BY THEODORE WOOD.

Perhaps in the entire range of insect anatomy there is no point more truly marvellous than the manner in which the respiratory system is modified, in order to suit it to the peculiar requirements of its owners.

In many ways the structure of the insects is wonderful enough. They are gifted with muscles of extraordinary strength, and are yet destitute of bones to which those muscles can be attached; they possess a circulatory system, and are yet without a heart; they perform acts involving the exercise of certain mental qualities, and are yet without a brain. But, more remarkable still, they breathe atmospheric air without the aid of lungs.

And this for a very good reason. It can be neither too often nor too strongly insisted upon that, throughout animated nature, Structure is in all cases subservient to Habit. If in any animal we find some singular development in bodily form, we may be quite sure that there is a peculiarity in the life-history which renders such development of particular service, and so may often gain very complete information with regard to the habits by a mere glance at external characteristics. If, for example, the general shape is cylindrical, the toes webbed, and the hair set closely against the body, we may safely conclude that the animal is one intended for a life in the water. If the form is conical, the limbs short, and the claws large and strong, that it is one which burrows in the earth. If the jaws are large and massive, the teeth long and sharply pointed, and the muscular power is concentrated principally into the fore-parts of the body, that it is a beast of prey. And so on with minor details.

And this rule holds equally good in the case of the insects, which are devoid of lungs for the very sufficient reason that those organs are necessarily weighty, and consequently unsuitable to the requirements of beings which are in great measure creatures of air. In all animals intended for a more or less aerial existence every particle of superfluous weight must be dispensed with, in order that the strain upon the muscles of flight may be reduced to the least possible degree. Take the bats, and see how the skeleton has been attenuated until it scarcely seems capable of affording the necessary rigidity to the frame. Take the birds, and see how a large portion of the body is occupied by supplementary air-cells, which permeate the very bones themselves, and thus minimize the weight without detracting from the strength. And so also with the insects, but in a different manner.

For in them the very lungs themselves are taken away, and replaced by a respiratory system of great simplicity, and yet of wonderful intricacy, which penetrates to every part of the structure, and simultaneously aerates the whole of the blood contained in the body. In other words, an insect is one large Lung.

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.

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.

When open, of course, it is instantly filled with water; when closed, the contents are driven out with some little force. Consequently, the action of the ejected fluid upon the surrounding water drives the insect sharply forward, just as a sky-rocket rises into the air owing to the action of the expelled gases upon the atmosphere. As soon as the effect of the first stroke is at an end a second contraction of the body takes place, and the operation is repeated as often as necessary. The water, while in the swimming tube, however, is exhausted of its oxygen, for the entrances to the respiratory system are inside instead of outside the body, and act in much the same manner as do the gills of a fish. The insect, therefore, is not obliged to visit the surface of the water at all, and can continue to search for prey without interruption.

Such are some of the many modifications brought about in insect structure by the requirements of the respiratory organs alone. Each, as will be noticed, is specially adapted to individual wants, and each is absolutely perfect in its own way, insuring a continual supply of oxygen for the purification of the blood, whatever the conditions under which life may be carried on.—Good Words.