Spiders (& Wind Scorpions)

Cave Spider (Meta menardi)
body up to 10-15 mm long

Like all arachnids, spiders differ from insects in having four (instead of three) pairs of legs and in having only two (instead of three) distinct body regions - a hard front part called the cephalothorax (or prosoma) and a soft hind part called the abdomen (or opisthosoma) which are joined by a narrow stalk called the pedicel. Spiders are fairly uniform in their general appearance (in most cases, apart from size, very like the example shown opposite) and are so well known to most people that there is probably little or no difficulty in their general recognition and overall distinction from most insects and other arthropods.

• Wind Scorpions
• True Spiders
• Spider Webs
• Spider Dispersal
• Spider Bites
• Spider Gallery

• Identification Guides & General Reading
   
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Wind Scorpions (spider-like arachnids)
Among the arachnids, only the spider-like wind scorpions (Order Solifuga), sometimes called barrel spiders or sun spiders, are likely to be confused with true spiders (Order Araneae). However, these two groups can be easily distinguished since both the cephalothorax and abdomen of wind scorpions are clearly segmented, whereas these body regions in true spiders are without obvious subdivisions. Wind scorpions also lack spinning organs at the tip of the abdomen and thus cannot produce 'silk' threads and spin webs as characteristic of true spiders. The illustration opposite shows a typical wind scorpion, Galeodes, compared with a typical true spider, Araneus. The latter is very similar to the cave spider pictured above. The wind scorpion shown here has extremely long palps which resemble an extra pair of legs (the palps of arachnids are a pair of sensory appendages, usually quite short, attached to the front of cephalothorax on either side of the mouthparts). All wind scorpions are ground-dwelling arachnids, found mainly in steppes and deserts, and largely confined to tropical and sub-tropical regions, whereas the true spiders are found in nearly all habitats and environments except for the polar regions of the world.

 

SOLIFUGA
Wind Scorpion
(Galeodes)
body up to 50 mm long

 

ARANEAE
True Spider
(Araneus)
body up to 15 mm long

True Spiders
True spiders (Araneae) represent by far the largest Order of the arachnids with about 35,000 species known worldwide. All are carnivorous predators with powerful fangs, called chelicerae, which are used to bite and crush the prey, and to inject venom. The chelicerae are attached to the front of the cephalothorax between the palps (a pair of jointed sensory appendages). The prey of spiders largely consists of insects and other arthropods (including other spiders), but some of the large, tropical species will also attack small frogs, lizards and even birds. Many of the spiders found commonly in farmland and gardens, particularly the so called money spiders (members of the family Linyphiidae) and wolf spiders (members of the family Lycosidae), are considered beneficial natural enemies, since they often prey on aphids, leafhoppers, gall midges, stem- and root-boring flies, cutworm moths and other undesirable insect pests of crop and garden plants.

Spider Webs
Apart from the construction of webs as snares for trapping prey, spiders also use silk for other purposes, e.g., for producing egg sacs, for wrapping and immobilizing prey, for constructing retreats in which to shelter, for dispersal on air currents and as safety lines when jumping or dropping to the ground. Spider silk is a protein material and is produced by glands in the spinning organs near the tip of the abdomen. The silk used for web-building is often provided with adhesive droplets which help trap and entangle any captured prey.

The webs familiar to most people are the large orb webs constructed by many of our commonest spiders and often seen adorning recessed corners of buildings, sheds and fences or suspended between the shoots and branches of garden and hedgerow plants. However, the webs of different spiders show great variation in structure - some species merely spin irregular sheets over soil, foliage and other structures (commonly called 'cob-webs'), many construct funnel-shaped webs often associated with holes in walls, fence posts and tree bark or with gaps under logs and stones, others produce tent-like structures among plant foliage, whilst a few species spin triangular or star-shaped webs. The variety is almost endless! Within a given species, no two webs are ever identical and even the webs of an individual spider may vary depending on its age and surroundings.

 

Example of an Orb Web
Photo: I. Britton - FreeFoto ©

Webs, especially orb webs, are often damaged by flying insect prey and bad weather, and sticky webs, even if undamaged, generally loose their adhesiveness after a day or two. Hence, the majority of spiders are forced to repair and rebuild their webs more or less on a daily base. However, nothing goes to waste in this process since spiders can rapidly recycle the protein of silk by eating the old or damaged web before a new one is made.

Spider Dispersal
Spiders generally move from place to place by walking and this is the only way that large spiders can disperse to find new habitats. However, the newly hatched young (spiderlings) of many large spiders, and both the adults and spiderlings of small species, can move more quickly to new areas by air. This arial dispersal is commonly called 'ballooning'. The spider first crawls to the top of vegetation or other convenient structure, stands on 'tiptoe' with the abdomen pointing skyward, and then releases strands of silk from its spinning organs. Eventually, perhaps after several tries, the silk threads are caught by the breeze and carried upwards on rising air currents. A small spiders is very light and when the pull of the air current is sufficiently strong, it lets go its foothold and sails aloft into the air. In this way spiders may be carried many miles before descending back to earth. Aerial dispersal, whilst undoubtedly very effective, does have its disadvantages. Ballooning spiders are fair game for many insectivorous birds, such as swallows, and may be eaten, or the spiders may be carried out to sea or into lakes and get drowned, or may land in some other unfavourable environment. Nevertheless, large numbers of ballooning spiders evidently manage to survive these risks, judging by their rapid colonisation of fresh habitats, as frequently observed in newly planted farm crops.

Spider Bites
All spiders bite and nearly all inject venom into their prey, but the fangs of the vaste majority of species are too small to penetrate human skin and very few will bite man, even when severely provoked. Of the 35,000 or so species throughout the world, about 500 are known to cause painful bites in humans, but only about a dozen of these spiders are highly venomous. The species which are particularly dangerous to man are largely confined to warmer parts of the world, although their common names will be familiar to most people. For example, the venomous bites of the black widows or red-back spiders (Latrodectus) and the Australian funnel-web spiders (Atrax) are well known, since both can be fatal in some circumstances. Likewise, the Brazilian wandering spider (Phoneutria) has a dangerous reputation as a very aggressive species with an extremely painful bite. Few, if any, of the spiders found in temperate regions are particularly venomous or likely to bite. Among the native spiders of Britain and northern Europe, for example, there are only one or two species that require some caution if handled. The water spider (Argyroneta) can give a fairly painful bite, but usually this is much less severe than the bites and stings of many common insects. Somewhat unpleasant bite wounds can also be inflicted by the larger species of Cheiracanthium and Steatoda. The latter species is often found in and around house and other buildings. You can find more about the recognition and biology of these and other spiders from the books listed below.

More pictures of true spiders (and other arachnids) at the Department of Entomology, University of Nebraska-Lincoln (UNL).

• Jones RD. 1983. The Country Life Guide to Spiders of Britain and Northern Europe. London, Hamlyn.
• Locket GH, Millidge AF. 1951. British Spiders, Vol I. London, Ray Society.
• Locket GH, Millidge AF. 1953. British Spiders, Vol II. London, Ray Society.
• Locket GH, Millidge AF., Merrett P. 1974. British Spiders, Vol III. London, Ray Society.
• Roberts MJ. 1993. The Spiders of Great Britain and Ireland. Compact Edition, Vols 1-3. Colchester, Hartley Books.
• Roberts MJ. 1995. Spiders of Britain and Northern Europe. Collins Field Guide. London, HarperCollins.

• Bristowe WS. 1958. The World of Spiders. New Naturalist Series. London, Collins.
• Chinnery M. 1993. Spiders. London, Whittet Books.
• Foelix RF. 1982. Biology of Spiders. London, Harvard University Press.
• Hillyard P. 1994. The Book of the Spider. London, Hutchinson.
• Preston-Mafham R. 1991. Spiders, an Illustrated Guide. Poole, Blandford.
• Preston-Mafham R, Preston-Mafham K. 1984. Spiders of the World. Poole, Blandford.