LETTER XXXVII.
INSECTA. (Insects.)
Before speaking of insects, my dear child, it will be necessary, in the first place, to tell you to what primary division they belong and on what characters this division has been established. And here I find myself in a difficulty. We have been but too learned already, and now we run the risk of becoming still more so, if we commence an attack on the three primary divisions which follow the vertebrates. We shall have to encounter terrible names and tedious details, besides having to take into account a thousand things of which we have not yet spoken. We are going on quietly with the history of the feeding machine which occupies the middle of the body, and learned men never looked in that direction for the establishment of their divisions; between ourselves, it was not accommodating enough. They have fallen back upon the locomotive apparatus (movement machine) which affects the body all over, and which they have proclaimed to be the leading feature of the animal organization, without noticing however that it is, after all, but the servant of the other. It is true that the great divisions are more easily established upon this point than the other, because the differences are more decided. It separates what the other unites, and thus it is that nature carries on that beautiful combination which the Germans have so accurately named "Unity in Variety" that is to say, she is always at work, as I have already told you, on the same canvas, but always embroidering it with a different pattern. Wait! I have something to promise, if you are very good, and if this history (that of the feeding machine) should have given you a taste for inquiry. I will tell you another time the history of the movement machine, and there the classification of our learned men will come in naturally very well. In the meantime we will do as they do, and just shut our eyes to their divisions, in which the feeding machine can have no interest, because they were established without reference to it. We will content ourselves, then, without further pretension to science, with modestly examining the last transformations of our pet machine in the principal groups of the inferior animals; of which groups I will now tell you the names in their proper order. They are as follows: Insects, Crustaceans, Mollusks, Worms, and Zoophytes. You must take these names on trust; those which you do not understand will be explained in their places.
1. Insects.—I know not where it was I once read that there are said to be something like a hundred thousand different species of insects; and I verily believe this is not all. Of course we shall not attempt to review the whole of this formidable battalion. Let us take one of those you are most familiar with—the cockchafer, for instance—and examine what goes on in his inside. The history is nearly that of all the others.
"Fly away, cockchafer, fly!" says the song; and surely it is a bird that we have here, and a bird which will appear to you even more wonderful than those of which I have already spoken, when you have considered the simplicity, and at the same time the strength, of his organization. His mode of flight is rather lumbering, it is true; he is, in comparison with the large flies, what the ox is to the deer; but when you contrast the weight of his thick body with the delicacy and narrow dimensions of the two membranes which sustain him in the air, you may well ask yourself how those little morsels of wings, thin as gold-beater's skin, can carry such a mass along. In fact, they only accomplish this feat of strength by dint of an excess of activity almost startling to think of. When you run as fast as you can, how many times, think you, do you move your legs in one second? You would be somewhat puzzled to say; and so should I: but I defy you to count ten. Now the bird makes his wing move much oftener when he beats the air with rapid blows as he flies; but even he does not strike a hundred strokes in a second: and what is this to the feats of the cockchafer's wing? It is not hundreds but thousands of times that he flaps his wings in a second; and here let me hint, by-the-by, that when people seriously wish to find out a method of travelling in the air, they will lay aside balloons, of which they can make nothing in their present condition, and will set to work to fabricate machines with wings which shall beat the air as fast as those of the cockchafer. This sounds extravagant, but I have seen an electric pile fixed in a stand with glass feet, which caused a little hammer to beat thousands of times in a second: and surely the hammer could have been made to communicate its movement to a small wing! Forgive me this little castle in the air! The idea came into my head a long while ago, and the cockchafer has just reminded me of it. I will not, however, pursue the subject, neither will I offer to explain the method used for counting the beats of an insect's wing. That would carry us farther than would be desirable.
To return to our little animal. I leave you to imagine the enormous amount of strength required for such precipitate motion. We have spoken of the rapid course of the blood in birds during flight: who shall calculate its comparative rate in this fabulously wonderful locomotive, the cockchafer? And if we lift up the cuirass which encases it, what do we behold? Not a single trace of all the complicated circulation-apparatus you have learnt to know so well; neither heart nor veins nor arteries; only a quantity of whitish liquid, equally distributed throughout the whole internal cavity. Not a trace of lungs, nor any apparent means of renovation for this seemingly motionless blood; for blood it is, in spite of its color, or, at any rate, blood in its first stage of formation. It also has its globules—ill-formed, it is true, and altogether in balls—like those found in the chyle with us; which chyle, be it observed, is the same color as the blood of insects, and may also be considered blood in its apprenticeship. By what magic, then, is this raw, imperfectly-formed steward, who seems altogether stationary, enabled to accomplish exploits which would stagger his higher-bred compeers, agile and perfected as they are? Where does he pick up the oxygen necessary for such repeated movements, it being an established fact that no animal can move at all without consuming oxygen, and that the quantity consumed is in proportion to the rate of motion? Look under his wings for an answer. There, all along his body, you will observe a number of small holes, pierced in a line, at regular distances, and furnished with shutters of two kinds. They are the mouths of what are called tracheæ, or breathing tubes: and from them branch out a multitude of little canals, which, spreading in endless ramifications through every part of the body, convey to the whole mass of the blood, from all directions, the air which makes its way into them through the tracheal holes. In this case, you see, it is not the blood which seeks the air, but the air which seeks the blood; whence arises a new system of circulation, whose action is all the more energetic because it is unintermitting, and makes itself felt everywhere at the same time. A little while ago we were wondering at the twofold respiration of birds; yet this is far less surprising than the universally-diffused respiration of insects, who may well be able to do without lungs, seeing that their whole body is one vast lung in itself.
For the rest, do not trust to appearances, nor imagine that the blood of our friend the cockchafer in reality remains motionless around the air-tubes, idly drinking in the oxygen which is brought to it. Though not flowing in enclosed canals, it is not the less continually displaced by regular currents, which sweep through and renew this apparently stagnant pool. Nor is this the only instance of such a current presented to us by nature.
Guess, however, if you can, where you will have to look for the counterpart to the circulation of the cockchafer. In ocean itself! But, remember, nothing is absolutely little or great in nature, who applies her laws indifferently to a world as to an atom. The blood of our world is water, which contains in itself all the germs of fertility, and without which, as I have already told you, life is impossible either in the animal or vegetable kingdom. The water of brooks, streams, and rivers, flows along in channels, which, when figured in a map, present to the eye of the beholder an exact picture of the system of circulation found in the vertebrated animals. But the waters of the sea are borne along, like the blood of insects, by a secret circulation, which cannot be represented on the map; i.e. by immense currents everlastingly in action, some on the surface, some in the mid-heart of the ocean, which drive it in ceaseless course from the equator to the poles, from the poles to the equator; so that the Supreme Intelligence, in His overruling providence, has ordained the same law to set in movement the immensity of ocean, and to effect circulation in the cockchafer's few drops of blood. In the latter we find the moving agent to be a long tube, which runs the whole length of the back, and is called the dorsal vessel (from the Latin dorsum, back). I told you that the cockchafer had no heart under his cuirass, but I spoke too hastily. The dorsal vessel is a true heart, but a heart devoid of veins or arteries, and thrown into the midst of the blood. It dilates and contracts like ours, sucks in the blood by means of side-valves, which act as our own do, and drives it back again into the mass by that valve at its extremities, which opens near the head. From thence arises a continued to-and-fro movement, which sends the blood from the head to the tail, and brings it back again from the tail to the head. But who would recognise, in this simple primitive organisation, where all seems to go on of its own accord, as it were, the same machine, with all its complicated movements, that we have been so long considering?
Well, in this apparently universal shipwreck of all the organs we know so well, there is yet one which survives, and remains the same as ever, namely, the digestive tube. I began by saying the insect is a bird. His digestive tube is formed upon the same pattern as that of birds, so that naturalists have bestowed the same names on the various parts in each of them. After the oesophagus comes a crop (jabot), very distinctly indicated; then a gizzard with thick coats, in which the food is ground down. The hen, if you remember, swallows small pebbles, which perform in her gizzard the office of the teeth in our mouths. The cockchafer has no need to swallow anything. His gizzard is furnished with little pieces of horn; real teeth, fixed in their places, which have a great advantage over the chance teeth picked up at random by the hen. I pointed out to you in birds, between the crop and the gizzard, a swelling or enlargement of the digestive tube, pitted with small holes, where the food is moistened by juices. The same enlargement is found here, covered all over with a multitude of small tubes, which might easily be mistaken for hairs, from which also falls a perfect shower of juices. The only difference is, that it comes after the gizzard, instead of before it, as in birds. Some naturalists, considering that the manufacture of chyle takes place here, have called it the chylific ventricle; [Footnote: The corresponding protuberance of the birds bears a name, somewhat similar, but stillmore barbarous. I had passed it over in silence, because, I make the confession in all humility, I do not understand it; but a remorse now seizes me: it is called the Ventricule succenturie.] a somewhat barbarous name, but one which explains itself, and might with truth be applied to the duodenum of the higher animals. Bile is poured in close to the hinder end of it, but you must not look for the liver; it has disappeared, or rather its form is entirely changed. You remember what the pancreas had become in fishes; i.e. a row of tubes giving out a salivary fluid. Such is exactly the appearance of the liver in the cockchafer.
Instead of that fleshy substance on which hitherto the office of preparing the bile had devolved, you see nothing but a floating bundle of long loose tubes, which, opening into the intestines, pour in their bile. The organ is transformed, but we recognise it again by the office it performs, which continues the same. As to the pancreas it is wanting here, as in the fish with salivary glands; but in its place in many insects other tubes, acting also as glands, pour saliva into the pharynx; i. e., the cavity at the back of the throat.
As you see, therefore, everything is found complete in this tube of a few inches long; and you can also distinguish there a small and a large intestine. We are speaking of the cockchafer, which feeds on the leaves of trees; and it is for this reason I name some inches as the length of the digestive tube. This would not be longer than the body itself, had it been destined, as in the case of many other insects, to receive animal food. In fact, the law which we have shown to exist with regard to the ox and the lion, rules also over the insect-world; and whilst a radical change seems to have been made in the rest of the organisation, here everything is in its place, and we find ourselves in the same system.
Was I not justified in asserting that the unity of the animal plan is to be found in the digestive tube? and that this is the unchanging basis upon which the Creator of the animal world had raised his varied constructions?
How would it be, then, if we were to take the insect from its starting-point when it is only a worm, that is to say, merely and simply a digestive tube? for I am only telling you a small portion of its history here; a history you must know, which reveals a miracle still more wonderful than the transformation of the little tadpole into the frog! There is a brilliant-colored fly which comes buzzing about the meat-safe—the bluebottle—do you know her? It is on her account that we put large covers of iron wire over the dishes of meat; but, perhaps, you never troubled yourself to think why.
But the truth is, she only comes there to deposit her eggs in the good roast-meat; and if she could get near enough to do so, you would soon afterwards see it swarming with little white worms, which would entirely take away all your appetite. These worms are only flies out at nurse, and they will find their wings by-and-by if you only give them time enough. Disgusting as they may appear on a dining-table, I assure you they deserve more interest than you may think. When we come to speak of worms, we will ask of them to let out the secret of the mysterious transformations of animals.
In the meantime, let us finish the observations we were making on the perfect insect, as this little creature is called when he has passed through the intermediate stages which separate him from the undeveloped condition. Forgive me, my dear child, here I am speaking to you as if you were a grown-up woman! This is because it is so difficult to explain things of this sort in any other way. And now that you have been introduced into the midst of the wonders of creation, you ought to familiarise yourself with the ideas and terms they have suggested to mankind. I began with you as a child, and great would be my triumph if I could leave you a grown-up girl! And I flatter myself that I have so far set your brain, to work, under pretence of amusing you, that this hope is not altogether unfounded. I found it necessary to say this to you in confidence, because I have just read over our first conversations, and perceive that I have insensibly put you on a different diet from the one I began with. I am obliged to comfort myself by remembering that you have grown older since, and that you are now acquainted with a great many things which you had never heard spoken of then. And this is the secret of all transformations. We crept on at first over ground that was quite unknown to us; but as we went along, our wings must have begun to grow, and we are now able to fly a little!
Do not be afraid, however; I will exercise your tiny butterfly-wings very carefully just at present. We have only to examine what becomes of the chyle of the cockchafer after it has been prepared in the pretty little tube so finely wrought. We men have chyliferous vessels which draw up chyle from the intestines and throw it within a short distance of the heart, into the torrent of blood, where its education is completed. But the cockchafer, who has no other vessels than his air-pipes, and the dorsal tube, which has no communication with the intestines, what is he to do? Do not distress yourself about him. Make a tube of a bit of linen, well sewn together, and fill it with water. Sew it together as firmly as you may on all sides, the water will have no difficulty in escaping through the meshes. And this is just what happens with the little tubes found in animals, the coats of which are formed of interwoven fibres. By-the-by, from thence comes their name of "tissue," which they share in common with all the solid substances of the body, for all were once supposed to have the same general structure. The intestine of the cockchafer floats, did I not say? in the lake of blood which fills the whole cavity of the body. Well, then, the chyle has only to penetrate through these coats, to go where it is wanted. Hence it is not at all surprising that this blood should be white; and I have very good reasons just now for comparing it to our chyle. It is, indeed, chyle arriving directly from the place of its manufacture, without undergoing any other process; by which you may see that this little machine (of the digestive organs of the cockchafer), though differing in appearance so entirely from our own, is reducible to the same elements of construction, and that life is maintained by the same process as with us; namely, by the action of the air upon the albumen extracted from food. The cockchafer, it is true, is much further removed from being a fellow-creature of ours than even the horse; but the principle of life is the same with him as with us. And this is quite enough to cause children, who can feel and reason, to think twice before they begin to torture, by way of amusement, a creature whose life the God of goodness has subjected to the same conditions as our own. I speak this to those miserable little executioners who make toys of suffering animals: but the case is different with agriculturists, who have necessarily to contend with the devourers of their harvests, and whom, I admit, it would not be reasonable to bind down by the maxim of Uncle Toby.
[Footnote: I have introduced my Uncle Toby, who really has nothing to do here, in order to make you acquainted with a few lines of Sterne, which I wish I could place before the eyes of every child in the world.
"Go!" said he, one day at table, to an enormous fly which had been buzzing around his nose and had cruelly tormented him all dinner time. After many attempts, he finally caught him in his hand. "Go! I will not do thee any harm," said my Uncle Toby, rising and crossing the room with the fly in his hand; "I would not hurt a hair of your head. Go!" said he, opening the window and his hand at the same moment, to let the fly escape; "go, poor little devil; away with you; why should I do you any harm? the world is certainly large enough to contain both of us!">[
But now to finish with the cockchafer. We have got to examine one very important part of his body, that which in other animals has been the one most talked about ever since we began our study: I mean the mouth. You know that this is the essentially variable point in the digestive tube; so that you will not be much surprised, should we find he has something altogether new. The mouth of the cockchafer is composed of a great number of small pieces placed externally round the entrance to the alimentary canal; but the names of these, as they would not interest you, I will not enter upon with you; more especially as they refer to such tiny morsels, that you would have great difficulty in finding them again on the owner. Of these pieces only two are worth our attention. These are two bits of extremely hard horn, placed one on each side of the animal, which are called "mandibles" and which serve the cockchafer to cut up the leaves which he eats. Fancy your share of teeth being two huge things fixed in the two corners of your mouth, each advancing alone against the other till they meet under the nose! You would then attack your tarts with the weapons of the cockchafer! You would not, however, be able to bite them straight through from the top to the bottom, as is done by all the animals whom we have yet seen. It is this which so peculiarly distinguishes the insect's manner of feeding; for we have already been taught by the bird and the tortoise, that it is possible to eat with two pieces of horn. The cockchafer now shows us how to eat sideways; but this is merely an accessory detail. It does not affect what happens after the mouthful is swallowed. All insects, however, have not this peculiarity. The cockchafer belongs to the category of grinding insects as they are called, who bite their food: but there is the category of the sucking insects (or suckers), whose food consists of liquids; and these insects are furnished in a different manner.
In the innocent butterfly, who lives on the juice of flowers, the digestive tube terminates externally in a sort of trunk, twisted in several convolutions, which is nothing more than an exaggerated elongation of the two jaws, which become hollow within, and form a tube when joined together. When the insect alights on a flower, he suddenly unrolls this trunk, and sucks in the juices from the depth of its "corolla," as you would drink up liquid with a straw from the bottom of a small vial. Amuse yourself some summer's day by watching a butterfly in his labors amongst the flowers: sometimes he stops still, but oftener he is contented to hover over them; and, as he does so, you will see a little loose thread, as it were, move backwards and forwards as fast as possible: this is his trunk, which he darts out, while flying, into the corolla of the flowers, but which scarcely seems to touch them, so delicate is its approach.
Less inoffensive far is the trunk of the mosquito-gnat, and of all the detestable troop of blood-sucking flies. It is always a tube; but this tube is no longer a simple straw, but a sheath furnished with stilettos of such exquisite delicacy and temper, that nothing is comparable to them; and these, as they play up and down, pierce the skin of the victim, like the lancets of the lamprey, and, like them, draw in blood as they retreat.
Finally, amongst the parasites, the last and lowest group of insects, the stiletto-sheath is reduced to the size of a kind of little tube-shaped beak, which, when not in use, folds down like the fangs of the rattlesnake.
You do not know, perhaps, what a parasite is. The word comes from the Greek, and signifies literally, "that which moves round the corn." The Greeks applied it to those shameless paupers who, to escape honest labor, made their way into the houses of the great, and enjoyed themselves at their expense. These parasites are little animals which settle themselves on large ones, to suck in, without having worked for it, the blood which the others have manufactured. The wolf hunts, fights, and tears its victim in pieces; and then, by means of that interior labor which I have spent so much time in describing, transforms it into nourishing liquid: and when all this is accomplished, the little flea, who lives hidden among his hairs, coolly draws out for his own use the valuable blood obtained with so much effort. There are many parasites in the world, my dear child—yourself, for instance, to begin with—who are perfectly happy to chew your bread without asking where the corn comes from which made it. But you have heart enough to see plainly that this indifference ought not to last, and that it is not honorable to go on living in this indefinite manner at other people's cost only.
You will some day have duties to fulfil, which you should accustom yourself to think of now, in order that you may prepare yourself for them beforehand, so that it may never hereafter be said of you that you passed through the midst of human society, taking from it all you needed, without giving it back anything in return, I advise you to conjure up this idea when the time comes to leave off playing and begin preparing to be of use. The sort of thing is not always very amusing, I admit, but you must look upon it as the ladder by which you will be enabled to rise from the degradation of a parasitical life. If you were in a well, and some one were to let down a real ladder for you to get up by, I do not think you would complain of the difficulty of using it. It is for you, then, to consider whether you would like to remain for ever in your present condition; for those who learn nothing, who submit to nothing, who are good for nothing, but to show off and amuse themselves—these remain parasites all their lives in reality, however little they may sometimes seem to suspect it.
At your age, however, there is still no disgrace in the matter. God shows us by the insects that little things are allowed to be parasitical; but on this subject I must return to a point in the history of animals which I touched upon before. I told you, in speaking of the crocodile, that the perfect state of the inferior animals is found represented in the infancy or less perfect state of those above them: and I may say the same again with regard to insects. All the young of the mammalia begin life as parasites, at least, as sucking animals: for they all live at first on their mother's milk, which is nothing more than blood in a peculiar state. But the name of parasite among insects is generally confined to those which take up their abode on the bodies of their hosts; though in common justice it might equally well be applied to the gnat and his relations, who, when once full, make their bow and are off, like the kitten when he has finished sucking. Well, without meaning to find fault, if we descend to the lower ranks of the mammals, we shall find among them many parasites in the received sense of the word. You remember the pouch to which the marsupials owe their odd name. The young kangaroo remains hidden for months in the pouch of its mother, feeding continually all the time; and it is then a strict parasite. During the four following months it goes in and out, and strolls about between meals, like other young ones of its class, and is then an animal at nurse affording thus a twofold example of the tendency of the great Creator to repeat Himself in His conceptions, here using for the infancy of the mammal the system invented for adult insects—elsewhere repeating the butterfly in the humming-bird, who may fairly be called a vertebrated butterfly, and reproducing the gnat in the vampire-bat, which I look upon as an enlarged and perfected revise of the original pattern, whence comes the scourge of our sweet summer nights.
And now, surely, I have said enough about these parasites, whose very name, I suspect, will make you shudder after my impertinent application of it. Never mind: it depends entirely upon yourself to get rid of whatever you find humiliating in the position I have hinted at. Do all you can to bring happiness to the parents on whom you live at present, and who give their life-blood so willingly for your good. God has made you very different from those little animals who have neither heart nor reason to guide them. Do not be like them, then, in conduct. By a little obedience and love—child as you are—you can pay them back what you owe, and they will never complain of the bargain.