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

by George Adams

I shall now proceed to illustrate the nature of the different transformations in insects, by giving an account of the musca chamæleon. In the worm or larva condition it lives in the water, breathes by the tail, and carries its legs within a little snout near its mouth. When the time arrives for its pupa state, it goes through the change without casting off the skin of the larva. Lastly, in the imago, or fly state, it would infallibly perish in the water, that element which had hitherto supplied it with life and motion, was not the larva by nature instructed where to choose a suitable situation for its approaching transformation.

This insect is characterized by Linnæus as “Musca chamæleon. Habitat larva in aquis dulcibus; musca supra aquam obambulare solet.” In a former edition of the Fauna Suecica he called it oestrus aquæ; but on a more minute examination, he found it was a musca; besides, the larvæ of all known oestri are nourished in the bodies of animals. The larva of this insect appears to consist of twelve annular divisions, see [Plate XI.] Fig. 1. by these it is separated into a head, thorax, and abdomen; but as the stomach and intestines lie equally in the thorax and abdomen, it is not easy to distinguish their limits until the insect approaches the pupa state. The parts most worthy of notice are the tail and snout. The tail is furnished with an elegant crown or circle of hair b, disposed quite round it in an annular form; by means of this the tail is supported on the surface of the water, while the worm or larva is moving therein, the body in the mean while hanging towards the bottom; it will sometimes remain in this situation for a considerable time, without the least sensible motion. When it is disposed to sink to the bottom by means of its tail, it generally bends the hairs of that part towards each other in the middle, but much closer towards the extremity; by these means a hollow space is formed, and the bladder of air pent up in it looks like a pearl, Fig. 2. [Plate XI.] It is by the assistance of this bubble, or little balloon, that the insect raises itself again to the surface of the water. If this bubble escape, it can replace it from the pulmonary tubes; sometimes large quantities of air may be seen to arise in bubbles from the tail of the worm to the surface of the water, and there mix with the incumbent atmosphere. This operation may be easily seen by placing the worm in a glass full of water, where it will afford a very entertaining spectacle. The snout is divided into three parts, of which that in the middle is immoveable; the two other parts grow from the sides of the former; these are moveable, vibrating in a very singular manner, like the tongues of lizards and serpents. The greatest strength of the creature is fixed in these lateral parts of the snout; it is on these that it walks when it is out of the water, appearing, as it were to walk on its mouth, using it to assist motion, as a parrot does its beak to climb, with greater advantage.

We shall now consider the external figure of this worm, as it appears with the microscope. It is small toward the head, larger about those parts which may be considered as the thorax; it then again diminishes, converging at the abdomen, and terminates in a sharp tail, surrounded with hairs in the form of the rays of a star.

This worm, the head and tail included, has twelve annular divisions, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, Fig. 3. [Plate XI.] Its skin resembles the covering of those animals that are provided with a crustaceous habit, more than it does that of naked worms or caterpillars; it is moderately hard, and like the rough skin called shagreen, being thick set with a number of grains, evenly distributed. The substance of the skin is firm and hard, and yet very flexible. On each side of the body are nine spiracula or holes, for the purpose of respiration; there are no such holes visible on the tail ring a, nor on the third ring counting from the head; for at the extremity of the tail there is an opening for the admission and expulsion of air; in the third ring the spiracula are very small, and appear only under the skin, near the place where the embryo wings of the future fly are concealed. It is remarkable that caterpillars, in general, have two rings without these spiracula; perhaps, because they change into flies with four wings, whereas this worm produces a fly that has only two. The skin has three different shades of colour; it is adorned with oblong black furrows, with spots of a light colour, and orbicular rings, from which there generally springs a hair, as in the figure before us, only the hair that grows on the insect’s side is represented; besides this, there are here and there some other larger hairs, c c. The difference of colour in this worm arises from the quantity of grains in the same space; for in proportion as there is a greater or lesser quantity of these, the furrows and rings are of a deeper or paler colour. The head d is divided into three parts, and covered with a skin, the grains on which are hardly discernible. The eyes are rather protuberant, and lie forwards near the snout. It has also two small horns i i, on the fore part of the head. The snout is crooked, and ends in a sharp point as at f; but what is altogether singular and surprizing, though no doubt wisely contrived by the great and almighty Architect, is, that this insect’s legs are placed near the snout, between the sinuses, in which the eyes are fixed. Each of these legs consists of three joints, the outermost of which is covered with hard and stiff hairs like bristles. From the next joint there springs a horny bone h h, which the insect uses as a kind of thumb; the joint is also of a black substance, between bone and horn in hardness; the third joint is of the same nature. To distinguish these particulars, the parts that form the upper sides of the mouth and the eyes must be separated by means of a small fine knife; you may then, by the assistance of the microscope, perceive that the leg is articulated, by means of some particular ligaments, with that portion of the insect’s mouth which answers to the lower jaw in the human frame. We may then also discern the muscles which serve to move the legs, and draw them up into a cavity that lies between the snout and those parts of the mouth which are near the horns i i.

This insect not only walks with these legs at the bottom of the water, but even moves itself on land by means of them; it likewise makes use of them to swim, while it keeps its tail on the surface contiguous to the air, and hangs downward with the rest of the body in the water: in this situation no motion is perceived in it, but what arises from its legs, which it moves in a most elegant manner. It is reasonable to conclude from what has been said, that the principal part of the creature’s strength lies in these legs; nor will it be difficult for those who are acquainted with the nature of the ancient hieroglyphics, which are now opening so clearly, to fix the rank of this insect in animated life, and point out those orders of being, and the moral state through which it receives its form and habits of life.

The snout is black and hard, the back part is quite solid, and somewhat of a globular form, whereas the front f, is sharp and hollow; on the back part three membranaceous divisions may be observed, by means of which, and the muscles contained in the snout, the insect can at pleasure expand or contract it.

The tail is constructed and planned with great skill and wisdom. The extreme verge or border, is surrounded by thirty hairs, and the sides adorned with others that are smaller; here and there the large hairs branch out into smaller ones, which may be reckoned as single hairs. These hairs are all rooted in the outer skin, which in this place is covered with rough grains, as may be seen by cutting it off, and holding it up, when dry, against the light, upon a thin plate of glass. By the same mode you will find, that at the extremities of the hairs there are also grains like those of the skin; in the middle of the tail there is a small opening, within it are minute holes, by which the insect inhales and expels the air it breathes. The hairs are very seldom disposed in so regular a manner as they are represented in Fig. 3. [Plate XI.] except when the insect floats with the body in the water, and the tail with its hairs a little lower than the surface, for they are then displayed exactly as delineated in the plate. The least motion downward of the tail produces a concavity in the water, and it then assumes the figure of a wine-glass, wide at the top, narrow at the bottom. The tail serves the larva both for the purposes of swimming and breathing, and it receives through the tail that which is the universal principle of life and motion in animals. By means of the hairs it can stop itself at pleasure when swimming, or remain suspended quietly in the water for any length of time. The motion of the insect in swimming is very beautiful, especially when it advances with its whole body floating on the surface of the water; after filling itself with air by the tail. To set out, it first bends the body to the right or left, and then contracts it in the form of the letter S, and again stretches it out in a strait line: by thus alternately contracting and then extending the body, it moves along on the surface of the water. It is of a very quiet disposition, and not to be disturbed by handling.

These larvæ are generally to be found in shallow standing waters, about the beginning of June, sooner or later, as the summer is more or less favourable; in some seasons they are to be found in great numbers, while in others, it is no easy matter to meet with them. They love to crawl on the plants and grass which grow in the water, and are often to be met with in ditches, floating on the surface of the water by means of their tail, the head and thorax at the same time hanging down; and in this situation they will turn over the clay and dirt with their snout and feet in search of food, which is generally a viscous matter that is common in small ponds and about the sides of ditches. This worm is very harmless, contrary to the opinion one might form at first sight, from the surprizing vibratory motion of the legs, which resembles the brandishing of an envenomed tongue or sting. They are most easily killed for dissection in spirit of turpentine.

After a certain period they pass into the pupa form; when they are about to change, they betake themselves to the herbs that float on the surface of the water, and creep gently thereon, till at length they lie partly on the dry surface, and partly on the water; when in the larva or pupa state, they can live in water, but can by no means inhabit there when changed into flies: indeed, man also, whilst in the uterus, lives in water, which he cannot do afterwards. When these worms have found a proper situation, they by degrees contract themselves, and in a manner scarce perceptible lose all power of motion. The inward parts of the worm’s tail now separate from the outmost skin, and become greatly contracted; this probably gives the insect considerable pain: by this contraction, an empty space is left in the exterior skin, into which the air soon penetrates.

Thus this insect passes into the pupa state under its own skin, entirely different from that of the caterpillar, which casts off the exterior skin at this time; this change may often be observed to take place in the space of ten or twelve hours, but in what manner it is performed we are ignorant, as it is effected in a hidden unknown way, inwardly within the skin, which conceals it from our view.