OF THE HABITATIONS OF CATERPILLARS.
Though the view which has already been given of the various proceedings of insects in forming their habitations, has extended to some length, I cannot with propriety omit noticing the wonderful art and industry which is manifested in these respects by the caterpillar; and more particularly so, as from the larva state the foundation of all our present knowledge of the natural history of insects has been obtained.
Some species of caterpillars form a kind of hammock, in which they eat and go through their various changes; while others erect a silken tent, under which they live until they have consumed the surrounding herbs. They then leave their abodes, and pitch their tents in a more fruitful spot.
Many associate together all their lives; these proceed from the same moth, who deposited her eggs near each other, or rather laid them in a heap, forming as it were a kind of nest. They are generally hatched on the same day, and, living together, constitute a new species of republic, in which all are brethren. They often amount to near six hundred in a family, though they are frequently to be found with only about two hundred. Of these social caterpillars there are some species which not only continue with the society while they are in a larva state, but even place their pupæ close together. There are other kinds who associate only for a short period.
Among the vast variety of insects which inhabit the oak, there is a species of caterpillar which live separate till they arrive at a certain age; they then assemble together, and do not quit each other till they attain their perfect state. As the number thus assembled is considerable, the nest is also very large. They remain in-doors during the day, not leaving their habitation till sun-set. When they go out, one of the body precedes the rest as a chief, whom they regularly follow; when the leader stops, the rest do the same, and wait till it goes on again, before they recommence their march. The first file generally consists of a single caterpillar, which is succeeded by a double file; these, by three in a row, which are then followed by files of five, and so on. They keep exceeding close to each other, not leaving any interval either between the ranks, or those in each rank; all of them following their captain in every direction, whether straight or crooked. After they have taken their repast, which is done on the march, they return to their nest in the same order in which they set out.
This mode is followed till they are full grown, when each forms a cone, in which it is changed into a chrysalis. M. Bonnet has shewn, that though these caterpillars proceed often very far from their nest, it is by no means difficult for them to get back again, because they spin over all the places in their rout. The first leads the way, the second follows spinning, the third spins after the first and second, and so on with the rest. All these threads form by degrees a small shining track, a little path; and all these paths meet at the nest. To be fully convinced of the use of these threads, let any one but break the continuation of them in some particular part, and he will see the little caterpillars turn back, as if they were at a loss, till one more daring than the rest restores the communication by spinning new threads.
The reader who is desirous of a fuller information concerning the habits of these, as well as many other insects, must be referred to the laborious and interesting memoirs of Reaumur. Happy if he should, like De Geer, be induced thereby to follow the steps of so great a master; he will derive from thence a continual source of new pleasures and increasing delights; and the more he extends the boundaries of his observations, the more he will be convinced that INFINITY is, as it were, impressed on all the works of the Creator.
Different species of caterpillars are often to be found in great numbers on the same tree or plant; but then as they seem to have no connection with each other, and the actions of the one have no influence on the rest, they may be considered as solitary; but there are others who seem still more independent of each other, and greater friends to solitude, constructing a lodging formed of leaves tied together with considerable ingenuity, in which they live as in a hermitage. The operation by which these tie the leaves together, is far surpassed by another kind, who fold and bend one part of the leaf till it meets the other. These are again exceeded by those who roll the leaves which they inhabit. For this purpose the caterpillar chooses a part of a leaf which it finds in some degree bent; here it establishes its abode, and begins its work, moving the head with great velocity in a curved line, or rather vibrating it like a pendulum, the middle of the body being the center on which it moves. At each motion of the head a thread is spun, and fixed to that part to which the head seems to be applied. The threads are extended from the bent to the flat part of the leaf, being always adjusted both in length and strength to the nature of the leaf, and the curvature which is to be given to it.
De Geer attending to the operations of a species of this kind of caterpillar, observed that at each new thread it spun, the edges of the leaf insensibly approached to each other, and were bent more and more, in proportion as the caterpillar spun new threads; when the last thread that was spun was tight, that which preceded it was loose and floating in the air. To effect this, the caterpillar, after it has fixed a thread to the two edges of the leaf, and before it spins another, draws it towards itself by the hooks of its feet, and by these means bends the leaf; it then spins another thread, to maintain the leaf in this position, which it again pulls towards itself, and repeats the operation, till it has bent the leaf in its whole direction. It now begins again, placing the threads further back upon the bent part of the leaf, and by proceeding in this manner, it is rolled up; when it has finished this business, it strengthens the work, by fastening the ends of the leaf together. The habitation thus formed is a kind of hollow cylinder, open to the light at both ends, the sides of it affording the insect food and protection, for within it the creature feeds in safety. In the same case they are also transformed; at the approach of the change the caterpillar lines the rolled leaf with silk, that the rough parts of it may not injure the chrysalis.
A great number of the smaller larvæ require an artificial covering, to protect them from the open air. Among these, some inhabit the interior parts of leaves, making their way between the superior and inferior membranes, living upon the parenchymous parts of the leaf; and as they are exceedingly small, a leaf affords them a spacious habitation. If the distance between the membranes be not large enough for them, they enlarge the space by forming different folds in one of them, in which they can move with ease: from these circumstances they have been named by Reaumur miners of leaves. This illustrious author has described these larvæ, the flies into which they are changed, and all the Various methods made use of by them in performing this work. Some mine a large oval or circular space; others form a kind of gallery, which is sometimes straight, sometimes crooked. They only leave a thin membrane on the upper side of the leaf; but they leave the under side more substantial. One species of moth which proceeds from these larvæ is very small but exceedingly beautiful.
The larvæ of the phryganea mostly live in little cases of their own building, which are formed of a variety of materials, that they train after them in the water wherever they go. These cases are generally cylindrical, and open at both ends; the inside is lined with silk spun by the larva, the outside formed of different substances, as bits of reed, stone, gravel, and some entirely of small shells, &c. which they arrange and manage with singular dexterity: they never quit this case. When they walk, they put out the head, and a few of the first rings of the body, training the case after them.
Having lived in the water for some time, they become inhabitants of the air. They assume the pupa form in the water, closing up the two ends of the case with bars of silk, by which it is secured from the attacks of its enemies; and at the same time there is a free passage for the water, which is still necessary for its existence. At a proper period the pupa forces its way through the case, and makes for the land, where its further change instantly commences, and is soon completed.
We shall close these specimens of the industry of insects with an account of that which is displayed by the larvæ of the tineæ. The greatest part of the body of these little creatures, except the head and six fore feet, is covered over with a thin tender skin; the body of the insect is cylindrical, and lodged in a tube which is open at both ends. Soon after they are born, they begin to cover themselves, and are, therefore, seldom to be found but in these tubes or cases. They are in general so small, that it is not easy to distinguish the cases without a magnifier; but as the body lengthens, the case becomes too short; it is, therefore, part of its daily employ to lengthen it. For this purpose it extends the head beyond the tube, and having found the materials which answer its purpose, it tears it off, and brings it to the end of the tube, and fixes it there, repeating this manoeuvre till it has sufficiently lengthened it. After it has finished one end, it turns itself round within the case, and performs the same operation at the other.
This does not terminate their labours, for the tube must also be increased in diameter, as it soon becomes too small for the body; the means they make use of to enlarge it, is precisely the same as we ourselves should adopt under similar circumstances. The insect slits the tube at the two opposite sides, at the same end, and inserts in the slit two pieces of the required size; it then performs the same at the other end. By these means they soon enlarge it sufficiently, without exposing themselves to the air during the operation. The outside of these cases is made of silk, hair, &c. the inside is of silk only. Their covering always partakes of the colour of the cloth or tree, &c. from whence it was taken; if it pass over a red piece, the colour will be red. When they are come to their perfect growth, they abandon the cloth, and seek for a proper place wherein they may pass from their present to a more perfect state.
I cannot conclude this long chapter better than in the words of Mr. Stillingfleet. “Many are apt to treat with contempt any man whom they see employed in poring over a moss, or examining an insect, from day to day, thinking that he spends his time and his life in unimportant and barren speculations; yet were the whole scene of nature laid open to our views, were we admitted to behold the connections and dependences of every thing on every other, and to trace the œconomy of nature through the smaller, as well as greater parts of this globe, we might, perhaps, be obliged to own that we were mistaken; that the Supreme Architect had contrived his works in such a manner, that we cannot properly be said to be unconcerned in any one of them; and, therefore, that studies, which seem upon a slight view to be quite useless, may in the end appear of no small importance to mankind. Nay, were we only to look back into the history of arts and sciences, we must be convinced that we are apt to judge over hastily of things of this nature. We should there find many proofs that he who gave this instinctive curiosity to some of his creatures, gave it for good and great purposes, and that he rewards with useful discoveries all these minute researches.
“It is true, this does not always happen to the searcher, or his contemporaries, nor even sometimes to the immediate succeeding generation; but I am apt to think, that advantages of one kind or other always accrue to mankind from such pursuits; some men are born to observe and record what perhaps by itself is perfectly useless, but yet of great importance to another who follows and goes a step further, still as useless; to him another succeeds, and thus by degrees, till at last one of a superior genius comes, who laying all that has been done before this time together, brings on a new face of things, improves, adorns, exalts human society.
“All those speculations concerning lines and numbers, so ardently pursued, and so exquisitely conducted by the Grecians, what did they aim at? or what did they produce for ages? a little arithmetic, and the first elements of geometry, were all they had need of. This Plato asserts; and though, as being himself an able mathematician, and remarkably fond of these sciences, he recommends the study of them; yet he makes use of motives that have no relation to the common purposes of life.
“When Kepler, from a blind and strong impulse, merely to find analogies in nature, discovered that famous one between the distance of the several planets from the sun, and the periods in which they complete their revolutions, of what importance was it to him or the world?
“Again; when Galileo, pushed on by the same irresistible curiosity, found out the law by which bodies fall to the earth, did he, or could he foresee that any good would come from his ingenious theorems; or was any immediate use made of them?
“Yet had not the Greeks pushed their abstract speculations so far, had not Kepler and Galileo made the above-mentioned discoveries, we never could have seen the greatest work that ever came from the hands of man, Sir Isaac Newton’s Principia.
“Some obscure person, whose name is not so much as known, diverting himself idly, as a stander-by would have thought, with trying experiments on a seemingly contemptible piece of stone, found out a guide for mariners on the ocean, and such a guide as no science, however subtil and sublime its speculations may be, however wonderful its conclusions, would ever have arrived at. It was mere curiosity that put Sir Thomas Millington upon examining the minute parts of flowers; but his discoveries have produced the most perfect and most useful system of botany that the world has yet seen.
“Other instances might be produced to prove, that bare curiosity in one age, is the source of the greatest utility in another; and what has frequently been said of chemists, may be applied to every other kind of vertuosi. They hunt, perhaps, after chimeras and impossibilities; they find something really valuable by the bye. We are but instruments under the Supreme Director, and do not so much as know, in many cases, what is of most importance for us to search after; but we may be sure of one thing, viz. that if we study and follow nature, whatever paths we are led into, we shall at last arrive at something valuable to ourselves and others, but of what kind we must be content to remain ignorant.”
CHAP. VI.
A GENERAL VIEW OF THE INTERNAL PARTS OF INSECTS, AND MORE PARTICULARLY OF THE CATERPILLAR OF THE PHALÆNA COSSUS—-A DESCRIPTION OF SUNDRY MISCELLANEOUS OBJECTS.
The interior part of insects includes four principal viscera; the spinal marrow, the intestinal bag, the heart, and tracheal vessels.
The spinal marrow, or principal trunk of the nerves of insects, is a whitish thread, extended the whole length from the head to the hindermost part, furnished at intervals with small knots or ganglions. From these knots proceed the nervous threads that are supposed to be the instruments of sensation and motion.
On the medullary thread is placed the intestinal bag, which is equal to it in length; it is a long gut, in which are contained the oesophagus, the stomach, and intestines.
Along the back, and parallel to the intestinal bag, runs a long thin vessel, in which may be perceived through the skin of the insect alternate contractions and dilatations; this part is supposed to perform the functions of the heart.
The tracheal vessels of insects are very similar to those of plants; are of the same structure, colour, and elasticity, and are, like them, dispersed through the whole body.
A clearer idea of these parts will be obtained by the short extract I shall give of M. Lyonet’s work; which, at the same time that it displays the wonderful organization of insects, shews how worthy it is of the attention of a rational being; and, though this description is confined to a particular species, it will be found to accord in general with a great number.
Of all the modifications of which matter is susceptible, the most noble is undoubtedly the organization thereof. In the structure of animals, the Sovereign Wisdom is exhibited to our view in the most striking manner. The body of an animal is a little particular system more or less complicated, and which, like the system of the universe at large, is the result of the combination and connection of a multitude of different parts, which all conspire to produce one general effect, the manifestation of the principle which we term life. So wonderful are these combinations that we are incapable of comprehending, or even of admiring sufficiently the astonishing apparatus of springs, levers, counter-weights, tubes of different diameters, &c. which constitute these organical machines. The interior parts of the insect, the most despicable in appearance, would absorb all the powers of the most able anatomist. He would be lost in the labyrinth as soon as he attempted to explore all its windings. A truth that will be evident to every one who considers only the small portion here introduced of the anatomy of the caterpillar inhabiting the trunk of the willow-tree. This caterpillar produces the phalæna cossus, or goat-moth. M. Lyonet in his admirable work entitled, “Traite Anatomique de la Chenille qui ronge le Bois de Saule,” has given an ample and minute description of this insect. In the following concise abstract enough will appear to convince the reader of the utility of microscopic glasses, in displaying the wonders of the creation, and to afford additional proof that the attention of the Almighty is not confined merely to objects of magnitude.
In a former edition of this work, I entered into a more minute detail of the several parts contained in the figures exhibited in [plate XII]. This account I have now omitted, as after all it could not convey a clear idea of the muscles alone, much less of the different parts of the caterpillar, without a reference to other plates of M. Lyonet’s work. I therefore concluded it would be better to let the figures speak for themselves, and then give a general description of the interior parts of the caterpillar; referring the reader for full particulars to the original.
Figures 1 and 2 represent the muscles of the caterpillar, when it is opened at the belly. Fig. 3 and 4 exhibit a view of the muscles when it is opened at the back. Fig. 5 and 6, an anatomical delineation of the head; so complex is this organ, that in order to give an adequate idea of its structure, M. Lyonet has employed no less than twenty figures. Fig. 7 is an out-line of the head more magnified than in the last figures. In order to obtain the views here exhibited, the muscles were freed as well from fat, as from the nerves and other vessels.
The BODY of the caterpillar in the [Plate] Fig. 2 and 3, is divided into twelve parts, corresponding to its rings marked by the numbers 1 to 12; to the first number the word RING is affixed. Each of these rings is distinguished from that which follows, and that preceding it, by a kind of neck or small hollow part. By conceiving a line to pass through these necks, and forming boundaries to the rings, we acquire twelve more divisions, Fig. 1 and 4; these are also marked with the numbers 1 to 12; to the first the word DIVISION is annexed. The several parts exhibited in the divisions, Fig. 1, are the muscles; those in Fig. 2, under the word ring, are also muscles, which appear when those in Fig. 1 are removed, lying under them.
The anatomical delineation of the muscles of the head, Fig. 5 and 6, should be considered as consisting of two figures, which join in the middle, being terminated by the superior and inferior lines. The head, as here represented, is magnified about three-hundred times. H H are the two palpi: the truncated muscles d, belong to the lower lip, and form a part of those which give it motion: K, the two ganglions of the neck united: I I, the two silk vessels: L, the oesophagus: M, the two dissolving vessels: the Hebrew letters denote the continuation of the cephalic arteries: S T U W and X are the ten abductor muscles of the jaw: under e e and f f are seen four occipital muscles: a a, a nerve of the first pair, belonging to the ganglion of the neck; b, a branch of this nerve.
Fig. 7 is an outline of the head magnified considerably more than in the last figure, exhibiting the nerves as seen from the under part. Excepting in two or three instances, only one nerve of each pair is shewn, as a greater number would have occasioned confusion. The nerves of the first ganglion of the neck are designed by capital letters; those of the ganglion a, are distinguished by Roman letters; those of the small ganglion, by Greek characters; and those of the frontal ganglion, except one, by numbers.