RESPIRATION OF THE IMAGO.
Perfect insects breathe. That this is so, the following experiments[Z] will satisfactorily prove. A spider and a fly were put into a glass jar, the mouth of which was closed all but a hole by which a tube was admitted. The tube was then connected with a pipe by which a supply of common coal-gas was poured into the jar, and when it was considered to be full of gas, and that all the air had been displaced, the tube was closed, and the insects were left shut up in an atmosphere of gas. In a few seconds both became very uneasy, the fly more particularly so; and in a few seconds more the fly began to agitate itself, to buzz against the sides of the glass, and to tumble over in the most extraordinary manner; the spider ran hurriedly about, as if astonished, and not knowing what to make of his new position; but presently he became very quiet, and, turning on his back, looked as if dead. For a minute or two the fly, which was a large, active, flesh-fly, continued its noisy evolutions, dashing itself, as if intoxicated, on every side of its transparent prison. At length, as if exhausted, it lay on its back, its limbs paralyzed, but the wings still moving with extreme rapidity, and causing it to spin round in the most singular manner. Ultimately it, too, became perfectly without motion. After the lapse of about ten minutes, fresh air was gradually let into the jar, and it became most amusing to watch the return of both these insects, which had previously been as it were in the very jaws of death, back to life again; twitchings of their limbs, and slight convulsive movements, were the first indications of returning activity; and in a few minutes more both insects, now placed in a perfectly pure atmosphere, were as lively as before the experiment, and were allowed to make their escape into the open air. With other insects the same effects were produced by saturating a piece of blotting paper with ether, and dropping it into the jar, which was immediately covered over. In several other experiments the newly-discovered powerful fluid chloroform, by means of which the operations of surgery are performed without pain, was employed; and it was remarkable to notice how quickly the insects were overpowered with the vapour of this potent liquid. In no instance was death produced by the gases or vapours employed. Wishing to destroy without pain a beautiful specimen of a goat-moth, it was first stupified with the gas, then transfixed with a pin, and again plunged in an atmosphere of gas all night long; but alas! for the unhappy insect, the morning dawned, and it was yet alive.
In these simple experiments, which were repeated sufficiently often to render the results certain, we have a most interesting proof that insects breathe. The first struggles and apparent intoxication can be compared to nothing more exactly than to the same phenomena when they take place in human beings breathing the vapours of ether, or chloroform. The subsequent state of insensibility was precisely similar to that induced in persons who are put under the influence of these vapours. An amusing instance of the restorative powers of fresh air occurred in the course of these experiments. A blue-bottle, which was purposely selected of as large a species as could be found, was first intoxicated and rendered insensible by coal gas, and was then as quickly as possible put into a net of the geometric spider: instantly the spider flew upon his bulky captive, but the fresh air began to take effect on the fly, and it commenced struggling in the most furious manner. Although the spider cast out a cloud of web, and in the most energetic manner endeavoured to envelope its prey, the fly seemed likely to be the victor; one of its large and powerful wings got free and flapped violently until the meshes of the web gave way, or was, perhaps, broken by the weight of the insect, and fly and spider, being suspended in the air, spun round so swiftly as to be quite indistinguishable. Ultimately, the spider conquered; and, suspending the huge body of its victim by a rope in the air, returned for a short time to its den.
If insects do not breathe, then all these results are quite unintelligible; for it would be immaterial to the insect what atmosphere it might happen to be placed in. A number of illustrious authors have recorded the result of their experiments upon the effects of other gases on insects. Bees, and other insects, have been placed under the exhausted receiver of an air-pump, and death ensued in a short space of time. It has also been found, that when the bodies of insects are covered with oil or grease, so as to stop up their breathing pores, they soon expire. It has been proved by carefully conducted experiments performed by Mr. Ellis, M. Sorg, and other observers, that on placing flies in a measured quantity of atmospheric air, at the end of a day or two almost the whole amount of the oxygen of the air had disappeared, and the same amount of carbonic acid had appeared in the chamber in which the insects were confined. We need not, however, after all that has been said on the subject of the respiration of insects in their various stages, enter into any further proofs that insects breathe not less perfectly in their imago condition than in any of the preceding states; in fact, they perform this function far more prominently in this their last state than in any of the foregoing. It will also be unnecessary to enter into a consideration of the minute particulars connected with this subject, as several of these have been previously glanced at.[AA]
The Air-tubes magnified.
The air enters the body of the insect at the spiracles, or breathing-pores, which are arranged along the abdomen, and situated at the side of the wings, and is received by a large tube on each side of the body, running its whole length, or nearly so, as we before mentioned. This tube, and those which it gives off as branches, are called tracheæ, or air-pipes. The tracheæ are formed in the most wonderful manner, so as to keep them always open, that the air can readily pass through them. Perhaps some of our readers have seen the contrivance adopted by sailors in order to blow air down into the hold and cabins of the ship; for those who have not, we may take the liberty of mentioning, that it is a large tube made of canvass, open at the side near the top, so as to catch the wind, while the bottom of it opens into the cabin. Now, the sailors always place the mouth of this tube towards the wind, which is forced down the tube into the cabin either by its own speed, or by that of the vessel, in the case of a steamer, and so fills the cabin below continually with fresh air. But, it will be asked, how do they contrive so to keep the sides of the bag from flapping together as to prevent them from closing? By imitating a contrivance which exists in the trachea or air-tube of the insect; they place at certain distances inside this canvass tube a series of wooden hoops, which entirely prevent its collapsing, as it otherwise would do. The most minute air-tubes of an insect are furnished, not, indeed, with a series of rings, but with what serves the same purpose, a firm but delicate coil of cartilage like a spiral spring, over which the membranous lining of the tube is stretched. Thus these tubes remain constantly open; and, whatever may be the movements of the insect, they retain this position. The tracheæ branch out in a most wonderful manner inside the insect's body: as thickly as in our bodies and in those of animals the fine blood-vessels divide until they form a mesh so close that we cannot prick ourselves with a pin without piercing some little tubes, and drawing blood; so in those of the insect, the minute air-pipes run in every direction, and divide into countless thousands of branches, which only become visible under the highest powers of a good microscope. The large tubes are seen without difficulty in many insects; and a drawing, which may give some idea of the immense mass of air-tubes existing in the body of an insect, is annexed: all the rest of the body is supposed to be cut away, and the larger tubes only are shown, as it is impossible to represent the smaller ones so as to give a clear view of their arrangement; they would cover the drawing like a cloud of the finest and most delicate lace-work. In short, in insects, there being no separate lungs in a particular place, as in the animal frame, the air is necessarily carried all over the body, so as to be supplied even to the most minute parts; thus the lungs may be said to extend through the whole body.
