THE members of the Arachnida series, to which belong spiders, scorpions, harvesters and mites, differ from the Crustacea and Insects in that their bodies consist of only two parts—the head and thorax are usually fused to form what is called a cephalothorax, and the abdomen. They have simple eyes, no antenna?, and eight legs and, except in mites, the life-history shows no metamorphosis. Their breathing is carried out by special devices situated in open pits on the under side of the cephalothorax. Into these pits thin, flat processes project like the pages of a book—whence they are called lung-books; the blood circulates in the leaves of the book and the air between them. Breathing tubes are present in spiders but differ from those of the insects. Very few members of the group live in water; most of them live on land and only rarely live in colonies. They are usually active by night and are carnivorous. The sexes are separate, the males usually being smaller than the females.
The largest Arachnida are the Scorpions which commonly live in holes in the sand and hunt insects. They may be as long as eight inches. The head and thorax are evident as separate units, as the head has no rings in contrast to the complete segmentation of the thorax and abdomen. The head carries a pair of strong claws which give the animal the appearance of a crustacean. The abdomen has six rings, the last one carrying a sting. When attacked, the scorpion adopts a menacing attitude, with its tail reared over its back with the sting pointing forwards. The prey are paralysed by the sting and pulled to pieces by the claws. The poison is strong and is immediately fatal to small creatures like insects; but if the scorpions inject their poison into each other, nothing amiss happens.
Although scorpions may look ferocious, they are really more retiring than pugilistic in habit and much prefer to pass unnoticed. This is true of their relations with all animals other than their own kind, and only courtship or hostilities
bring them together. The young are well developed when born, and are carried on the mother’s back for a time, during which time they never feed. Later they leave the mother and live their lives quite independently.
Spiders are the big order within the Arachnida. In addition to the general characters of the group, they possess silk-producing organs or spinnerets at the tip of the abdomen. The production of silk plays a most important part in the habits of the spiders. Much has been written about spiders, and, in spite of the fact that a study of their habits reveals a marvellous world of instinct and by-play, imagination on the part of observers has no doubt added colour to the already vivid pictures of the spiders’ private lives.
SPIDERS THAT BUILD THEIR OWN AIRCRAFT A NEWLY hatched spider climbs up to a point of vantage on a plant, and there it sits and drops a thread of liquid silk which is drawn out by the breeze; the spider feels the tug, releases its hold of the leaf and is carried away by the wind, supported by its gossamer thread. It may go many miles before landing. With the exception of the burrowing spiders, most spiders adopt this means of aerial transport.
The wolf-spiders of the fields are speedy and hunt their prey on foot; others hide under plants and pounce upon passing insects. The crab-spiders which walk sideways like crabs may hide in flowers of the same colour as themselves. Some have the capacity for changing their colours to match the surroundings and they catch insect visitors.
The garden spider traps prey in webs; the struggling of the victim causes the threads to vibrate, and the spider, feeling these vibrations, rushes to the spot and proceeds to tie up the prey still further with bands of silk. As a rule the web-building spiders tackle larger prey than the hunting spiders which have no silken threads with which to bind their victims.
The poison of spiders is injected into the prey through the fangs. Certain exotic spiders are dangerous to man. The spider usually is regarded as the aggressor, but quite often it is preyed upon by birds and mammals; its cocoons may be parasitised by the ichneumon-flies. Consequently spiders employ various protective devices, such as protective colouration, mimicry, etc. Some are streaked with the warning colours of yellow and black; others display striking colours when they run about, but conceal them when they crouch, so
that to all intents and purposes they become invisible. They may mimic other animals like snails, beetles and ants—the last being the commonest type of mimicry. In addition to these imitations, we all know how some spiders feign death when touched.
Tropical spiders are parasitised by ichneumon parasitic flies and birds attack them in their webs; it is up to the spider to protect itself and this it does by introducing imitation spiders into its webs ! One method is to place a number of pellets shaped and coloured like itself on the threads in such a way that its enemies do not recognise the real thing when they see it. Others manufacture mats of insects bound together by silk, place them at the centres of their webs and conceal themselves behind them. About twenty different methods involving the use of protective devices of this kind have been observed.