Kitabı oku: «Eclectic Magazine of Foreign Literature, Science, and Art, March 1885», sayfa 17

HOW INSECTS BREATHE

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.

PIERRE'S MOTTO: A CHACUN SELON SON TRAVAIL

A TALK IN A PARISIAN WORKSHOP ABOUT THE UNEQUAL DISTRIBUTION OF WEALTH

“A chacun selon son travail, To each man according to his work, that's my way of looking at it. Go by that motto and things will soon come right.”

I heard this said, with great emphasis, by Pierre Nigaud to some of his mates as I entered the workshop. I went there every month to collect the contributions to a Provident Insurance Club, to which several of the men belonged. Pierre was on the whole an industrious as well as clever workman, and had joined the club readily, as he thought it right to save something for his wife and children, and to provide for a rainy day, as the saying is.

I had observed, however, that Pierre on the last occasion when I saw him was less frank than he used to be, and did not hand over his money with the same cheerful goodwill as formerly. What was the cause I did not know, but he soon made it plain. He had been listening to some plausible people, or reading some shallow treatises that made him discontented with his lot.

“I was just saying when you came in,” he began, “A chacun selon son travail, To each man according to his work. Don't you think that a good motto?”

“Well, it sounds good, but it depends how you apply it, and what you are talking about.”

“I was talking, I and my mates, about the great inequality among people. Riches are distributed in a very strange and, I say, unjust fashion. Is it not unjust that, while so many poor fellows have to work hard to gain a few pence a day, there are wealthy Nabobs who haul in gold by shovelfuls? I read in a paper the other day that the English Duke of Westminster has an income of twenty millions of francs, which brings him at least 50,000 francs a day!”

“Quite true, and he is far from being the most wealthy man you might name, I believe the Californian Mackay has about seventy millions of income. Rothschild, of Frankfort, left more than a milliard. Astor and Vanderbilt, of New York, and other millionaires on both sides the ocean, have untold wealth.”

“There, you see,” said Pierre; “and what appears to me the worst wrong of all is that these huge incomes belong to people who do next to nothing, while poverty is oftenest the lot of those who work and toil the hardest. I call this downright injustice. A chacun selon son travail. The riches ought to be with those that work. That's my way of looking at it.”

“All right, Pierre,” said I; “there is a good deal of truth in what you say. It is quite true that in regard to the distribution of wealth, as in regard to many other things, this world is far from being perfect. But do you think that if you had the re-arrangement of society, and the redistribution of riches, you could proceed on some other and better plan?”

“Certainly. I believe, without any presumption, that I could,” said Pierre. “What seems to me difficult is not to make things better, but to make them any worse than they are now!”

One of the workmen here said that nothing was simpler than to take the surplus wealth of these rich men, and divide it amongst the deserving poor.

“That plan is just a little too simple,” I remarked. “All the millions of a Rothschild would go a very little way, if divided among the population of Paris alone, and we should soon have to resort to other schemes to redress the ever-renewed inequalities. No; no; what I want Pierre to show us is some better system of society, and he thinks he has the key to the problem in his favorite motto, A chacun selon son travail. But just let me remind you that in ancient times there was a king of Spain who was a bit of an astronomer; and looking at the heavens, and wondering at the complicated movements of the stars, he said that if he had been consulted in the matter he could have made a much better and simpler arrangement. Your purpose is not so ambitious and presumptuous as his, for the heavens are the work of the Almighty, who has imposed upon Nature certain fixed laws; whereas the laws of society are the work of men, and men are liable to err. Let us then hear what improvement you can suggest in the laws and usages which regulate the distribution of wealth.”

Pierre was somewhat taken aback, for he felt that the existing arrangements of society were very complex, and it was not easy to determine where the reform should begin.

“Well,” said I, “let us suppose that a number of persons were set on shore upon an island, where none had any rights or property beyond the others. Let us suppose that there are as yet no laws, that there is no government, no past history: all are free and equal, and you have full power to organise the distribution of wealth in this new society, and to decide what is to be the share of each. Come now, you have a carte blanche, let us hear what you would do.”

“Well,” said Pierre, “I should begin by deciding that every one was to do what he would and what he could, and that every one should keep what he was able by his work and industry to obtain. A chacun selon son travail: behold my fundamental rule!”

“It is an excellent rule,” I said, “and I do not think any one could find a better. It appears to me to be just, and also eminently practical, for it would stimulate every one to produce by his industry as much as he could. I see by this that you are no advocate of Communism.”

“Certainly not,” said Pierre. “Communism is a very good thing in a family, where every one exerts himself to work for those he loves, and accepts without murmur his share of work, certain that the mother, or whoever is housekeeper, manages the common purse with thrift, and in the interest of all. But in a large society, I do not think that men are equally willing to exert themselves for those whom they have no knowledge of and no special attachment to. Besides, in Communism under the State, the manager holding the purse strings would be no other than the Government, and I would not have confidence in its management being wise and economical.”

“I quite agree with you. But let us return to your plan. After establishing your principle, “to each one the produce of his labor,” what would you do then?”

“Nothing at all; every one would then stand on his own bottom. He that works well would have sufficient, and he who did no work would have nothing.”

“You do not imagine,” I observed, “that you would obtain equality by these conditions? Since every one has to take his part in the work, it is evident that these parts will be small or great, according as each is industrious or not. You would soon come to have in your new society the rich and the poor.”

“Well, perhaps; but at all events there would be none too rich or too poor.”

“How do you know that? Here are two families: in one the habits of work, of order, of economy, are hereditary; the other is given, from father to son, to idleness, improvidence, and dissipation. The distance that separates these families, small at first, must go on increasing, till in the natural course of things, sooner or later, there would come to be the same inequality as between Rothschild and a beggar. It would only be a question of time.”

Pierre's companions, who were listening attentively to the discussion, here murmured assent, or what would correspond to the “Hear, hear!” of more formal debates. Pierre, however, merely remarked that this result might seem opposed to his views, but that he nevertheless accepted it; “because,” said he, “in this case the inequality of riches would at least be the result of work and of the efforts of each worker. There would be no injustice.”

“Pardon me, Pierre, but I think that your motto is still causing you to cherish some illusions. Let me show you my way of looking at it. A chacun selon son travail, you say, To every one the product of his own industry. But what is the proprietor to do with the product of his labor? He will no doubt sell all that is over and above what he needs for his own use, and the price of what is sold will form his income. But the price of things depends on a variety of conditions independent of our personal labor and our own will; such for instance, as the vicissitudes of seasons and the variations of the markets. Out of a difference of ten francs in the price of wine may result the fortune or the ruin of a proprietor, and that proves nothing as to his having himself labored well or ill. The revenue or net profit is rarely in exact proportion to the labor bestowed, in farming or vine-growing or any other industry. What we call chance will always play its part in the affairs of this world, and in the new world which you are planning you cannot hinder Fortune from dispensing her favors in an unequal fashion; it is not without reason that she is represented with a bandage over her eyes!”

“Ah, bah!” exclaimed Pierre; “you disconcert me with your suppositions. What do you want? I firmly believe that in my colony, as everywhere, there will be good and bad luck, but while the chances are equal for all, and there is no place for wrong-doing or trickery, I console myself. At least you will admit that my principle, A chacun le produit de son travail, will have this good result, that it will render impossible the existence of rich idle people who pass their life in doing nothing.”

“Are you quite sure of that, Pierre? If any one after working ten or twenty years has produced enough property to suffice for his wants during the remainder of his days, do you pretend to hinder him from spending in his own way, in idleness if he pleases, what he had amassed by his labors?”

“Certainly not, because such a one would be living on the product of his own toil. Let a man rest in the evening after having worked hard in the morning, and let him live in ease in his old age after having produced enough by the toil of his youth; I see no harm in that. I have no wish to condemn the members of my colony to forced labor in perpetuity. The only idlers that I wish to exclude are those who live without ever having worked at all or produced anything – the rentiers, as they call them, or idle people, who live on their income, or the interest of their money.”

“Stop now, Pierre; do you admit that a man who has obtained anything by his labor has the right to do what he pleases with it?”

“Assuredly.”

“Here is a man who has made a loaf of bread. You admit his right to eat it all if he is hungry, or to set part of it aside if he has not appetite at the time for all of it, or even to throw some of it away, as he pleases.”

“Yes, it is a consequence of my principle, A chacun le produit de son travail. He who creates wealth has the right to dispose of it as he pleases. But what has that to do with your argument?”

“Just this. If he who produces a thing can do what he pleases with it, he can surely give it where he pleases. If, then, it suits me to make every day a loaf for you, and to give it to you; still more, if it pleases me to give to you out of my property or to bequeath to you after my death enough bread, or, what comes to the same thing, enough money to support you during your life, you will have acquired the means of walking about with your hands in your pockets like an idle gentleman. You will, in fact, have become a rentier.”