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

by George Adams

OF THE RESPIRATION OF INSECTS.

As respiration is one of the most important actions in the life of every animal, great pains have been taken by many naturalists to investigate the nature of this action in insects; to prove its existence, and explain in what manner it is carried on. Malpighi, Swammerdam, Reaumur, and Lyonet have discovered in the caterpillar two air-vessels placed the whole length of the insect, these they have called the tracheæ; they have also shewn that an infinite number of ramifications proceed from these, and are dispersed through the whole body; that the tracheal vessels communicate with particular openings on the skin of the caterpillar, termed spiracula; there are nine of these on each side of the body. These vessels seem calculated for the reception of air; they contain no fluids, are of a cartilaginous nature, and when cut preserve their figure, and exhibit a well-terminated opening. Notwithstanding this discovery, respiration has not been proved to exist in many species of insects, and the mechanism thereof is very obscure in all; nor is the absence of it more surprising in the caterpillar or embryo state of insects, than in that of other animals, where we find that respiration is by no means necessary to existence previous to their birth, though indispensably so afterwards.

Reaumur thought that the air entered by the spiracula into the trachea, but was not expelled by the same orifice, and consequently that the respiration of insects was carried on in a manner totally different from that of other animals; that the air was expired through a number of small holes or pores which are to be found in the skin of the caterpillar, after having been conducted to them through the extremities of the finer ramifications of the tracheal vessels; whereas Bonnet, in consequence of a great variety of experiments, supposed that the inspiration and expiration of the air was through the spiracula, and that there was no expiration of air through the pores of the skin. These experiments were made either by plunging the caterpillars into water, or anointing them with fat and greasy substances, some all over, others only partially. The number of small bubbles which are observed to cover the surface of their bodies, when they are immerged in water, does not arise from the air which is included within, and then proceeding from them, but they are formed by the air which is lodged near the surface of their bodies, in the same manner that it is about all other substances. To render the experiments more accurate, and prevent the air from adhering to the skin, before he plunged the caterpillars in water he always brushed them over with an hair pencil; after this, very few air bubbles were found on their bodies when immerged in water. Caterpillars will remain a considerable time under water, without destroying the principle of life; and they also recover, in general, soon after they are taken out. To know whether a few only of the spiracula might not be sufficient for the purposes of respiration, he plunged some partially in water, so that only two or more spiracula remained in the open air: in these cases the caterpillar did not become torpid as it did when they were all immerged in water. One caterpillar, upon which Bonnet made his experiments, lived eight days suspended in water, with only two of its anterior spiracula in the air; during this time he observed, that when the insect moved itself, little streams of bubbles issued from the anterior spiracula on the left side; from this, and many other experiments, it appeared to him, that amongst all the eighteen spiracula, the two anterior and the two posterior are of the greatest use in respiration.[79] Sometimes when the apertures of these have been stopped with oil, the caterpillar has fallen into convulsions. If the posterior part had been oiled, that part became paralytic. Notwithstanding these experiments, and many more which have been made, the subject is far from being decided, and many still doubt whether there is any respiration in insects similar to ours, at least at certain periods of their life. This opinion seems to be further confirmed by the experiments of M. Lyonet. He confined several large musk beetles under a glass for more than half an hour, exposed to the fumes of burning sulphur; and, though during their continuance there the vapour was so thick that he could not see them, yet on their being liberated, they did not seem at all effected thereby.[80]

[79] Philos. Trans. vol. xlv. p. 300.

[80] Lesser Theologie des Insectes, tom. 1, p. 124. Ibid. p. 126.

Supposing respiration to be absolutely necessary to the existence of the pupæ of different insects, when we reflect on the great solidity of their cases or cones, it is not easy to conceive how they can live several months under the earth, in spaces so confined, and almost impervious to the air: and indeed if they did respire, the same situation seems to preclude a continuance of the operation, as the air would soon be corrupted, and unfit for the offices of life. As the tracheæ are divided and subdivided to a prodigious degree of minuteness, it has been conjectured by some writers, that they may act as so many sieves, which, by separations properly contrived, filtrate the air, and so furnish it to the body of different degrees of purity and subtilty, agreeable to the purposes and nature of the various parts. The experiments that have been made with the air-pump are by no means conclusive; the injury which the insect sustains when the atmospheric pressure is taken from the body, does not prove that it inspired and expired the air that we have removed; it only shews that an incumbent pressure is necessary to their comfortable existence, as it prevents the fluids from disengaging themselves in an aerial form, and as it counterbalances and re-acts on the principle of life, and, by keeping the action thereof in proper tone and order, confines and regulates its energies.

Though it is difficult to ascertain whether some insects respire, at least at certain periods of their existence, yet there are others to whom the inspiration and expiration of air seems absolutely necessary: there are many aquatic insects which are obliged to keep their tails suspended on the surface of the water for this purpose. To prove this, keep the tail under water, and you will perceive the insect to be agitated and uneasy, and to seek for some opening to expose this part to the air; if it find none, it soon goes to the bottom and dies. Some aquatic beetles resist the trial for a considerable time, while their larvæ can only support it for a few minutes. There is a circumstance which renders all experiments on this subject with insects doubtful and difficult, namely, the vast tenaciousness of the life principle in the lower orders of animated nature, and its dissemination through their whole frame.

Musschenbroeck inclosed the pupa of a moth in a glass tube, very little larger than the moth itself, and of the following figure.

The end A of the tube was drawn out in a capillary form, the other end was covered with a piece of wet bladder to exclude the air; the capillary end B was then plunged in water, which rose to D. He placed the capillary part of the tube before a microscope, on a small micrometer, in order to observe any motion or change in the situation of the water; as it is evident the expiration or inspiration of air by the insect would make it rise or fall alternately. In the first experiment he observed a small degree of motion at distant intervals, not above two or three times in an hour; in a second experiment on another subject, he could observe no motion at all. He then placed some pupæ under the receiver of an air-pump, in water which he had previously purged of its air; on exhausting the air from the receiver, he observed one bubble to arise in a part near the tail, and a few near the wings. The pupæ did not swell under the operation; on the contrary, on letting in the air, it was found to be diminished in a small degree, but in less than a quarter of an hour it recovered its former figure. M. Martinet published at Leyden, in 1753, a dissertation, in which, it is said, he has clearly proved by a number of experiments that the pupæ of caterpillars and some other insects do not respire.