Understand spider behaviour

Spiders are one of the easiest groups of invertebrates to watch. Professor Steve Harris describes the key behaviour you need to look out for. 

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Background

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  • Spiders are arachnids. They differ from insects in having only two parts to the body, eight legs not six, six or eight eyes (two in insects) and spinnerets on their abdomens that produce silk.
  • A study of an undisturbed grass field in Sussex found 5.5 million spiders per hectare.
  • Many species have become highly specialised to reduce competition with others. Some are nocturnal; others diurnal. Some build fixed webs (sheets, orbs or tunnels); others throw webs to entangle prey. Some hunt prey; others sit and wait in ambush. Look for spiders under stones and logs, in sheds, at ground level, among plants and bushes and on walls.
 
Silk production
 
  • Silk is used to build webs and egg sacs, wrap up prey, help dispersal of young and as safety lines when escaping predators. Water spiders also use silk to hold an underwater air supply.
  • Spiderlings disperse using silk. They travel to a high point, raise their abdomens and let out one or more strands. On warm days with rising air currents, the spiderlings are lifted into the air and carried away.
 
Hunting
 
  • Spiders eat large numbers of insects. Exceptionally, one spider may take hundreds of very small flies in one day.
  • Crab spiders are sit-and-wait predators. They are often seen perched on garden flowers with their long front legs held out, crablike, to seize insects visiting the plant.
  • Wolf spiders are brown and furry, and on sunny days large numbers can be seen running through vegetation (on the edge of a pond, for instance) hunting prey.
  • On sunny walls, black-and-white striped jumping spiders can be seen stalking and pouncing on prey.
  • The very distinctive nocturnal spider Dysdera crocota, which has a reddish-brown body and legs, a pale abdomen and powerful fangs, hunts woodlice under stones and flowerpots.
  • Spiders avoid unpalatable insects. Burnet and cinnabar moths, for instance, lie still in spiders’ webs and are thrown out by the host.
  • Edible prey is wrapped up in silk. The remains can often be seen attached to the web.
 
Breeding
 
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  • Mating usually involves some form of courtship. In web-building spiders, the male vibrates the web of the female; in hunting spiders, he uses his legs to signal to the female in a form of semaphore.
  • Eggs are laid in a silken sac. They start developing straightaway or remain dormant over the winter.
  • The egg sac may be left, guarded by the female or even carried or rolled around.
  • Wolf spiders carry their spiderlings around for a week. Some feed their young on liquified food, others kill prey and leave it for them.
  • Garden spiders each lay one egg sac in a sheltered spot and stay with it until they die in autumn. The spiderlings emerge early the following summer. They spin a web, then cluster into a ball on it. If disturbed, the youngsters scurry in all directions.