives in our company without us being aware of it.
The marauding male you see running across the floor is seeking out a female who has still to moult to maturity. In this way he ensures he will be the first to mate with her. Before he does so, he has to prepare. He spins a tiny web and deposits on it a drop of sperm from an opening on his abdomen. He then sucks the sperm into each of his two palps – they act almost like hypodermic syringes – and, fully primed, he is ready for action. When he finds a web he can tell instantly whether the female is suitable from chemical cues (pheromones) she has laid down in her silk, which he detects with his feet and palps. If the signs are right he starts to court, but very carefully – he does not want to be mistaken for potential food. He pulls at the web with his fangs, taps it with his legs and bobs his abdomen, all of which tell the female that he is a male and of the right species. If accepted, the male begins a period of co-habitation and both can be found tucked well down in the tubular retreat of the female’s sheet web. As soon as the female undergoes her final moult mating takes place. The male inserts his palps, one at a time and repeatedly, pumping sperm into special receptacles within the female’s abdomen. Afterwards, the male remains in the female’s web, mating frequently and guarding her against the attentions of other suitors. As winter approaches the male dies.
The following spring, as temperatures rise and flying insects increase, the female starts to feed and produces a series of silk-covered egg sacs, using the sperm she has been nurturing within her body all winter. The white egg sacs, hung near the web, are about the size of a small-finger nail and are often decorated (camouflaged?) with fly husks, the remains of past meals. Depending on temperature the eggs take from 30 to 50 days to develop. Eventually the spiderlings cut a neat hole in the egg-sac wall and escape, 50 or more of them from each sac. On emergence spiderlings are just a couple of millimetres long and pale bluish-grey in colour. For a week or so they cluster around the empty egg sac but gradually disperse to construct their own webs and follow independent lives. Initially the spiderlings feed on tiny insects such as midges and fruitflies but as they grow they expand their diets to include larger items like wasps, moths and blowflies. They, like all spiders, are adapted for a ‘feast and famine’ lifestyle and only need to eat occasionally to survive.
A mated female can produce up to 10 eggs sacs (in captivity, at least) but then usually dies before the next winter, although a few might possibly make it through to the next spring. The young develop over the summer and autumn and are half-grown by the time they over-winter. The following spring they complete their development, with males maturing before females. Unlike some other species, these large house spiders can mature after a variable number of moults. What determines the size at which they mature isn’t known, but probably depends on the amount of food available during their development. As a result, it is not uncommon to see really tiny adults, as well as the monsters. However, size matters. On average, the larger the females the more eggs she produces per egg-sac and tiny males are probably less able to defend their female partners from larger rivals.
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