Having discovered that a mason-bee that was engaged in the process of construction would go on building to an useless or even injurious extent, Fabre tried another experiment to ascertain whether a bee that was engaged in the process of provisioning the nest, would do so in conditions that rendered its work futile. Taking away a nest with completely built cell that a bee was storing with food, he substituted for it one in which the cell was only commenced, and therefore incapable of containing food; when the bee with its store of provisions reached this should-be receptacle it appeared to be very perplexed, tested the imperfect cell with its antennae, left the spot and returned again; repeating this several times it finally went to the cell of some stranger to deposit its treasure. In other cases the bee broke open a completed cell, and having done so went on bringing provisions to it, although it was already fully provisioned and an egg laid therein: finally, the little creature having completed the bringing of this superfluous tale of provisions, deposited a second egg, and again sealed up the cell. But in no case does the bee go back from the provisioning stage to the building stage until the cycle for one cell of building, provisioning, and egg-laying is completed: but when this is the case, the building of a fresh cell may be again undertaken. This is a good example of the kind of consecutive necessity that seems to be one of the chief features of the instinct of these industrious little animals. Another equally striking illustration of these peculiarities of instinct is offered by interfering with the act of putting the provisions into the cell. It will be recollected that when the bee brings provisions to add to the stock, it carries both honey and pollen; in order to deliver these it begins by entering head first into the cell and disgorging the honey, then emerging it turns round, enters backwards and scrapes off the pollen from its body. If after the honey has been discharged, the bee be interfered with and gently removed to a slight distance with a straw, it returns to complete its task, but instead of going on with the actions at the point at which the interruption took place, it begins the series over again, going in—at any rate partially—head first, although it has no honey to discharge, and having performed this useless ceremony it then emerges, turns round and adds the pollen. This illustration is in some respects the reverse of what might have been expected, for the Insect here does not continue the act at the interrupted point, but begins the series of actions afresh.
It would be reasonable to suppose that an Insect that takes the pains to provide for the safety of its progeny by constructing a complex edifice of cement, secures thereby the advantage of protection for its young. But this is far from being the case. Notwithstanding the cement and the thick dome of mortar, the Chalicodoma is extremely subject to the attacks of parasites. The work performed by the creature in constructing its mass of masonry is truly astounding; Fabre calculated from measurements he made that for the construction and provisioning of a single cell, the goings and comings of the bee amounted to 15 kilometres, and it makes for each nest sometimes as many as fifteen cells. Notwithstanding all this labour, it would appear that no real safety for the larvae is obtained by the work. Some sixteen—possibly more—other species of Insects get their living off this industrious creature. Another bee, Stelis nasuta, breaks open the cells after they have been completely closed and places its own eggs in them, and then again closes the cells with mortar. The larvae of this Stelis develop more rapidly than do those of the Chalicodoma, so that the result of this shameless proceeding is that the young one of the legitimate proprietor—as we human beings think it—is starved to death, or is possibly eaten up as a dessert by the Stelis larvae, after they have appropriated all the pudding.
Another bee, Dioxys cincta, is even more audacious; it flies about in a careless manner among the Chalicodoma at their work, and they do not seem to object to its presence unless it interferes with them in too unmannerly a fashion, when they brush it aside. The Dioxys, when the proprietor leaves the cell, will enter it and taste the contents; after having taken a few mouthfuls the impudent creature then deposits an egg in the cell, and, it is pretty certain, places it at or near the bottom of the mass of pollen, so that it is not conspicuously evident to the Chalicodoma when the bee again returns to add to or complete the stock of provisions. Afterwards the constructor deposits its own egg in the cell and closes it. The final result is much the same as in the case of the Stelis, that is to say, the Chalicodoma has provided food for an usurper; but it appears probable that the consummation is reached in a somewhat different manner, namely, by the Dioxys larva eating the egg of the Chalicodoma, instead of slaughtering the larva. Two of the Hymenoptera Parasitica are very destructive to the Chalicodoma, viz. Leucospis gigas and Monodontomerus nitidus; the habits of which we have already discussed (vol. v. p. 543) under Chalcididae. Lampert has given a list of the Insects attacking the mason-bee or found in its nests; altogether it would appear that about sixteen species have been recognised, most of which destroy the bee larva, though some possibly destroy the bee's destroyers, and two or three perhaps merely devour dead examples of the bee, or take the food from cells, the inhabitants of which have been destroyed by some untoward event. This author thinks that one half of the bees' progeny are made away with by these destroyers, while Fabre places the destruction in the South of France at a still higher ratio, telling us that in one nest of nine cells, the inhabitants of three were destroyed by the Dipterous Insect, Anthrax trifasciata, of two by Leucospis, of two by Stelis, and of one by the smaller Chalcid; there being thus only a single example of the bee that had not succumbed to one or other of the enemies. He has sometimes examined a large number of nests without finding a single one that had not been attacked by one or other of the parasites, and more often than not several of the marauders had attacked the nest.
It is said by Lampert and others that there is a passage in Pliny relating to one of the mason-bees, that the Roman author had noticed in the act of carrying off stones to build into its nest; being unacquainted with the special habits of the bee, he seems to have supposed that the insect was carrying the stone as ballast to keep itself from being blown away.
Fig. 20—Anthidium manicatum, Carder-bee. A, Male; B, female.
The bees of the genus Anthidium are known to possess the habit of making nests of wool or cotton, that they obtain from plants growing at hand. We have one species of this genus of bees in Britain; it sometimes may be seen at work in the grounds of our Museum at Cambridge: it is referred to by Gilbert White, who says of it, in his History of Selborne: "There is a sort of wild bee frequenting the garden-campion for the sake of its tomentum, which probably it turns to some purpose in the business of nidification. It is very pleasant to see with what address it strips off the pubes, running from the top to the bottom of a branch, and shaving it bare with the dexterity of a hoop-shaver. When it has got a bundle, almost as large as itself, it flies away, holding it secure between its chin and its fore legs." The species of this genus are remarkable as forming a conspicuous exception to the rule that in bees the female is larger than the male. The species of Anthidium do not form burrows for themselves, but either take advantage of suitable cavities formed by other Insects in wood, or take possession of deserted nests of other bees or even empty snail-shells. The workers in cotton, of which our British species A. manicatum is one, line the selected receptacle with a beautiful network of cotton or wool, and inside this place a finer layer of the material, to which is added some sort of cement that prevents the honied mass stored by the bees in this receptacle from passing out of it. A. diadema, one of the species that form nests in hollow stems, has been specially observed by Fabre; it will take the cotton for its work from any suitable plant growing near its nest, and does not confine itself to any particular natural order of plants, or even to those that are indigenous to the South of France. When it has brought a ball of cotton to the nest, the bee spreads out and arranges the material with its front legs and mandibles, and presses it down with its forehead on to the cotton previously deposited; in this way a tube of cotton is constructed inside the reed; when withdrawn, the tube proved to be composed of about ten distinct cells arranged in linear fashion, and connected firmly together by means of the outer layer of cotton; the transverse divisions between the chambers are also formed of cotton, and each chamber is stored with a mixture of honey and pollen. The series of chambers does not extend quite to the end of the reed, and in the unoccupied space the Insect accumulates small stones, little pieces of earth, fragments of wood or other similar small objects, so as to form a sort of barricade in the vestibule, and then closes the tube by a barrier of coarser cotton taken frequently from some other plant, the mullein by preference. This barricade would appear to be an ingenious attempt to keep out parasites, but if so, it is a failure, at any rate as against Leucospis, which insinuates its eggs through the sides, and frequently destroys to the last one the inhabitants of the fortress. Fabre states that these Anthidium, as well as Megachile, will continue to construct cells when they have no eggs to place in them; in such a case it would appear from his remarks that the cells are made in due form and the extremity of the reed closed, but no provisions are stored in the chambers.
The larva of the Anthidium forms a most singular cocoon. We have already noticed the difficulty that arises, in the case of these Hymenopterous larvae shut up in small chambers, as to the disposal of the matters resulting from the incomplete assimilation of the aliment ingested. To allow the once-used food to mingle with that still remaining unconsumed would be not only disagreeable but possibly fatal to the life of the larva. Hence some species retain the whole of the excrement until the food is entirely consumed, it being, according to Adlerz, stored in a special pouch at the end of the stomach; other Hymenoptera, amongst which we may mention the species of Osmia, place the excreta in a vacant space. The Anthidium adopts, however, a most remarkable system: about the middle of its larval life it commences the expulsion of "frass" in the shape of small pellets, which it fastens together with silk, as they are voided, and suspends round the walls of the chamber. This curious arrangement not only results in keeping the embarrassing material from contact with the food and with the larva itself, but serves, when the growth of the latter is accomplished, as the outline or foundations of the cocoon in which the metamorphosis is completed. This cocoon is of a very elaborate character; it has, so says Fabre, a beautiful appearance, and is provided with a very peculiar structure in the form of a small conical protuberance at one extremity pierced by a canal. This canal is formed with great care by the larva, which from time to time places its head in the orifice in process of construction, and stretches the calibre by opening the mandibles. The object of this peculiarity in the fabrication of the elaborate cocoon is not clear, but Fabre inclines to the opinion that it is for respiratory purposes.
Other species of this genus use resin in place of cotton as their working material. Among these are Anthidium septemdentatum and A. bellicosum. The former species chooses an old snail-shell as its nidus, and constructs in it near the top a barrier of resin, so as to shut off the part where the whorl is too small; then beneath the shelter of this barrier it accumulates a store of honey-pollen, deposits an egg, and completes the cell by another transverse barrier of resin; two such cells are usually constructed in one snail-shell, and below them is placed a barricade of small miscellaneous articles, similar to what we have described in speaking of the cotton-working species of the genus. This bee completes its metamorphosis, and is ready to leave the cell in early spring. Its congener, A. bellicosum, has the same habits, with the exception that it works later in the year, and is thus exposed to a great danger, that very frequently proves fatal to it. This bee does not completely occupy the snail-shell with its cells, but leaves the lower and larger portion of the shell vacant. Now, there is another bee, a species of Osmia, that is also fond of snail-shells as a nesting-place, and that affects the same localities as the A. septemdentatum; very often the Osmia selects for its nest the vacant part of a shell, the other part of which is occupied by the Anthidium; the result of this is that when the metamorphoses are completed, the latter bee is unable to effect its escape, and thus perishes in the cell. Fabre further states with regard to these interesting bees, that no structural differences of the feet or mandibles can be detected between the workers in cotton and the workers in resin; and he also says that in the case where two cells are constructed in one snail-shell, a male individual is produced from the cell of the greater capacity, and a female from the other.
Osmia is one of the most important of the genera of bees found in Europe, and is remarkable for the diversity of instinct displayed in the formation of the nests of the various species. As a rule they avail themselves for nidification of hollow places already existing; choosing excavations in wood, in the mortar of walls, and even in sandbanks; in several cases the same species is found to be able to adapt itself to more than one kind of these very different substances. This variety of habit will render it impossible for us to do justice to this interesting genus within the space at our disposal, and we must content ourselves with a consideration of one or two of the more instructive of the traits of Osmia life. O. tridentata forms its nest in the stems of brambles, of which it excavates the pith; its mode of working and some other details of its life have been well depicted by Fabre. The Insect having selected a suitable bramble-stalk with a cut extremity, forms a cylindrical burrow in the pith thereof, extending the tunnel as far as will be required to allow the construction of ten or more cells placed one after the other in the axis of the cylinder; the bee does not at first clear out quite all the pith, but merely forms a tunnel through it, and then commences the construction of the first cell, which is placed at the end of the tunnel that is most remote from the entrance. This cavity is to be of oval form, and the Insect therefore cuts away more of the pith so as to make an oval space, but somewhat truncate, as it were, at each end, the plane of truncation at the proximal extremity being of course an orifice.