Common objects of the microscope

by J. G. Wood

The eggs of insects are all very beautiful, and three of the most curious forms are given on Plate VIII.

Fig. [2] is the empty egg of the gad-fly, as it appears when fastened to a hair of the horse. Fig. [5] represents the pretty ribbed egg of the common tortoise-shell butterfly; and Fig. [7] is the very beautiful egg of the very horrid bed-bug, worthy of notice on account of the curious lid with which its extremity is closed, by means of which the young larva creeps out as soon as it is hatched.

The feathers of birds, and the fur of animals, will furnish many examples of the eggs of parasites, some of which are of extreme beauty. The feather or hair may be mounted in a cell without disturbing the eggs, which should, however, be heated sufficiently to kill the embryo if present.

Fig. [9] shows the penetrating portions of the sting of the wasp. The two barbed stings, which seem to be the minute prototypes of the many-barbed spears of the South Sea islanders, are seen lying one at each side of their sheath, and a single barb is drawn a little to the left on a very much larger scale. It is by reason of these barbs that the sting is always left adhering to the wound, and is generally drawn wholly out of the insect, causing its death in a short while.

The sting is only found in female insects, and is supposed to be analogous to the “ovipositor” of other insects, i.e. the instrument by which the eggs are deposited in their places. Fig. [20] shows the curious egg-placing apparatus of one of the saw-flies. The backs of these “saws” work in grooves, and they work alternately, so that the fly takes but a very short time in cutting a slit in the young bark of a tender shoot, and laying her eggs in the slit. When she has completed one of these channels, she sets to work upon another, and in the early spring the young branches of the gooseberry bushes may be seen plentifully covered with these grooves and the eggs. When hatched, black caterpillar-like grubs from the eggs issue, and devastate the bushes sadly, turning in process of time into blackish flies, which are seen hovering in numbers over the gooseberries, and may be killed by thousands.

The scales and hairs of other animals deserve great attention. Fig. [23] is a single hair of the human beard, as it often appears when tied in a knot—by Queen Mab and her fairies, according to Mercutio. Fig. [22] is a portion of the same hair as it appears when splitting at its extremity. The structure of the hair is not, however, so well seen in this object as in that represented on Fig. [24], which is a beautiful example of white human hair that once adorned the head of the victor of Waterloo. It formed one of a tiny lock given to me by a friend, and is so admirable an example of human hair, that I forthwith mounted it for the microscope. In this hair the cells may be seen extending down its centre, and the peculiar roughened surface produced by the flattened cells which are arranged around its circumference are also seen. By steeping in caustic potash, these scales can be separated, but generally they lie along the hair in such a manner that if the hair be drawn through the fingers from base to point, their projecting ends permit it to pass freely; whilst if it be drawn in the reverse direction, they cause it to feel very harsh to the touch.

In the sheep’s wool (Fig. [30]) this structure is much more developed, and gives to the fibres the “felting” power that causes them to interlace so firmly with each other, and enables cloth—when really made of wool—to be cut without unravelling. Fig. [37] is the smooth hair of the badger; and Fig. [34] is the curious hair of the red deer, which looks as if it had been covered with a delicate net.

Fig. [28] is the soft, grey, wool-like hair of the rat; and Fig. [29] is one of the larger hairs that protrude so plentifully, and form the glistening brown coat of that animal. Fig. [38] is the curiously knobbed hair of the long-eared bat, the knobs being formed of protuberant scales that can easily be scraped off. Fig. [31] shows a hair of the common mole; and Fig. [32] is one of the long hairs of the rabbit. Fig. [27] is a flat hair of the dormouse, slightly twisted, the difference in the breadth showing where the twist has taken place. The hair of the mouse is beautifully ribbed, so as to look like a ladder. Fig. [26] is one of the very long hairs that so thickly clothe the tiger moth caterpillar; and Fig. [25] is a beautifully branched hair taken from the common humble bee.

All hairs should be examined by polarised light, with a plate of selenite, when most gorgeous colour effects may be obtained.

The four fibres mostly used in the manufacture of apparel are: wool, Fig. [30], which has already been described; linen, Fig. [39]; cotton, Fig. [40]; and silk, Fig. [41]. The structure of each is very well marked and easily made out with the microscope; so that an adulterated article can readily be detected by a practised eye. Cotton is the most common adulteration of silk and linen fabrics, and may at once be detected by its flat twisted fibre. Silk is always composed of two parallel threads, each proceeding from one of the spinnerets of the caterpillar, and it may be here remarked that if these threads are not quite parallel the silk is of bad quality. Silken fibre is always covered, when new, with a kind of varnish, usually of a bright orange colour, which gives the undressed “floss” silk its peculiar hue, but which is soluble and easily washed away in the course of manufacture.