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

by Various

With so wonderful a respiratory system, it naturally follows that an insect must be particularly susceptible to the effects of any poisonous vapor, which, being immediately carried to all parts of the body, must speedily be attended by fatal results. And this is the case in a very marked degree. A moth or beetle, which will live for hours, and even days, after receiving an injury which would cause instant death to a more highly organized being, will yet succumb in a few seconds to the fumes of ether or chloroform, owing to the fact that the deadly influence is simultaneously exerted upon all the nerve-centres of the body, instead of being confined to one or two alone.

So much for the respiratory system of insects as a group. We have seen how air is admitted into the body, how the entire bulk of the blood is continuously aerated, and how every particle of needless weight has carefully been dispensed with. There are many species, however, whose mode of life renders necessary certain further developments, in order that respiration may be carried on under circumstances which would otherwise render it impossible. Such, for example, are the various aquatic insects, which, while spending the greater part of their existence beneath the surface of the water, must yet be enabled to command a continual supply of atmospheric air. They are not, as a rule, furnished with gills like the fish, for it is necessary that they should be able to leave their ponds and streams at will, and become for the time terrestrial or aerial beings, subject to the same conditions as others of their class. But they are, nevertheless, provided with certain modifications of structure, which enable them to breathe with equal ease, whether submerged in the water, crawling upon the ground, or flying through the air.

Even in these modifications there is considerable variety, dependent in all cases upon the requirements of the individual species. The Water-beetles, for instance, which must be able to lurk concealed among the weeds, &c., until a victim comes within their reach, and then to pursue and overtake it, carry down with them a supply of air in a kind of reservoir, situated between the body and the wing-cases. The former of these is concave and the latter convex, so that a chamber of considerable size is formed, containing sufficient for their requirements during a tolerably long period of time. And in these insects the spiracles, instead of being situated along the sides of the body, are placed upon the upper surface of the abdomen, so that they open into the air-chamber itself, and allow the respiration to be carried on without the slightest difficulty or inconvenience.

There is only one drawback to this arrangement, and that is, that the increased buoyance prevents the insect from remaining beneath the water excepting at the expense of active exertion, unless it can find some submerged object to which to cling. Even this disadvantage, however, is more apparent than real, for, while on the watch for prey, it is necessary for the insect to remain as motionless as possible, and, when engaged in swimming, the peculiar action of the oar-like limbs neutralizes the tendency to rise towards the surface.

Upon an average, a water-beetle remains from fifteen to twenty minutes without requiring to breathe; this period being capable of considerable extension should occasion arise. I have forced one of these insects, for instance, to stay beneath the surface for nearly an hour and a half, by alarming it as often as it attempted to rise. Generally speaking, however, before the first half hour is over, the beetle allows itself to float to the surface, protrudes the tips of the wing-cases, and expels the exhausted air from the cavity beneath them; a fresh supply is then taken in, and the insect again dives, the entire operation occupying barely a second of time.

The Water Scorpion affords us an instance of a perfectly different structure.

Here we have a being, feeding upon living prey, which it must capture for itself, and yet sluggish and slow of foot. By stratagem alone, therefore, can it hope to succeed, and it accordingly lies hidden among the dead leaves, sticks, &c., at the bottom of the water until some luckless insect passes within reach of its jaw-like fore-limbs. But this may not occur for hours, and it is imperatively necessary that no alarm should be given by frequent journeys to the surface in search of air. So, the extremity of the body is furnished with a curious organ consisting of two long filaments, which are, in reality, tubular, and which serve to convey air to the spiracles. The extreme tips of these project slightly above the surface when the insect is at rest at the bottom of the pond, so that respiration can be carried on without difficulty, and without necessitating the slightest change of position.

A still more curious structure, although of very much the same character, is afforded us by the grubs of the common Drone-fly. These are inhabitants of the thickest and most fetid mud, dwelling entirely beneath its surface, and consequently cut off from all personal communication with the atmosphere. But from the end of the body proceeds a long tube, which can be lengthened or shortened at will, somewhat after the manner of a telescope, and which conveys air to the spiracles just as do the tail filaments of the water scorpion. Unable to change their position, these “rat-tailed maggots,” as they are popularly called, are yet independent of any alteration in the depth of the water above them, for the air-tube can be instantly regulated to the required length, and so insure an uninterrupted supply of air.

Yet another system we find employed in the case of the grub of the Dragon-fly, which stands almost alone among insects in its power of extracting the necessary oxygen from the water itself. This is one of the most rapacious of living beings, ever upon the watch for prey, and securing its victims, not by stealth and fraud, but by open attack. Its swimming powers, consequently, are of a very high order, and are due to an organ which serves the double purpose of locomotion and respiration, and which is one of the most wonderful pieces of structure to be found in the whole of the insect world.

If a dragon-fly grub be even casually examined, a curious five-pointed appendage will be noticed at the extremity of the body. If these five points be carefully separated they will be seen to surround the entrance to a tubular passage, of about the diameter of an ordinary pin. This passage runs throughout almost the entire length of the body, and, by the expansion and contraction of the abdominal muscles, can be opened and closed at will.