Fig. 21.—Osmia tricornis, ♀. Algeria.
The first cell thus made is stored with pollen and honey, and an egg is deposited. Then a barrier has to be constructed to close this chamber; the material used for the barrier is the pith of the stem, and the Insect cuts the material required for the purpose from the walls of the second chamber; the excavation of the second chamber is, in fact, made to furnish the material for closing up the first cell. In this way a chain of cells is constructed, their number being sometimes as many as fifteen. The mode in which the bees, when the transformations of the larvae and pupae have been completed, escape from the chain of cells, has been the subject of much discussion, and errors have arisen from inference being allowed to take the place of observation. Thus Dufour, who noted this same mode of construction and arrangement in another Hymenopteron (Odynerus nidulator), perceived that there was only one orifice of exit, and also that the Insect that was placed at the greatest distance from this was the one that, being the oldest of the series, might be expected to be the first ready to emerge; and as the other cocoons would necessarily be in the way of its getting out, he concluded that the egg that was last laid produced the first Insect ready for emergence. Fabre tested this by some ingenious experiments, and found that this was not the case, but that the Insects became ready to leave their place of imprisonment without any reference to the order in which the eggs were laid, and he further noticed some very curious facts with reference to the mode of emergence of Osmia tridentata. Each Insect, when it desires to leave the bramble stem, tears open the cocoon in which it is enclosed, and also bites through the barrier placed by the mother between it and the Insect that is next it, and that separates it from the orifice of exit. Of course, if it happen to be the outside one of the series it can then escape at once; but if it should be one farther down in the Indian file it will not touch the cocoon beyond, but waits patiently, possibly for days; if it then still find itself confined it endeavours to escape by squeezing past the cocoon that intervenes between it and liberty, and by biting away the material at the sides so as to enlarge the passage; it may succeed in doing this, and so get out, but if it fail to make a side passage it will not touch the cocoons that are in its way. In the ordinary course of events, supposing all to go well with the family, all the cocoons produce their inmates in a state for emergence within a week or two, and so all get out. Frequently, however, the emergence is prevented by something having gone wrong with one of the outer Insects, in which case all beyond it perish unless they are strong enough to bite a hole through the sides of the bramble-stem. Thus it appears that whether a particular Osmia shall be able to emerge or not depends on two things—(1) whether all goes well with all the other Insects between it and the orifice, and (2) whether the Insect can bite a lateral hole or not; this latter point also largely depends on the thickness of the outer part of the stem of the bramble. Fabre's experiments on these points have been repeated, and his results confirmed by Nicolas.
The fact that an Osmia would itself perish rather than attack the cocoon of its brother or sister is certainly very remarkable, and it induced Fabre to make some further experiments. He took some cocoons containing dead specimens of Osmia, and placed them in the road of an Osmia ready for exit, and found that in such case the bee made its way out by demolishing without any scruple the cocoons and dead larvae that intervened between it and liberty. He then took some other reeds, and blocked the way of exit with cocoons containing living larvae, but of another species of Hymenoptera. Solenius vagus and Osmia detrita were the species experimented on in this case, and he found that the Osmia destroyed the cocoon and living larvae of the Solenius, and so made its way out. Thus it appears that Osmia will respect the life of its own species, and die rather than destroy it, but has no similar respect for the life of another species.
Some of Fabre's most instructive chapters are devoted to the habits and instincts of various species of the genus Osmia. It is impossible here to find space even to summarise them, still more impossible to do them justice; but we have selected the history just recounted, because it is rare to find in the insect world instances of such self-sacrifice by an individual for one of the same generation. It would be quite improper to generalise from this case, however, and conclude that such respect for its own species is common even amongst the Osmia. Fabre, indeed, relates a case that offers a sad contrast to the scene of self-sacrifice and respect for the rights of others that we have roughly portrayed. He was able to induce a colony of Osmia tricornis (another species of the genus, be it noted) to establish itself and work in a series of glass tubes that he placed on a table in his laboratory. He marked various individuals, so that he was able to recognise them and note the progress of their industrial works. Quite a large number of specimens thus established themselves and concluded their work before his very eyes. Some individuals, however, when they had completed the formation of a series of cells in a glass tube or in a reed, had still not entirely completed their tale of work. It would be supposed that in such a case the individual would commence the formation of another series of cells in an unoccupied tube. This was not, however, the case. The bee preferred tearing open one or more cells already completed—in some cases, even by itself—scattering the contents, and devouring the egg; then again provisioning the cell, it would deposit a fresh egg, and close the chamber. These brief remarks will perhaps suffice to give some idea of the variety of instinct and habit that prevails in this very interesting genus. Friese observes that the variety of habits in this genus is accompanied as a rule by paucity of individuals of a species, so that in central Europe a collector must be prepared to give some twenty years or so of attention to the genus before he can consider he has obtained all the species of Osmia that inhabit his district.
As a prelude to the remarks we are about to make on the leaf-cutting bees of the genus Megachile it is well to state that the bee, the habits of which were described by Réaumur under the name of "l'abeille tapissière," and that uses portions of the leaves of the scarlet poppy to line its nest, is now assigned to the genus Osmia, although Latreille, in the interval that has elapsed since the publication of Réaumur's work, founded the genus Anthocopa for the bee in question. Megachile is one of the most important of the genera of the Dasygastres, being found in most parts of the world, even in the Sandwich Islands; it consists of bees averaging about the size of the honey-bee (though some are considerably larger, others smaller), and having the labrum largely developed; this organ is capable of complete inflection to the under side of the head, and when in the condition of repose it is thus infolded, it underlaps and protects the larger part of the lower lip; the mandibles close over the infolded labrum, so that, when the Insect is at rest, this appears to be altogether absent. These bees are called leaf-cutters, from their habit of forming the cells for their nest out of pieces of the leaves of plants. We have several species in Britain; they are very like the common honey-bee in general appearance, though rather more robustly formed. These Insects, like the Osmiae, avail themselves of existing hollow places as receptacles in which to place their nests. M. albocincta frequently takes possession of a deserted worm-burrow in the ground. The burrow being longer than necessary the bee commences by cutting off the more distant part by means of a barricade of foliage; this being done, it proceeds to form a series of cells, each shaped like a thimble with a lid at the open end (Fig. 22, A). The body of the thimble is formed of large oval pieces of leaf, the lid of smaller round pieces; the fragments are cut with great skill from the leaves of growing plants by the Insect, which seems to have an idea of the form and size of the piece of foliage necessary for each particular stage of its work.
Fig. 22—Nidification of leaf-cutting bee, Megachile anthracina. A, one cell separated, with lid open; the larva (a) reposing on the food; B, part of a string of the cells. (After Horne.)
Horne has given particulars as to the nest of Megachile anthracina (fasciculata), an East Indian species.[[30]] The material employed was either the leaves of the Indian pulse or of the rose. Long pieces are cut by the Insect from the leaf, and with these a cell is formed; a circular piece is next cut, and with this a lid is made for the receptacle. The cells are about the size and shape of a common thimble; in one specimen that Horne examined no less than thirty-two pieces of leaf disposed in seven layers were used for one cell, in addition to three pieces for the round top. The cells are carefully prepared, and some kind of matter of a gummy nature is believed to be used to keep in place the pieces forming the interior layers. The cells are placed end to end, as shown in Fig. 22, B; five to seven cells form a series, and four or six series are believed to be constructed by one pair of this bee, the mass being located in a hollow in masonry or some similar position. Each cell when completed is half filled with pollen in the usual manner, and an egg is then laid in it. This bee is much infested by parasites, and is eaten by the Grey Hornbill (Meniceros bicornis).
Megachile lanata is one of the Hymenoptera that in East India enter houses to build their own habitations. According to Horne both sexes take part in the work of construction, and the spots chosen are frequently of a very odd nature. The material used is some kind of clay, and the natural situation may be considered to be the interior of a hollow tube, such as the stem of a bamboo; but the barrel of a gun, and the hollow in the back of a book that has been left lying open, have been occasionally selected by the Insect as suitable. Smith states that the individuals developed in the lower part of a tubular series of this species were females, "which sex takes longer to develop, and thus an exit is not required for them so soon as for the occupants of the upper cells which are males." M. proxima, a species almost exactly similar in appearance to M. lanata, makes its cells of leaf-cuttings, however, and places them in soft soil.