SPIDERS MASTER HUNTERS

P. Alluster

Spiders are a species of arachnids in the family of animals called anthropods. All spiders are predatory, feeding mainly on insects, and are very efficient hunters. Many (not all) weave webs or traps to catch their victims, then secrete a poison from behind their fangs to stun or even kill them outright.

The spider’s web is woven from a special silk. This is a fibrous protein first secreted as a fluid and then stretched into strands which, because of their strength and elasticity, are extraordinarily resistant to breakage.

Garden spiders (Arena diedemata) make their webs from two different silks. The threads of the main structure are woven from a strong silk which can be stretched further (by as much as 20%) but then loses its strength. By contrast, the other kind of silk, used between the main threads, is lighter and stickier and can be expanded three times without losing its original characteristics. Under a microscope drops containing a reserve of silk can be seen at intervals on these thinner ‘hunting silks’.

After its web is complete the spider hides out of sight, somewhere on the outer strands of the web. When an insect flies into the web and struggles, the spider is alerted by the vibrations and runs out. It rapidly contains the victim’s struggle to escape by tying it up with the silk set aside for this purpose in the drops: the elasticity of the hunting silks is vital in this task. While tying it up, the spider injects the victim with the poison from behind its jaws which both paralyses the insect and acts as a digestive juice softening up the now helpless corpse. The spider then goes on injecting and sucking back fluids until the soft parts of the corpse have been digested - any skeletal parts left over are simply discarded.

Spiders put their weaving skills to a number of different uses. As well as making the insect traps we call spider webs, they weave draglines’ that help them to locate themselves and to break their fall if they should slip. Small spiders spin a sort of ‘parachute’ thread that allows them to be carried on the wind.

Some species of spider make active traps. Menneus spins an elastic net between its legs and sweeps it through the air to catch passing insects. Cledomelea dangles from one leg a blob of sticky silk at the end of a long thread and swings it out to attach its prey. Trapdoor spiders (Ctenizidae) dig a burrow closed by a silken door; when an insect ventures near, the spider darts out to capture the imprudent victim.

Spider webs are beautiful, intricate constructions: threads which serve as scaffolding during the construction process are removed once the web, a mesh of sticky and non-sticky lines, has been completed. The skill of producing webs is clearly instinctive, but the irregularity and variety of web forms shows that the skill is adapted by individual species to serve different functions and suit different circumstances - some webs hang in the air to catch insects as they fly, others are laid across the ground, both at angles calculated (presumably by experience) to lure and intercept prey.

Recent research has shown that some A. Glomosus spiders use ultraviolet rays to attract their prey. In one experiment fruit flies (Drosophila) were set free between two webs lit up by a white beam. One web was that of a A. Glomosus spider and radiated ultraviolet rays; the other was not: the flies were attracted to the former

Another remarkable species are the Dolmedes spiders which have long legs (8-10 cm) and striped, brown bodies. They live near water ponds where they have learnt, despite having very poor sight, to catch fish. Their hunting-gathering technique is of awe-inspiring dexterity and patience, rivaling that of any human fisherman. First the spider walks around on the bank to pick a site suitable for laying a web. Once that is done, it waits patiently, standing partly on water and partly on land. A special sticky secretion helps secure its hold on the surface of the water. While waiting, it prepares its poison in its mouth. When a small fish happens by, the spider plunges forward to seize it, releasing its poison into the water as it does so. As the poison begins to work, the spider turns over making its own body a sort of float for the struggling fish, carries it to land and there consumes it.

Some species of spiders do not make webs to ensnare their prey. Instead, they actively pursue their prey or lie in ambush for it. They are endowed with specially keen sight or touch sense, used respectively for hunting in daylight or in the dark. The ambushing varieties are remarkably well camouflaged - the colour and shape of their bodies making them almost invisible against the immediate background of leaves or bark or stones and sand.

One of the night-hunting spiders of the Amazon jungle spends the day hiding in crevices or in holes in trees, emerging into the jungle at night to stalk its food. Its legs spread the width of a human hand and move with utmost stealth until, when near enough, the spider makes a sudden, final dash, seizing small mammals (humming-birds, for example), stunning them with its poison, then dragging and shaking them to death. The detestation and horror this species arouses in human beings is hardly justified - its poison is not more troubling to a human than a bee sting.

Reputation and significance

Spiders have a very negative image among human beings. Perhaps the number of legs, the grotesque facial expression, the hairiness of some species, the fact that they carry a poison, but most of all, the fact that they hide in corners and come out unexpectedly - have contributed to the spiders’ bad reputation. The poison of spiders, with just two exceptions (the ‘black widow’ and the ‘brown recluse’), is relatively harmless to humans.

Spider silk cannot economically be converted into silk cloth for human use. However, it has been used for the cross-hairs of optical instruments. More recently, the silk of the tropical species Nephila has been employed in the manufacture of bullet-proof jackets. The Nephila spin huge webs strung across trees, as long as 2 metres or more, and of a silk so strong and elastic that the local peoples make very effective fishing nets from it.

On balance, it is high time human beings overcame their irrational detestation of spiders. We should be grateful to them for all the good they do for us in preserving our persons and properties, especially our crops, against devastation by insects. One authority calculated the spider population of England and Wales as of the order of 2.5 billions at any one time. This means that if (at a most conservative estimate) each spider eats 100 insects a year, then the total number of insects consumed by spiders is 250 billions annually.

REFERENCES

‘Spiders’ Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright (c) 1994 Funk & Wagnall’s Corporation.

Buton, M. & Buton, R. (1975) Enevlopedia of Insects and Arachnids, BPC Publishing Ltd, London.

Gerald, L. & Wood, F.Z.S. (1982) The Guinness Book of Animal Facts and Feats, Guinness Superlative Ltd, London.

Waterson, AR. (ed.) (t975), Collins Enevlopedia of Animals, William Collins Sons & Co Ltd, London and Glasgow.

SPIDERS IN GUINESS BOOK OF RECORDS

The largest and the heaviest spider: The Guyanan ‘bird-eating’ spider (Theraphosa

blondi) of South America has long been credited with the ‘largest spider’ title. A male specimen with a leg-span of 254mm (10 in) and a body length of 89mm (3.5 in) weighed just under 57g (2 oz).

The smallest spiders are the midget spiders (Symphytognathidae), the tiniest of which is the pale yellow Patu marplesi of Western Samoa, S.W. Pacific. A male specimen found in moss at an altitude of 610m (2000 if) measured 0.43mm (0.07 in), which means it is half the size of a full-stop on this page!

The largest spider webs are the aerial ones spun by the tropical orb weavers of the genus Nephila. Several examples found in the Karrakpur Hills near Monghyr, central Bihar, India measured 1.5m (5ft) in diameter (about 4.79m (1 5ft 9in) in circumference) and had long supporting guy-lines up to 6.1 m (2Oft) in length.

The smallest webs in the world are the aerial ones spun by midget spiders. That of the orb weaver Chasmoeephaion armaturn of New Zealand measures about 9-10mm (O.35-0.39in.) in diameter which means it is half the size of a small postage stamp.

The highest speed recorded for a spider on a level surface is 53cm/s (1 .73ft/s) (= 1.90km/h; 1.18 miles/h) for a female house spider, Tegenaria atrica. This may not seem very fast, but the spider covers a distance equivalent to 330 times its own body length in ten seconds.

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