CHAPTER VIII
ANIMAL LIFE AND THE MICROSCOPE
There are few more interesting animals than spiders and we may spend many an hour learning details of their structure, which only the microscope can show, and studying their habits, for only by doing so is it brought home to us how astonishingly clever they are. The spider, of course, is not an insect; it has eight legs, whereas the insect has only six, its head and thorax are fused, but in the case of insects head and thorax are separate. There are many other, less evident, points of difference as we shall see.
For the microscope, there are few better objects in animal land than the feet of spiders. Their study will give us plenty of occupation for they are modelled on various plans, according to the different kinds of spider. Taking the common garden spider as our first example we shall find that its foot is a most ingenious contrivance. Our microscope will show us that the foot is armed with a pair of comb-like claws. A little study of the habits of the spider will enlighten us concerning the uses of these combs. At this point we may remark that the examination of living creatures beneath the microscope should, whenever possible, go hand in hand with a study of habits. Over and over again in our microscopical investigations we shall come across structures which appear to be useless as far as we can surmise. A careful observation of the living owners of these puzzling structures will probably clear up the whole matter. Well, let us watch a garden spider; if we do so intelligently we shall see two uses of these combs and may guess the third. The spider uses its combs as we do, to straighten its hair; they also clean its body. It uses them to obtain a firm grasp of the threads of which its web is composed and, though we cannot see this, so quick are the movements of the creature, the combs serve a very useful purpose in holding captured prey.
The garden spider and its relatives are distinguished by the fact that, in addition to the two large comb-like claws, they possess a third smaller claw and some toothed spines. The small claw and toothed spines are movable and, when pressed against very firm grasp. With these cleverly contrived feet she—it is always the lady spider who makes the web and does all the work—hauls in the slack of the combs of the larger claws afford their owner a her web and owing to their firm grasp she can run readily over its meshes.
The house spider, which spins a web seemingly in a disordered tangle and quite unlike the beautiful web of the garden spider, has feet of a different pattern. The most interesting feature about the legs of this creature is the wonderful double comb with which it teases out the threads of its web as they are formed. This comb takes the form of a double row of minute, curved spines on the last joint but one of the hind legs; it must certainly be examined under our microscope and we should try to see the combs being used by the spider.
We must also make a point of examining the feet of a wolf spider for they are constructed on a different plan to those of the spiders we have mentioned. Wolf spiders are the creatures which spin no proper web but lurk in holes in walls or in the ground and dash out from their hiding places to seize their prey. They usually line their lairs with silk. We shall have more to say about wolf spiders in a moment.
The Zebra spider, which belongs to the family of jumping spiders, has very curious feet, not so much on account of its claws as because of the curious clubbed hairs which adorn them. This little spider is black, with white stripes on body and legs; the peculiar habit, for a spider, of leaping upon its remarkable hairs on its feet render it exceedingly sure-footed and it has need to be, for it exhibits prey.
There are many other spiders which we may examine with the certainty of finding some features of interest, the Drassid spiders which lurk beneath bark and stones; the crab spiders usually brightly coloured little fellows with the habit of living in flowers; the little money-spinners and the harvest-men; these last are not true spiders but they are none the less interesting, they are the small-bodied, very long legged creatures which occasionally find their way into our houses.
Having taken our fill of the spiders’ feet we may well turn our attention to their heads. If we have caught a spider in the act of killing a struggling fly, it must have struck us that one bite from the spider is sufficient to kill its victim. Let us see if we can find the jaws which so quickly bring death even to large insects. We shall require a steady hand and some little skill to examine them properly but the task is not beyond our powers. Having killed our spider we must snip off its head, place it on a slide and examine it with a low magnification. Looking straight at the face, we can plainly see the sharply pointed, hinged jaws; in nearly all spiders they work from side to side and they can be closed on their hinges like pocket knives. With a pair of mounted needles and two steady hands, let us dissect the head of our spider, so that we obtain one of the jaws quite free from its surroundings. At the base we shall find a little sac, the poison gland, and if we now magnify the jaw much more highly we shall observe a tiny hole very near the tip. When the spider has grasped her prey in her jaws she causes the poison from the poison glands to pass into the body of her victim, by way of the little hole in her jaw; the poison causes paralysis and the victim struggles no more.
The only excuse we can make for spending so much time with the spiders is that they are of the greatest interest to the microscopist. Returning to our friend the garden spider we must examine the spinning organs, known as spinnerets. These are to be found near the tip of the abdomen on the under side. There are six pairs in all in the garden spider but the middle pair are shorter than the others and, in consequence, not easily seen. The tips of the fleshy little spinnerets should be highly magnified and we shall notice that the structure of the tips differs in each pair of spinnerets. On the foremost pair, there is a fairly large projection and numerous small ones; on the middle pair three large projections and many smaller ones, whilst on the hind pair, in addition to the small projections there are five large ones. The large projections are called spigots and the small ones are known as spools; from the former is derived the strong silk of the web, from the spools the fine threads issue.