Chapter VII Carpentry of Tree-Hoppers and Saw-Flies
The operations of an insect in boring into a leaf or a bud to form a lodgment for its eggs appear very simple. The tools, however, by which these effects are performed are very complicated and curious. In the case of gall-flies (Cynips), the operation itself is not so remarkable as its subsequent chemical effects. These effects are so different from any others that may be classed under the head of Insect Architecture, that we shall reserve them for the latter part of this volume, although, with reference to the use of galls, the protection of eggs and larvæ, they ought to find a place here. We shall, however, at present confine ourselves to those which simply excavate a nest, without producing a tumour.
The first of these insects which we shall mention is celebrated for its song, by the ancient Greek poets, under the name of Tettix. The Romans called it Cicada, which we sometimes, but erroneously, translate “grasshopper;” for the grasshoppers belong to an entirely different order of insects. We shall, therefore, take the liberty of calling the Cicadæ Tree-hoppers, to which the cuckoo-spit insect (Tettigonia spumaria, Oliv.) is allied; but there is only one of the true Cicadæ hitherto ascertained to be British, namely, the Cicada hæmatodes (Linn.), which was discovered in the New Forest, Hampshire, by Mr. Daniel Bydder.
M. Réaumur was exceedingly anxious to study the economy of these insects; but they not being indigenous in the neighbourhood of Paris, he commissioned his friends to send him some from more southern latitudes, and he procured in this way specimens not only from the South of France and from Italy, but also from Egypt. From these specimens he has given the best account of them yet published; for though, as he tells us, he had never had the pleasure of seeing one of them alive, the most interesting parts of their structure can be studied as well in dead as in living specimens. We ourselves possess several specimens from New Holland, upon which we have verified some of the more interesting observations of Réaumur.
Virgil tells us that in his time “the cicadæ burst the very shrubs with their querulous music;”BH but we may well suppose that he was altogether unacquainted with the singular instrument by means of which they can, not poetically, but actually, cut grooves in the branches they select for depositing their eggs. It is the male, as in the case of birds, which fills the woods with his song; while the female, though mute, is no less interesting to the naturalist on account of her curious ovipositor. This instrument, like all those with which insects are furnished by nature for cutting, notching, or piercing, is composed of a horny substance, and is also considerably larger than the size of the tree-hopper would proportionally indicate. It can on this account be partially examined without a microscope, being, in some of the larger species, no less than five linesBI in length.
The ovipositor, or auger (tarière), as Réaumur calls it, is lodged in a sheath which lies in a groove of the terminating ring of the belly. It requires only a very slight pressure to cause the instrument to protrude from its sheath, when it appears to the naked eye to be of equal thickness throughout, except at the point, where it is somewhat enlarged and angular, and on both sides finely indented with teeth. A more minute examination of the sheath demonstrates that it is composed of two horny pieces slightly curved, and ending in the form of an elongated spoon, the concave part of which is adapted to receive the convex end of the ovipositor.
When the protruded instrument is further examined with a microscope, the denticulations, nine in number on each side, appear strong, and arranged with great symmetry, increasing in fineness towards the point, where there are three or four very small ones, beside the nine that are more obvious. The magnifier also shows that the instrument itself, which appeared simple to the naked eye, is, in fact, composed of three different pieces; two exterior armed with the teeth before mentioned, denominated by Réaumur files (limes), and another pointed like a lancet, and not denticulated. The denticulated pieces, moreover, are capable of being moved forwards and backwards, while the centre one remains stationary; and as this motion is effected by pressing a pin or the blade of a knife over the muscles on either side at the origin of the ovipositor, it may be presumed that those muscles are destined for producing similar movements when the insect requires them. By means of a finely-pointed pin carefully introduced between the pieces, and pushed very gently downwards, they may be, with no great difficulty, separated in their whole extent.
The contrivance by which those three pieces are held united, while at the same time the two files can be easily put in motion, is similar to those of our own mechanical inventions, with this difference, that no human workman could construct an instrument of this description so small, fine, exquisitely polished, and fitting so exactly. We should have been apt to form the grooves in the central piece, whereas they are scooped out in the handles of the files, and play upon two projecting ridges in the central piece, by which means this is rendered stronger. M. Réaumur discovered that the best manner of showing the play of this extraordinary instrument is to cut it off with a pair of scissors near its origin, and then, taking it between the thumb and the finger at the point of section, work it gently to put the files in motion.
Beside the muscles necessary for the movement of the files, the handle of each is terminated by a curve of the same hard horny substance as itself, which not only furnishes the muscles with a sort of lever, but serves to press, as with a spring, the two files close to the central piece, as is shown in the lower figure.
M. Pontedera, who studied the economy of the tree-hoppers with some care, was anxious to see the insect itself make use of the ovipositor in forming grooves in wood, but found that it was so shy and easily alarmed, that it took to flight whenever he approached; a circumstance of which Réaumur takes advantage, to soothe his regret that the insects were not indigenous in his neighbourhood. But of their workmanship, when completed, he had several specimens sent to him from Provence and Languedoc by the Marquis de Caumont.
The gall-flies, when about to deposit their eggs, select growing plants and trees; but the tree-hoppers, on the contrary, make choice of dead, dried branches, for the mother seems to be aware that moisture would injure her progeny. The branch, commonly a small one, in which eggs have been deposited, may be recognised by being covered with little oblong elevations caused by small splinters of the wood, detached at one end, but left fixed at the other, by the insect. These elevations are for the most part in a line, rarely in a double line, nearly at equal distances from each other, and form a lid to a cavity in the wood about four lines in length, containing from four to ten eggs. It is to be remarked that the insect always selects a branch of such dimensions that it can get at the pith, not because the pith is more easily bored, for it does not penetrate into it all, but to form a warm and safe bed for the eggs. M. Pontedera says, that when the eggs have been deposited, the insect closes the mouth of the hole with a gum capable of protecting them from the weather; but M. Réaumur thinks this only a fancy, as, out of a great number which he examined, he could discover nothing of the kind. Neither is such a protection wanted; for the woody splinters above mentioned furnish a very good covering.
The grubs hatch from these eggs (of which, M. Pontedera says, one female will deposit from five to seven hundred), issue from the same holes through which the eggs have been introduced, and betake themselves to the ground to feed on the roots of plants. They are not transformed into chrysalides, but into active nymphs, remarkable for their fore limbs, which are thick, strong, and furnished with prongs for digging; and when we are told, by Dr. Le Ferve, that they make their way easily into hard stiff clay, to the depth of two or three feet, we perceive how necessary to them such a conformation must be.
Saw-flies.
An instrument for cutting grooves in wood, still more ingeniously contrived than that of the tree-hopper, was first observed by Vallisnieri, an eminent Italian naturalist, in a four-winged fly, most appropriately denominated by M. Réaumur the saw-fly (Tenthredo) of which many sorts are indigenous to Great Britain. The grubs from which these flies originate are indeed but too well known, as they frequently strip our rose, gooseberry, raspberry, and red currant trees of their leaves, and are no less destructive to birch, alder, and willows; while turnips and wheat suffer still more seriously by their ravages. These grubs may readily be distinguished from the caterpillars of moths and butterflies by having from sixteen to twenty-eight feet, by which they usually hang to the leaf they feed on, while they coil up the hinder part of their body in a spiral ring. The perfect flies are distinguished by four transparent wings; and some of the most common have a flat body of a yellow or orange colour, while the head and shoulders are black.
In order to see the ovipositor, to which we shall for the present turn our chief attention, a female saw-fly must be taken, and her belly gently pressed, when a narrow slit will be observed to open at some distance from the apex, and a short, pointed, and somewhat curved body, of a brown colour and horny substance, will be protruded. The curved plates which form the sides of the slit are the termination of the sheath, in which the instrument lies concealed till it is wanted by the insect. The appearance of this instrument, however, and its singular structure, cannot be well understood without the aid of a microscope.
The instrument thus brought into view is a very finely-contrived saw, made of a horny substance, and adapted for penetrating branches and other parts of plants where the eggs are to be deposited. The ovipositor-saw of the insect is much more complicated than any of those employed by our carpenters. The teeth of our saws are formed in a line, but in such a manner as to cut in two lines parallel to, and at a small distance from, each other. This is effected by slightly bending the points of the alternate teeth right and left, so that one-half of the whole teeth stand a little to the right, and the other half a little to the left. The distance of the two parallel lines thus formed is called the course of the saw, and it is only the portion of wood which lies in the course that is cut into sawdust by the action of the instrument. It will follow that in proportion to the thinness of a saw there will be the less destruction of wood which may be sawed. When cabinet-makers have to divide valuable wood into very thin leaves, they accordingly employ saws with a narrow course, while sawyers who cut planks use one with a broad course. The ovipositor-saw being extremely fine, does not require the teeth to diverge much; but from the manner in which they operate, it is requisite that they should not stand, like those of our saws, in a straight line. The greater portion of the edge of the instrument, on the contrary, is towards the point somewhat concave, similar to a scythe, while towards the base it becomes a little convex, the whole edge being nearly the shape of an Italic f.
The ovipositor-saw of the fly is put in motion in the same way as a carpenter’s hand-saw, supposing the tendons attached to its base to form the handle, and the muscles which put it in motion to be the hand of the carpenter. But the carpenter can only work one saw at a time, whereas each of these flies is furnished with two, equal and similar, which it works at the same time—one being advanced and the other retracted alternately. The secret, indeed, of working more saws than one at once is not unknown to our mechanics; for two or three are sometimes fixed in the same frame. These, however, not only all move upwards and downwards simultaneously, but cut the wood in different places; while the two saws of the ovipositor work in the same cut, and consequently, though the teeth are extremely fine, the effect is similar to a saw with a wide set.
It is important, seeing that the ovipositor-saws are so fine, that they be not bent or separated while in operation—and this, also, nature has provided for, by lodging the backs of the saws in a groove, formed by two membranous plates, similar to the structure of a clasp-knife. These plates are thickest at the base, becoming gradually thinner as they approach the point, which the form of the saws requires. According to Vallisnieri, it is not the only use of this apparatus to form a back for the saws, he having discovered, between the component membranes, two canals, which he supposes are employed to conduct the eggs of the insect into the grooves which it has hollowed out for them.
The teeth of a carpenter’s saw, it may be remarked, are simple, whereas the teeth of the ovipositor-saw are themselves denticulated with fine teeth. The latter, also, combines at the same time the properties of a saw and of a rasp or file. So far as we are aware, these two properties have, never been combined in any of the tools of our carpenters. The rasping part of the ovipositor, however, is not constructed like our rasps, with short teeth thickly studded together, but has teeth almost as long as those of the saw, and placed contiguous to them, on the back of the instrument, resembling in their form and setting the teeth of a comb, as may be seen in the figure. Of course, such observations are conducted with the aid of a microscope.
When a female saw-fly has selected the branch of a rose-tree, or any other, in which to deposit her eggs, she may be seen bending the end of her belly inwards, in form of a crescent, and protruding her saw, at the same time, to penetrate the bark or wood. She maintains this recurved position so long as she works in deepening the groove; but when she has attained the depth required, she unbends her body into a straight line, and in this position works upon the place lengthways, by applying the saw more horizontally. When she has rendered the groove as large as she wishes, the motion of the tendons ceases, and an egg is placed in the cavity. The saw is then withdrawn into the sheath for about two-thirds of its length, and at the same moment a sort of frothy liquid, similar to a lather made with soap, is dropped over the egg, either for the purpose of gluing it in its place or sheathing it from the action of the juices of the tree. She proceeds in the same manner in sawing out a second groove, and so on in succession, till she has deposited all her eggs, sometimes to the number of twenty-four. The grooves are usually placed in a line, at a small distance from one another, on the same branch; but sometimes the mother-fly shifts to another, or to a different part of the branch, when she is either scared or finds it unsuitable. She commonly, also, takes more than one day to the work, notwithstanding the superiority of her tools. Réaumur has seen a saw-fly make six grooves in succession, which occupied her about ten hours and a half.
The grooves, when finished, have externally little elevation above the level of the bark, appearing like the puncture of a lancet in the human skin; but in the course of a day or two the part becomes first brown and then black, while it also becomes more and more elevated. This increased elevation is not owing to the growth of the bark, the fibres of which, indeed, have been destroyed by the ovipositor-saw, but to the actual growth of the egg; for when a new-laid egg of the saw-fly is compared with one which has been several days enclosed in the groove, the latter will be found to be very considerably the larger. This growth of the egg is contrary to the analogy observable in the eggs of birds, and even of most other insects; but it has its advantages. As it continues to increase, it raises the bark more and more, and consequently widens, at the same time, the slit at the entrance; so that, when the grub is hatched, it finds a passage ready for its exit. The mother-fly seems to be aware of this growth of her eggs, for she takes care to deposit them at such distances as may prevent their disturbing one another by their development.
Another species of saw-fly, with a yellow body and deep violet-coloured wings, which also selects the rose-tree, deposits her eggs in a different manner. Instead of making a groove for each egg, like the preceding, she forms a large single groove, sufficient for about two dozen eggs. These eggs are all arranged in pairs, forming two straight lines parallel to the sides of the branch. The eggs, however, though thus deposited in a common groove, are carefully kept each in its place; for a ridge of the wood is left to prevent those on the right from touching those on the left—and not only so, but between each egg of a row a thin partition of wood is left, forming a shallow cell.
The edges of this groove, it will be obvious, must be farther apart than those which only contain a single egg, and, in fact, the whole is open to inspection; but the eggs are kept from falling out, both by the frothy glue before mentioned, and by the walls of the cells containing them. They were observed also, by Vallisnieri, to increase in size like the preceding.
[In the middle of summer, plenty of these grooves may be seen, by looking at the under lid of leaf-stalks or delicate young twigs. Row upon row of the grooves are sometimes found, so the all-destructive power of the insects must indeed be great. The larvæ, when full fed, dispose of themselves in various ways. Those of the gooseberry-fly, for example (Nematus Ribesii), after they have stripped the bush of its leaves, either seek the ground or remain on the branches, and spin a series of cocoons, attaching them to each branch by their ends. Those, therefore, who wish to destroy these little pests, must know both localities of the cocoons, or they will allow one half to escape while destroying the other.]
This insect has a flat yellow body and four pellucid wings, the two outer ones marked with brown on the edge. In April it issues from the pupa, which has lain under ground from the preceding September. The female of the gooseberry saw-fly does not, like some of the family, cut a groove in the branch to deposit her eggs;—“of what use, then,” asks Réaumur, “is her ovipositor-saw?” In order to satisfy himself on this point, he introduced a pair of the flies under a bell-glass along with a branch bent from a red-currant bush, that he might watch the process. The female immediately perambulated the leaves in search of a place suited to her purpose, and passing under a leaf began to lay, depositing six eggs within a quarter of an hour. Each time she placed herself as if she wished to cut into the leaf with her saw; but, upon taking out the leaf, the eggs appeared rather projecting than lodged in its substance. The caterpillars are hatched in two or three weeks; and they feed in company till after midsummer, frequently stripping both the leaves and fruit of an extensive plantation. The caterpillar has six legs and sixteen pro-legs, and is of a green colour mixed with yellow, and covered with minute black dots raised like shagreen. In its last skin it loses the black dots and becomes smooth and yellowish white. The Caledonian Horticultural Society have published a number of plans for destroying these caterpillars.
[Another remarkable mode of disposing of the pupa is shown in the accompanying illustration; it represents the nest of an exotic saw-fly, named Deilocenes Ellisii. In this instance, the numerous larvæ unite in spinning for themselves a common envelope of considerable strength; it is seen as it appears when attached to the branch of a tree. The material of which it is composed is the tough silken fibre spun by the larvæ of so many insects, which may be seen in perfection in the cocoons of the Microgaster. Two species of this curious group will be described in a future page.
By the side of the branch is seen a diagram of the same nest, as it would appear in section. The irregularly angular cells are seen in the centre, and around them is the common envelope composed of fibres. As may be seen from the upper figure, as soon as the insects have attained their perfect form, they gnaw their way out of the cell and the covering also. The insect is shown as it appears when flying.
We will conclude this chapter by a few remarks upon some exotic insects, whose nests are not only remarkable in their form, but are valuable to the entomologist in affording grounds for the reception or rejection of certain familiar theories upon the subject of this volume—Insect Architecture. Several of these nests are of comparatively late discovery, and are therefore found in this work.
The curious series of cells shown in the left-hand figure is made by a hymenopterous insect belonging to the genus Icaria, and the specimens from which the drawing was taken may be seen in the British Museum. They are made of a paper-like substance, much resembling in look the material of which the common wasp builds its cells, but as they are exposed to the air, they are necessarily tougher and stronger than ordinary wasp cells, which are shielded from the elements. The insects belonging to this genus make nests of very diverse forms, some of which are stuck on leaves in a most curious manner, reminding the observer of the parasitic mollercoids that cover the stems and fronds of large seaweeds. Others, however, are not dependent upon leaves for their support, but stand out boldly from the branches to which they are fixed, supported entirely by a footstalk composed of the same material as the cells, though necessarily of a harder and more compact substance.
As many of these nests have been found in India, it is easy to trace the manner in which they were made. The mother insect began by kneading woody fibre into a paste, and making the footstalk of the future nest. One end of this footstalk is attached very strongly to the branch, and to the other end is fastened the first cell. As soon as the Icaria has made the first beginning of the cell, and raised—or rather lowered—the walls to a fourth or so of their complete dimensions, she inserts an egg into the yet imperfect cell, and adds to the walls while the egg is being hatched. Her next duty is, to add a second cell, and this is quickly followed by a third, all these cells being fastened to each other on three or four of their sides, leaving the others free and unattached. It is evident that by this mode of construction the cells nearest the branch must be the longest, because they are begun the soonest, and this will always be found to be the case.
Now, there is a point respecting which the attention of the reader must be specially solicited. On looking at the cells, he will see that they are partly cylindrical and partly angular, and may perhaps think that this fact goes towards proving that the hexagonal shape of bee cells is owing to mutual pressure, the outer sides of the cells being rounded, while the inner are angular. But, there are other cells in existence, built by allied insects, and formed in an analogous manner, and which are either angular or cylindrical, exactly according to the instinctive powers of the insect which built them.
On the right hand of the Icarian nest may be seen a singular-looking structure pendent at the end of a long footstalk. This is the nest of an insect called Mischocyttarus labiatus, one of the Polistidæ. In this case, the cells are built so as to be defended from the rain by a sort of penthouse, over which all the rain-drops would run, and so fall harmless to the ground. The cells of this insect are soft in texture, and are more cylindrical than angular, the angles being but very slightly marked.
Here, however, is the nest of an insect called Raphigaster Guiniensis, which is built in a manner similar to that of the Icaria, the cells being closely in contact with each other. The material of which they are made is peculiarly soft. something like very thin and flimsy grey paper. Consequently, they must press strongly upon each other, and we might reasonably expect to find that their angles are well and boldly developed. But, instead of that, we find that they have no angles at all, but remain smooth and rounded throughout their length.
Perhaps the most powerful argument against the equal pressure theory is to be found in the nest of a species of Icaria, which is shown in the accompanying illustration.
As may be seen by reference to the illustration, the material of which they are made is so soft, that they bend over by their own weight, and therefore we might expect to find that they would follow the shape of the Raphigaster and the Mischocyttarus. But, we find that all the cells are boldly angular, and that the angles are just as sharp on the exterior of each cell as on the sides which cement the cells together. It is clear that the bold lines and decided angles of these cells cannot have been produced mechanically, and that they must have been intentionally formed by the insect architect.
One single cell, such as is here shown, is sufficient to overthrow the theory of “equal pressure,” by which insects were deprived of all mechanical skill, and supposed to labour like so many animated machines, without caring or knowing anything about the work on which they were engaged. According to the equal pressure theory, each of these cells would have required six similar cells around it before it could have assumed the hexagonal form, and yet we find that a cell which is only connected with its neighbour by one side, has its other five sides angular, and with the angles boldly defined.]