Essays on the Microscope / Containing a Practical Description of the Most Improved Microscopes, a General History of Insects, etc., etc.

by George Adams

Of all the modifications of which matter is susceptible, the most noble is undoubtedly the organization thereof. In the structure of animals, the Sovereign Wisdom is exhibited to our view in the most striking manner. The body of an animal is a little particular system more or less complicated, and which, like the system of the universe at large, is the result of the combination and connection of a multitude of different parts, which all conspire to produce one general effect, the manifestation of the principle which we term life. So wonderful are these combinations that we are incapable of comprehending, or even of admiring sufficiently the astonishing apparatus of springs, levers, counter-weights, tubes of different diameters, &c. which constitute these organical machines. The interior parts of the insect, the most despicable in appearance, would absorb all the powers of the most able anatomist. He would be lost in the labyrinth as soon as he attempted to explore all its windings. A truth that will be evident to every one who considers only the small portion here introduced of the anatomy of the caterpillar inhabiting the trunk of the willow-tree. This caterpillar produces the phalæna cossus, or goat-moth. M. Lyonet in his admirable work entitled, “Traite Anatomique de la Chenille qui ronge le Bois de Saule,” has given an ample and minute description of this insect. In the following concise abstract enough will appear to convince the reader of the utility of microscopic glasses, in displaying the wonders of the creation, and to afford additional proof that the attention of the Almighty is not confined merely to objects of magnitude.

In a former edition of this work, I entered into a more minute detail of the several parts contained in the figures exhibited in [plate XII]. This account I have now omitted, as after all it could not convey a clear idea of the muscles alone, much less of the different parts of the caterpillar, without a reference to other plates of M. Lyonet’s work. I therefore concluded it would be better to let the figures speak for themselves, and then give a general description of the interior parts of the caterpillar; referring the reader for full particulars to the original.

Figures 1 and 2 represent the muscles of the caterpillar, when it is opened at the belly. Fig. 3 and 4 exhibit a view of the muscles when it is opened at the back. Fig. 5 and 6, an anatomical delineation of the head; so complex is this organ, that in order to give an adequate idea of its structure, M. Lyonet has employed no less than twenty figures. Fig. 7 is an out-line of the head more magnified than in the last figures. In order to obtain the views here exhibited, the muscles were freed as well from fat, as from the nerves and other vessels.

The BODY of the caterpillar in the [Plate] Fig. 2 and 3, is divided into twelve parts, corresponding to its rings marked by the numbers 1 to 12; to the first number the word RING is affixed. Each of these rings is distinguished from that which follows, and that preceding it, by a kind of neck or small hollow part. By conceiving a line to pass through these necks, and forming boundaries to the rings, we acquire twelve more divisions, Fig. 1 and 4; these are also marked with the numbers 1 to 12; to the first the word DIVISION is annexed. The several parts exhibited in the divisions, Fig. 1, are the muscles; those in Fig. 2, under the word ring, are also muscles, which appear when those in Fig. 1 are removed, lying under them.

The anatomical delineation of the muscles of the head, Fig. 5 and 6, should be considered as consisting of two figures, which join in the middle, being terminated by the superior and inferior lines. The head, as here represented, is magnified about three-hundred times. H H are the two palpi: the truncated muscles d, belong to the lower lip, and form a part of those which give it motion: K, the two ganglions of the neck united: I I, the two silk vessels: L, the oesophagus: M, the two dissolving vessels: the Hebrew letters denote the continuation of the cephalic arteries: S T U W and X are the ten abductor muscles of the jaw: under e e and f f are seen four occipital muscles: a a, a nerve of the first pair, belonging to the ganglion of the neck; b, a branch of this nerve.

Fig. 7 is an outline of the head magnified considerably more than in the last figure, exhibiting the nerves as seen from the under part. Excepting in two or three instances, only one nerve of each pair is shewn, as a greater number would have occasioned confusion. The nerves of the first ganglion of the neck are designed by capital letters; those of the ganglion a, are distinguished by Roman letters; those of the small ganglion, by Greek characters; and those of the frontal ganglion, except one, by numbers.

A GENERAL VIEW OF THE INTERIOR PARTS OF THE CATERPILLAR.

The MUSCLES have neither the exterior form, nor the colour of those of larger animals. In their natural state they are soft, and have the appearance of a jelly; they are of a greyish blue, and the silver-coloured appearance of the aerial or pulmonary vessels, which creep over and penetrate their substance, exhibits under the microscope a most beautiful spectacle. When the caterpillar has been soaked for some time in spirit of wine, they lose their elasticity and transparency, and become firm, opake, and white; the aerial vessels disappear. At first sight they might be taken for tendons, as they are of the same colour and possess almost the same lustre. They are generally flat, and of an equal size throughout; the middle seldom differs either in colour, substance, or size, from the extremities. The ends are fixed to the skin; the rest of the muscle is generally free and floating; several of them branch out considerably; the branches extend sometimes so far, that it is not always easy to discover whether they are distinct and separate muscles, or parts of another. They are of a moderate strength; those that have been soaked in spirit of wine, when examined by the microscope, will be found to be covered with a membrane which may be separated from them; they then appear to consist of several parallel bands, disposed according to the length of the muscle. These, when divided by the assistance of very fine needles, appear to be composed of still smaller bundles of fibres, in the same direction; which, when examined by a very deep magnifier, and in a favourable light, appear twisted like a small cord. The muscular fibres of the spider, which are much larger than those of the caterpillar, are found on examination to consist of two substances, one soft, and the other hard; the last is twisted round the former spirally, and thus gives to it the afore-mentioned cord-like appearance. If the muscles are separated by means of very fine needles, in a drop of some fluid, we find that they are not only composed of fibres, membranes, and aerial vessels, but also of nerves; and, from the drops of oil that may be seen floating on the fluid, that they are also furnished with many unctuous particles. The muscles in a caterpillar are very numerous, exceeding by much those of the human body; the reader may form some idea of their number by inspecting Fig. 1 2 3 and 4 of [Plate XII.] They occupy the greatest part of the head; there is an amazing number at the oesophagus, the intestines, &c. the skin is as it were lined by different beds of them, placed one under the other, and ranged with very great symmetry. The number of muscles that our observer has been able to distinguish is truly astonishing; he found 228 in the head, 1647 in the body, and 2066 in the intestinal tube, making in all 3941!

The SPINAL MARROW, and the brain of the caterpillar, if it can be said to have any, seem to have very little relation to those of man; in the last, the brain is inclosed in a bony cavity; it occupies the greatest part of the head, and is anfractuose, and divided into lobes. There is nothing similar to this in the caterpillar; we find indeed in the head of that which we are describing, a part which seems to answer the purpose of the brain, because the nerves that are disseminated through the head are derived from it; but then this part is unprotected, and so small, that it does not occupy one-fifth part of the head; the surface is smooth, and has neither lobes nor anfractuousness; and if we must call this a brain, the caterpillar may be said to have thirteen, as there are twelve more such parts following each other in a line; they are nearly of the same size with that in the head, and of the same substance, and it is from them that the nerves are distributed through the whole body. Lest the idea of thirteen brains might be disagreeable to his readers, Lyonet has called these parts ganglions. The spinal marrow in the human species descends down the back, inclosed in a bony case; is large with respect to its length, and not divided into branches, diminishing in thickness in proportion as it is removed further from the brain. In the caterpillar, the spinal marrow goes along the belly, is not inclosed in any tube, is very small, forks out at intervals, and is nearly of the same thickness throughout, except at the ganglions. For a description of the numerous vessels, and curious texture of these parts, reference must be had to the original work of Lyonet. The substance of the spinal marrow, and of the ganglions, is not near so tender and easily separated as in man; it has a very great degree of tenacity, and does not break without considerable tension. The substance of the ganglions differs from that of the spinal marrow, as no vessels can be discovered in the latter, whereas the former are full of very delicate ones. The patient anatomist of the caterpillar has counted forty-five pair of nerves, and two single ones; so that there are ninety-two principal nerves, whose ramifications are innumerable.

The TRACHEAL ARTERIES of the caterpillar are two large aerial elastic vessels, which with their numerous ramifications may be pressed close together, and drawn out considerably, but return immediately to their usual size when the tension ceases; they creep under the skin close to the spiracula, one at the right side of the insect, the other at the left, each of them communicating with the air, by means of nine spiracula; they are nearly as long as the body, beginning at the first spiraculum, and going a little farther than the last, terminating in some branches which extend to the extremities of the body. Round about each spiraculum the tracheal artery pushes forth a great number of branches, which are again divided into smaller ones; these further subdivide, and spread through the whole body of the caterpillar. This vessel and its principal branches are composed of three coats, which may be separated one from the other. The exterior covering is a thick membrane, furnished with a great number of fibres, which describe a vast variety of circles round it, communicating with each other by numerous shoots. The second is very thin and transparent; no particular vessel is distinguished in it. The third is composed of scaly threads, which are generally turned in a spiral form, and come so near each other, as scarce to leave any interval; these threads are curiously united with the membrane which occupies the intervals, and form a tube which is always open, notwithstanding the flexure of the vessel. There are also many other peculiarities in its structure, which cannot be well explained without more plates. The principal tracheal vessels branch out into 236 smaller ones, from which there spring 1326 different ramifications.