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.

On the underside of our garden spider there is a dark patch and, just in front of this dark spot, are a pair of slits. These we must open up very carefully, in a dead specimen of course, and within, if we have succeeded in our dissection, we shall see from fifteen to twenty little flaps resembling the leaves of a book, in fact they are known as lung books and, by means of them, the spider breathes.

One more word and we must leave the spiders. The eyes must be examined in every specimen. Most spiders have eight eyes, set like little gems in the front part of the head; some have six eyes, some only two and a few kinds are eyeless, but these last spend all their lives in dark caves, so eyes would be useless to them. When we examine the eyes of wolf spiders we shall observe that they are placed on the tops of little projections so that their owners may better be enabled to see all around them.

The hairs and scales of many spiders make beautiful objects for the microscope. We must make a point of examining the hairs of the water spider also the scales from the Zebra spider. The latter with their feathery form and iridescent colouring, are particularly beautiful. We may advantageously spend a moment or two in the examination of the spider’s web and the threads of which it is made. The strands radiating from the centre of the web differ from those which are arranged spirally. The latter are covered with a sticky substance as may be seen under the microscope. When these spiral threads are laid down by the spider, the sticky substance covers their whole length in a thin film, but the little architect adds a finishing touch, by pulling the thread as a bowman pulls his bow and then releasing it suddenly. The result of this performance is that the sticky substance forms a series of minute globules over the whole length of the thread.

In order to be in a position thoroughly to master the details of animal structure it is necessary to have acquired sufficient skill to cut sections. They cannot, however, be cut so easily as is the case with plant sections. The various parts of animals are either so hard, e.g., bones and teeth, that they must be treated almost as pieces of rock and rubbed down till they are transparent, or they are so soft that they require soaking in various chemicals to make them harder and even then it is usually necessary to imbed them, i.e., surround them with some easily melted substance which sets moderately hard, such as paraffin wax. Cutting sections of animal parts is beyond the average amateur.

The feathers of birds make beautiful objects for the microscope. For those microscopists who desire beauty of colour rather than details of structure it is hard to beat the glorious shades of certain feathers beneath the microscope. To obtain the best effect a fairly low magnification should be used and all manner of lighting thrown upon the object, for we have all seen the feather which appears drab at one angle is of the greatest brilliance at another. Various feathers and from various parts of birds should be examined, if we desire to understand their structure. Each feather consists of a vast number of cells but it is improbable that we shall be able to prove this statement by the examination of any large feather. We must take a down feather, notice carefully the arrangement of its various parts, for it will be interesting to compare this soft, weak feather with a comparatively strong flight feather from a wing. Now, under a higher magnification, we can plainly see the little cells of which the down feather is built up.

One of the strong wing feathers of such a bird as a pigeon is of the greatest interest as a microscopic object. We must take a few of the barbules, the slender, flattened portions of the feather which fringe either side of the barb. A moderately high magnification will show how ingeniously they are contrived. The hinder side of each barbule is a moderately thick upwardly curved edge whilst, on the forward side, there is a row of curved hooks. When the feather is neat and tidy, and its owner when in good health usually sees to it that its feathers are well kept, the hooks of one barbule engage with the curved edge of the next barbule. The feather, by means of this ingenious locking device, becomes much more nearly a solid structure than would be the case if the barbules did not hook on to one another. The arrangement for hooking together the fore and hind wings of bees and wasps is very similar. We may examine a number of flight feathers but we shall not find any very striking differences between those of various birds. All, apparently, follow a common design.

From feathers to hair and from hair to horns and some scales is not a very far cry. We have talked about the examination of hair in another chapter, so we will not repeat ourselves here. Scales we shall most of us have opportunities of examining in plenty. We have just mentioned that some scales are comparable to hairs and feathers. Such scales are to be found in snakes and lizards. The scales of fish are of a different order but they are equally or even more interesting when examined under the microscope.

If we live in a district where many and various fish are caught we shall soon discover that their scales differ in a remarkable degree. Some are of the texture of horn, some are gristly, some bony and some covered with enamel, after the manner of teeth. Not only do they differ in texture but in design as we shall see in a moment.

Certain fishes, the eel is one, the mackerel another, are said to be scaleless. As a fact their scales are very thin and transparent and so arranged that they are less evident than those of other fish. By taking a little of the skin of one of these fish we can easily detach a few scales for examination. Those of the eel we shall find are very thin and delicate and quite transparent. These and all other fish-scales may be made into permanent slides by mounting in Canada Balsam, as described in our last chapter.

The carp, whiting, salmon, sprat, herring and many other fish have scales called cycloid or circular; the term is rather a misnomer because they are not truly circular, but the name is used to distinguish them from other scales. The structure is easily made out with a moderate magnification. Many of these scales, however, exhibit portions more dense than the rest; these dense spots are caused by little particles of lime which may be seen under a higher magnification.

Perch, pike, sole and some other fish have much more peculiar scales, known as ctenoid or combed for the reason that their unattached, that is to say their hinder margins, are toothed like a comb.

The scales of sharks, dog-fish and rays are called placoid for they are toothed; not only so but their arrangement is frequently quite dissimilar to the scales of ordinary fish. Taking the herring as our example, but a salmon or any other fish would serve equally well, and examining the arrangement of its scales with the help of our pocket lens, we shall find that the scales are fixed to the fish by their forward edges and that each scale partly overlaps its neighbour, as do tiles on a roof. In the shark family, however, the scales are often relatively wide apart, they do not overlap but are imbedded separately in the skin. The scales of rays have each a hard spine projecting from the centre, those of sharks and dog-fish have teeth, and they are teeth not only in appearance but also in structure.

The ganoid scales of sturgeon we are hardly likely to meet with. Sometimes these fish are on sale in London and other large towns and a specimen of their scales may be procured. They are bony in structure and, though interesting, require a considerable amount of preparation to render them sufficiently transparent to be examined under the microscope.

It is interesting to note that all the fossil fish which are discovered from time to time have either ganoid or placoid scales, a fact which shows that the sharks, rays and sturgeon are directly descended from creatures which swam the seas thousands of years ago.

The shells of shell-fish are not easy to examine microscopically, but frequently their plates may be detached from the edges of such shells as oysters and mussels and these should be examined. If the outer part of the shell be taken we can easily see its honeycomb structure and, by adding a little acid and waiting till all action has ceased, we shall have a structure remaining which is remarkably like a number of plant cells. The inner layer of many of these shells is composed of beautifully iridescent mother-of-pearl. Now such iridescence is usually caused by surfaces furrowed with many very fine lines and mother-of-pearl is no exception. Under the microscope, with a moderately high magnification, we can see minute striations all practically parallel to one another.

The cuttlefish is peculiar in having a skeleton which is a moderately soft plate. These plates can often be found washed up by the tide, may be cut out from a dead cuttlefish or bought from a chemist’s as cuttlefish bone. However we secure the material we shall find that one side of the “bone” is hollow and that across this hollow, delicate plates run parallel to one another at intervals. Between these parallel plates there appear to be a number of fibres but, if we cut a thin slice of the structure and examine it under the microscope, we shall see that the apparent fibres are really very thin plates of bone which wind and double upon themselves in a beautiful manner. The structure of these plates gives strength to the bone without adding to its weight.

Snails are sure to attract the microscopist sooner or later, so too are slugs. Many of the latter have shells, small flat or ear-shaped shells, quite different to the portable homes of snails. In many young snails, which may be killed by dropping into boiling water, we can find the shells so transparent that they form good objects for our microscope. Sometimes they are composed of six-sided cells, sometimes of beautiful star-shaped cells.

From the microscopist’s point of view the most interesting feature of the snail is its rasping organ, often wrongly termed the tongue. To find this organ it is necessary to open up the mouth of a dead snail, and if we seek the assistance of our lens while doing so, we shall have no difficulty in finding the rasp—it may be recognised by the minute teeth with which it is furnished. The whole structure should be carefully removed and mounted upon a slide. In some kinds of snails there are but a hundred teeth, other kinds, however, possess as many as twenty-six thousand eight hundred. The snail makes use of this remarkable organ to procure its food. Vegetation is pressed against a plate at the top of the creature’s mouth and literally filed into small pieces by the rasping organ. Captive water snails may be watched while using their rasp upon the water plants or upon the green slime which soon accumulates in aquaria. The eggs of snails are easily found and should be examined, the queer little inmates may be studied through the transparent shells, in all stages of development.

The naturalist whose inclinations lead him towards the study of animal life will find plenty to occupy his time and his microscope. All kinds of eggs of small creatures may be watched as they develop. Frogs’ eggs are of interest in this respect, so too are tadpoles which hatch from them. The whole blood circulation in a young tadpole may easily be studied under the microscope, the structure of the external gills, the gradual change to internal gills, the development of legs, the absorption of the tail. The tadpole and its marvellous changes will afford sufficient microscopic material to last for many weeks.