Megachile centuncularis, M. Willughbiella, and other species of the same family, like the preceding, cover the walls of their cells with a coating of leaves, but are content with a more sober colour, generally selecting for their hangings the leaves of trees, especially of the rose, whence they have been known by the name of the leaf-cutter bees. They differ also from M. Papaveris in excavating longer burrows, and filling them with several thimble-shaped cells composed of portions of leaves so curiously convoluted, that, if we were ignorant in what school they have been taught to construct them, we should never credit their being the work of an insect. Their entertaining history, so long ago as 1670, attracted the attention of our countrymen Ray, Lister, Willughby, and Sir Edward King; but we are indebted for the most complete account of their procedures to Reaumur.
The mother bee first excavates a cylindrical hole eight or ten inches long, in a horizontal direction, either in the ground or in the trunk of a rotten willow-tree, or occasionally in other decaying wood. This cavity she fills with six or seven cells wholly composed of portions of leaf, of the shape of a thimble, the convex end of one closely fitting into the open end of another. Her first process is to form the exterior coating, which is composed of three or four pieces of larger dimensions than the rest, and of an oval form. The second coating is formed of portions of equal size, narrow at one end but gradually widening towards the other, where the width equals half the length. One side of these pieces is the serrate margin of the leaf from which it was taken, which, as the pieces are made to lap one over the other, is kept on the outside, and that which has been cut within. The little animal now forms a third coating of similar materials, the middle of which, as the most skilful workman would do in similar circumstances, she places over the margins of those that form the first tube, thus covering and strengthening the junctures. Repeating the same process, she gives a fourth and sometimes a fifth coating to her nest, taking care, at the closed end or narrow extremity of the cell, to bend the leaves so as to form a convex termination. Having thus finished a cell, her next business is to fill it to within half a line of the orifice, with a rose-coloured conserve composed of honey and pollen, usually collected from the flowers of thistles; and then having deposited her egg, she closes the orifice with three pieces of leaf so exactly circular, that a pair of compasses could not define their margin with more truth; and coinciding so precisely with the walls of the cell, as to be retained in their situation merely by the nicety of their adaptation. After this covering is fitted in, there remains still a concavity which receives the convex end of the succeeding cell; and in this manner the indefatigable little animal proceeds until she has completed the six or seven cells which compose her cylinder.
The process which one of these bees employs in cutting the pieces of leaf that compose her nest is worthy of attention. Nothing can be more expeditious: she is not longer about it than we should be with a pair of scissors. After hovering for some moments over a rose-bush, as if to reconnoitre the ground, the bee alights upon the leaf which she has selected, usually taking her station upon its edge so that the margin passes between her legs. With her strong mandibles she cuts without intermission in a curve line so as to detach a triangular portion. When this hangs by the last fibre, lest its weight should carry her to the ground, she balances her little wings for flight, and the very moment it parts from the leaf flies off with it in triumph; the detached portion remaining bent between her legs in a direction perpendicular to her body. Thus without rule or compasses do these diminutive creatures mete out the materials of their work into portions of an ellipse, into ovals or circles, accurately accommodating the dimensions of the several pieces of each figure to each other. What other architect could carry impressed upon the tablet of his memory the entire idea of the edifice which he has to erect, and, destitute of square or plumb-line, cut out his materials in their exact dimensions without making a single mistake? Yet this is what our little bee invariably does. So far are human art and reason excelled by the teaching of the Almighty[771].
Other insects besides bees construct habitations of different kinds for their young, as various species of burrowing wasps (Fossores), Geotrupes, &c., which deposit their eggs in cylindrical excavations that become the abode of the future larvæ. In the procedures of most of these, nothing worth particularizing occurs; but one species called by Reaumur the mason-wasp, (Odynerus muraria,) referred to in a former letter, works upon so singular a plan, that it would be improper to pass it over in silence, especially as these nests may be found in this country in most sandy banks exposed to the sun. This insect bores a cylindrical cavity from two to three inches deep, in hard sand which its mandibles alone would be scarcely capable of penetrating, were it not provided with a slightly glutinous liquor which it pours out of its mouth, that, like the vinegar with which Hannibal softened the Alps, acts upon the cement of the sand, and renders the separation of the grains easy to the double pickaxe with which our little pioneer is furnished. But the most remarkable circumstance is the mode in which it disposes of the excavated materials. Instead of throwing them at random on a heap, it carefully forms them into little oblong pellets, and arranges them round the entrance of the hole so as to form a tunnel, which, when the excavation is completed, is often not less than two or three inches in length. For the greater part of its height this tunnel is upright, but towards the top it bends into a curve, always however retaining its cylindrical form. The little masses are so attached to each other in this cylinder, as to leave numerous vacuities between them, which give it the appearance of filagree-work. You will readily divine that the excavated hole is intended for the reception of an egg, but for what purpose the external tunnel is meant is not so apparent. One use, and perhaps the most important, would seem to be to prevent the incursions of the artful Ichneumons, Chrysidæ, &c. which are ever on the watch to insinuate their parasitic young into the nests of other insects: it may render their access to the nest more difficult; they may dread to enter into so long and dark a defile. I have seen, however, more than once a Chrysis come out of these tunnels. That its use is only temporary, is plain from the circumstance that the insect employs the whole fabric, when its egg is laid and store of food procured, in filling up the remaining vacuity of the hole; taking down the pellets, which are very conveniently at hand, and placing them in it until the entrance is filled[772].—Latreille informs us, that a nearly similar tunnel, but composed of grains of earth, is built at the entrance of its cell by a bee of his family of pioneers[773].
Under this head, too, may be most conveniently arranged the very singular habitations of the larvæ of the Linnæan genus Cynips, the gall-fly, though they can with no propriety be said to be constructed by the mother, who, provided with an instrument as potent as an enchanter's wand, has but to pierce the site of the foundation, and commodious apartments, as if by magic, spring up and surround the germe of her future descendants. I allude to those vegetable excrescencies termed galls, some of which resembling beautiful berries and others apples, you must have frequently observed on the leaves of the oak, and of which one species, the Aleppo gall, as I have before noticed, is of such importance in the ingenious art "de peindre la parole et de parler aux yeux[774]." All these tumours owe their origin to the deposition of an egg in the substance out of which they grow. This egg, too small almost for perception, the parent insect, a little four-winged fly, introduces into a puncture made by her curious spiral sting, and in a few hours it becomes surrounded with a fleshy chamber, which not only serves its young for shelter and defence, but also for food; the future little hermit feeding upon its interior and there undergoing its metamorphosis. Nothing can be more varied than these habitations. Some are of a globular form, a bright red colour, and smooth fleshy consistence, resembling beautiful fruits, for which indeed, as you have before been told, they are eaten in the Levant: others, beset with spines or clothed with hair, are so much like seed-vessels, that an eminent modern chemist has contended respecting the Aleppo gall, that it is actually a capsule[775]. Some are exactly round; others like little mushrooms; others resemble artichokes; while others again might be taken for flowers: in short, they are of a hundred different forms, and of all sizes from that of a pin's head to that of a walnut. Nor is their situation on the plant less diversified. Some are found upon the leaf itself; others upon the footstalks only; others upon the roots; and others upon the buds[776]. Some of them cause the branches upon which they grow to shoot out into such singular forms, that the plants producing them were esteemed by the old botanists distinct species. Of this kind is the Rose-willow, which old Gerard figures and describes as "not only making a gallant shew, but also yeelding a most cooling aire in the heat of summer, being set up in houses for the decking of the same." This willow is nothing more than one of the common species, whose twigs, in consequence of the deposition of the egg of a Cynips in their summits, there shoot out into numerous leaves totally different in shape from the other leaves of the tree, and arranged not much unlike those composing the flower of a rose, adhering to the stem even after the others fall off. Sir James Smith mentions a similar lusus on the Provence willows, which at first he took for a tufted lichen[777]. From the same cause the twigs of the common wild rose often shoot out into a beautiful tuft of numerous reddish moss-like fibres wholly dissimilar from the leaves of the plant, deemed by the old naturalists a very valuable medical substance, to which they erroneously gave the name of Bedeguar. None of these variations is accidental or common to several of the tribe, but each peculiar to the galls formed by a single and distinct species of Cynips.
How the mere insertion of an egg into the substance of a leaf or twig, even if accompanied, as some imagine, by a peculiar fluid, should cause the growth of such singular protuberances around it, philosophers are as little able to explain, as why the insertion of a particle of variolous matter into a child's arm should cover it with pustules of small pox. In both cases the effects seem to proceed from some action of the foreign substance upon the secreting vessels of the animal or vegetable: but of the nature of this action we know nothing. Thus much is ascertained by the observations of Reaumur and Malpighi—that the production of the gall, which however large attains its full size in a day or two[778], is caused by the egg or some accompanying fluid: not by the larva, which does not appear until the gall is fully formed[779]; that the galls which spring from leaves almost constantly take their origin from nerves[780]; and that the egg, at the same time that it causes the growth of the gall, itself derives nourishment from the substance that surrounds it, becoming considerably larger before it is hatched than it was when first deposited[781].—When chemically analysed, galls are found to contain only the same principles as the plant from which they spring, but in a more concentrated state.
No productions of nature seem to have puzzled the ancient philosophers more than galls. The commentator on Dioscorides, Mathiolus, who agreeably to the doctrine of those days ascribed their origin to spontaneous generation, gravely informs us that weighty prognostications as to the events of the ensuing year may be deduced from ascertaining whether they contain spiders, worms, or flies. Other philosophers, who knew that except by rare accident no other animals are to be found in galls, besides grubs of different kinds which they rationally conceived to spring from eggs, were chiefly at a loss to account for the conveyance of these eggs into the middle of a substance in which they could find no external orifice. They therefore inferred that they were the eggs of insects deposited in the earth, which had been drawn up by the roots of trees along with the sap, and after passing through different vessels had stopped, some in the leaves, others in the twigs, and had there hatched and produced galls! Redi's solution of the difficulty was even more extraordinary. This philosopher, who had so triumphantly combated the absurdities of spontaneous generation, fell himself into greater. Not having been able to witness the deposition of eggs by the parent flies in the plants that produce galls, he took it for granted that the grubs which he found within them could not spring from eggs: and he was equally unwilling to admit their origin from spontaneous generation,—an admission which would have been fatal to his own most brilliant discoveries. He therefore cut the knot, by supposing that to the same vegetative soul by which fruits and plants are produced, is committed the charge of creating the larvæ found in galls[782]! An instance truly humiliating, how little we can infer from a man's just ideas on one point, that he will not be guilty of the most pitiable absurdity on another!
Though by far the greater part of the vegetable excrescencies termed galls, are caused by insects of the genus Cynips, they do not always originate from this tribe. Some are produced by weevils belonging to Schüppel's genus Ceutorhynchus; as those on the roots of kedlock (Sinapis arvensis), which I have ascertained to be inhabited by the larvæ of Curculio contractus Marsh., Rhynchænus assimilis, F. From the knob-like galls on turnips called in some places the anbury, I have bred another of these weevils, (Curculio pleurostigma, Marsh., Rhynchænus sulcicollis, Gyll.) and I have little doubt that the same insects, or species allied to them, cause the clubbing of the roots of cabbages. It seems to be a beetle of the same family that is figured by Reaumur[783], as causing the galls on the leaves of the lime-tree. Others owe their origin to moths, as those resembling a nutmeg which Reaumur received from Cyprus[784]; and others again to two-winged flies, as the woody galls of the thistle caused by Trypeta Cardui[785], and the cottony galls found on ground ivy, wild thyme, &c. as well as a very singular one on the juniper resembling a flower, described by De Geer[786], all which are the work of minute gall-gnats (Cecidomyiæ, Latr.). Some of these last convert even the flowers of plants into a kind of galls, as T. Loti of De Geer[787], which inhabits the blossoms of Lotus corniculatus; and one which I have myself observed to render the flowers of Erysimum Barbarea like a hop blossom. A similar monstrous appearance is communicated to the flowers of Teucrium supinum by a little field-bug, Tingis Teucrii of Host[788], and to another plant of the same genus by one of the same tribe described by Reaumur[789]. In these two last instances, however, the habitations do not seem strictly entitled to the appellation of galls, as they originate not from the egg, but from the larva, which, in the operation of extracting the sap, in some way imparts a morbid action to the juices, causing the flower to expand unnaturally: and the same remark is applicable to the gall-like swellings formed by many Aphides, as A. Pistaciæ, which causes the leaves of different species of Pistacia to expand into red finger-like cavities; A. Abietis, which converts the buds or young shoots of the fir into a very beautiful gall, somewhat resembling a fir-cone, or a pine-apple in miniature; and A. Bursariæ, which with its brood inhabits angular utriculi on the leafstalk of the black poplar, numbers of which I have observed on those trees by the road-side from Hull to Cottingham.—The majority of galls are what entomologists have denominated monothalamous, or consisting of only one chamber or cell; but some are polythalamous, or consisting of several.
Having thus described the most remarkable of the habitations constructed by the parent insects for the accommodation of their future young, I proceed to the second kind mentioned, namely, those which are formed by the insect itself for its own use. These may be again subdivided into such as are the work of the insects in their larva state; and such as are formed by perfect insects.