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.