Eclectic Magazine of Foreign Literature, Science, and Art, March 1885

by Various

If we take any moderately large insect, say a wasp or a hornet, we can see, even with the naked eye, that a series of small spot-like marks runs along either side of the body. These apparent spots, which are generally eighteen or twenty in number, are in fact the apertures through which air is admitted into the system, and are generally formed in such a manner that no extraneous matter can by any possibility find entrance. Sometimes they are furnished with a pair of horny lips, which can be opened and closed at the will of the insect; in other cases they are densely fringed with stiff, interlacing bristles, forming a filter, which allows air, and air alone, to pass. But the apparatus, of whatever character it may be, is always so wonderfully perfect in its action that it has been found impossible to inject the body of a dead insect with even so subtle a medium as spirits of wine, although the subject was first immersed in the fluid, and then placed beneath the receiver of an air-pump.

The apertures in question, which are technically known as “spiracles,” communicate with two large breathing-tubes, or “tracheæ,” which extend through the entire length of the body. From these main tubes are given off innumerable branches, which run in all directions, and continually divide and subdivide until a wonderfully intricate network is formed, pervading every part of the structure, and penetrating even to the antennæ and claws.

Physiologists tell us that if in the human frame the nerves, the muscles, and the veins and arteries could be separated from one another, while retaining their own relative positions, each would be found to possess the perfect human form. In other words, there would be the nerve-man, the muscle-man, and the blood-vessel-man, as well as the bone-man which supplies the framework of the whole. In the same way we may speak of the tracheal, or breathing-tube insect; for the two main tubes and the endless ramifications of their branches, if they could be detached from the surrounding tissues while themselves suffering no displacement, would exhibit to us the form of the insect from which they were taken, and that so exactly that in many cases we should almost be able to recognize the species.

In the smaller branches of these air-vessels considerable variety is to be found. Some retain their tubular character to their very termination. Others assume a curious beaded form, dilating at short intervals into small chambers; while yet others abruptly resolve themselves into sac-like reservoirs, in which a comparatively large quantity of air is stored up. From the larger vessels are thrown off vast numbers of exceedingly delicate filaments, so small that a very powerful microscope is necessary in order to detect them, which float loosely in the blood, and furnish it with the constant supply of oxygen necessary for its purification.

Now, we may well ask ourselves how it is that these tubes, which are of almost inconceivable delicacy, should remain open during the various movements of which the flexible body is capable. Why is it, for instance, that the air-supply of the lower leg is not cut off when the limb is bent at the knee-joint? or from the head, when that important part of the frame is tucked away beneath the body? How does the Earwig contrive to breathe while folding its wings by the aid of its tail-forceps? or many of the Cocktail-beetles when curled up in their peculiar attitude of repose?

The answer to these questions is simple enough, and may be discovered by a glance at one of the most familiar of our own inventions—the flexible gas-tube. This preserves its tubular form no matter to what degree it may be bent or twisted, for coiled closely within it is a spiral wire, which obliges the interior of the pipe to retain its diameter almost unaltered alike when straight or curved. And as with this, so with the tracheæ of the insect, whose walls are formed of a double layer, the one lying inside the other, while between the two, and surrounding the inner, is coiled a fine but very strong elastic thread, whose convolutions allow the vessel to be bent in any required direction without losing its cylindrical form. By the exercise of a little care the anatomist can often unwind an inch or two of this spiral thread from a single branch of the tracheæ of a tolerably large insect, so closely is it coiled, and so elastic its character.

It will thus be seen that each expansion of the respiratory muscles causes the air to rush to every part of the body, the entire bulk of the blood being consequently aerated at each respiration. This fact is a most important one, for, as it is not necessary that the blood should be brought to a definite centre, as in the higher animals, before it can be re-vivified, and then despatched through another series of vessels upon its errand of invigorating the frame, the necessity for a circulatory system is almost wholly at an end, and a large amount of weight consequently dispensed with. Insects have neither veins nor arteries, one principal vessel running along the back, and the blood passing slowly through this, and flowing between the various organs of the body until it again enters it at the opposite extremity to that from which it emerged.

Nor is this all. With ourselves, as with the higher animals in general, nearly one-half of the blood, the venous, is always effete and useless, requiring to pass through the lungs before it can again be rendered fit for service. When this is vivified and pumped back by the heart into the system, that which was before arterial becomes venous in its turn; and so on. But not in the case of the insects. The whole bulk of their blood is arterial, if we may use the expression in speaking of animals which do not possess a vascular system. In other words, being incessantly vivified throughout the body, owing to the comprehensive character of the respiratory apparatus, no portion of it becomes at any time effete from the exhaustion of the contained oxygen. Blood so thoroughly and continually aerated, therefore, can practically perform double work, and need be far less in volume than in beings whose circulation is conducted upon different principles. The tracheal structure, consequently, while itself detracting from rather than adding to the substance of the body, permits of the abolition, not only of lungs, but also of veins and arteries and of a considerable proportion of the blood, so that the weight of the insect is reduced to the least possible degree.

There is yet another point to be considered, and that a very curious and at present unexplained one. Upon careful investigation we find that the tracheæ extend beyond the limits of the circulation, showing that they must serve some secondary purpose in addition to that generally attributed to them. For nature provides nothing in vain, and would not without good and sufficient reason have carried the breathing-tubes farther than necessary for their primary object of regenerating the blood. As to what this purpose may be, however, we have no certain knowledge, and can only conjecture that it is in some way connected with the olfactory system. It is well known that the sense of scent is in many insects very highly developed, enabling them to ascertain the position of their food while yet at a considerable distance. Burying-beetles and blowflies, for instance, will detect the faintest odor of putrid carrion, and will wing their way without hesitation to the spot whence it proceeds. Ivy-blossom, again, will attract almost every butterfly and moth in the neighborhood, and this clearly by reason of its peculiar fragrance.

It may be, therefore, that the perfection of the organs of scent in insects is due to the fact that they are distributed throughout the body, instead of being localized as is the case with animals higher in the scale. That they must be connected with the respiratory apparatus would seem, judging by analogy, to be indisputable, for, so far as we know, an odor cannot be appreciated unless the air containing it be allowed to pass more or less rapidly over the olfactory nerves. And in no other part of an insect's structure could this requisite so well be observed as in the tracheæ themselves, through which a stream of air is continually passing, and which penetrate to the remotest parts of the body.