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The ingenious traps of cannibalistic spiders

Posted in Insects, Nature, Wildlife on Thursday, 29 September 2011

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This edited article about spiders originally appeared in Look and Learn issue number 828 published on 26 November 1977.

Water Spider, picture, image, illustration

The Water Spider

Spiders are strange and fascinating creatures – and there is none stranger or more fascinating than the trapdoor spider. Although it spends most of its life underground, it’s first venture in the world is to travel by air.

A few weeks after a family of spiderlets have hatched out of their eggs and are strong enough to move about, they form up in single file and march off to a tall bush or tree. Then they climb up to the top, throw out some silken threads and are carried away by the breeze.

Just how far they fly or balloon along in this way depends upon the air currents. Sometimes they travel for many kilometres, but usually they drop back to earth after a flight of only a few hundred metres.

Spiderlets’ air voyages are a wise precaution by nature to make certain that they have a reasonable chance of growing up. Adult trapdoor spiders are ardent cannibals and most of the youngsters would be eaten by their parents if they did not leave home.

Wherever it may land, the young trapdoor spider immediately sets to work and digs a shaft or tunnel into the ground. It does the job with it own built-in tool: a rake consisting of thin but very strong bristles arranged like the teeth of a comb and growing from the jaws.

As the grains of earth are loosened, the spider spins silk thread round them to make up tiny parcels which it carries some distance away from the tunnel. This prevents wasps, centipedes and other enemies from finding the excavation.

When the tunnel has been dug to the depth of a 50-pence piece, the spider covers the opening with a lid made from the grains of earth held together with a silk web. The lid has a bevelled edge so that it fits tightly into the hole like the plug of a bath.

Next, silk thread is spun along one edge of the top of the lid. The other ends of the thread are fixed to the ground to make a hinge.

On the underside of the trapdoor, the spider drills two holes. These form handles into which the spider places its legs to pull the door shut.

The holes are also used to lock the door. If an enemy tries to force its way into the burrow, the spider slips its legs through the holes, so bolting the door.

Immediately it has finished the trapdoor, the spider digs its tunnel deeper and deeper. As it works its way down, it plasters the wall with earth mixed with saliva. This makes the shaft waterproof.

The plastered wall sets hard and smooth and is then “papered” with spider web.

Very often a second tunnel is dug to branch off from the main one. The second tunnel also has a trapdoor. If an intruder gets past the first door it has to tackle the second door before it can get within reach of its quarry.

Sometimes, too, there is a third trapdoor opening on to the surface of the ground. This provides the spider with an escape hatch if the first and second doors are broken down.

Throughout the day, the spider sits just outside its closed trapdoor, waiting for the insects upon which it feeds. Immediately one comes along, the spider snaps it up, opens the trap and disappears into the tunnel for dinner.

There are more than a hundred species of trapdoor spiders. They range in size from midgets about ten millimetres across to giants bigger in diameter than a ten-penny-piece.

All true trapdoor spiders are native to warm climates, such as those of Africa, Indonesia, Malaysia and South America. But Britain is not without its ingenious spiders.

In the south of England there lives a rare species called the raft spider, which builds tiny boats of leaves and twigs lashed together with silken cables. These are launched on the surface of a pond when the spiders wish to make a raid upon the water insects.

The spider skipper often leaves his ship and runs over the surface of the pond to catch flies, or even dives below to seize a water creature.

Even more ingenious is the true water spider which lives among the plants at the bottom of clear, quiet ponds.

There it builds a thimble-shaped dome of waterproof silk and plant material fastened mouth downwards to the edge of the plant or wedged in the crevice of a stone. Then it goes to the surface and catches air bubbles and carries them down, brushing them off into its cell until it is full of air. To this home, the water spider brings whatever prey it catches. Here, too, its eggs are laid and hatched.

Truly, spiders are wonderful creatures. They are not insects. Insects have six legs. Spiders belong to the family known as arachnids and they all have eight legs.

Each of these legs consists of seven segments and ends in two or three sharp claws. And the number of their eyes varies from two to eight, depending upon their species. But what really makes a spider distinctive is the way it has adpated its behaviour to ensure its survival, whether this means making a trapdoor, sailing on a raft or making an air-filled home in the depths of a pond.

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