The leaf-cutter bees (Megachile)—which differ from Andrena and Halictus and agree with Osmia, Apis and Bombus in having elongate tongues—cut neat circular disks from leaves, using them for lining the cells of their underground nests. The carpenter-bees (Xylocopa and allied genera), unrepresented in the British Islands, though widely distributed in warmer countries, make their nests in dry wood. The habits of X. violacea, the commonest European species, were minutely described in the 18th century in one of R.A.F. de Réaumur’s memoirs. This bee excavates several parallel galleries to which access is gained by a cylindrical hole. In the galleries are situated the cells, separated from one another by transverse partitions, which are formed of chips of wood, cemented by the saliva of the bee.
Among the solitary bees none has more remarkable nesting habits than the mason bee (Chalicodoma) represented in the south of France and described at length by Fabre. The female constructs on a stone a series of cells, built of cement, which she compounds of particles of earth, minute stones and her own saliva. Each cell is provided with a store of honey and pollen beside which an egg is laid; and after eight or nine cells have been successively built and stored, the whole is covered by a dome-like mass of cement. Fabre found that a Chalicodoma removed to a distance of 4 kilometres from the nest that she was building, found her way back without difficulty to the exact spot. But if the nest were removed but a few yards from its former position, the bee seemed no longer able to recognize it, sometimes passing over it, or even into the unfinished cell, and then leaving it to visit again uselessly the place whence it had been moved. She would accept willingly, however, another nest placed in the exact spot where her own had been. If the unfinished cell in the old nest had been only just begun, while that in the substituted nest were nearly completed, the bee would add so much material as to make the cell much larger than the normal size, her instinct evidently being to do a certain amount of building work before filling the cell with food. The food, too, is always placed in the cell after a fixed routine—first honey disgorged from the mouth, then pollen brushed off the hairs beneath the body (fig. 7, c) after which the two substances are mixed into a paste.
Inquilines and Parasites.—The working bees, such as have been mentioned, are victimized by bees of other genera, which throw upon the industrious the task of providing for the young of the idle. The nests of Andrena, for example, are haunted by the black and yellow species of Nomada, whose females lay their eggs in the food provided for the larva of the Andrena. According to H. Friese, the relations between the host and the inquiline are quite friendly, and the insects if they meet in the nest-galleries courteously get out of each other’s way. D. Sharp, in commenting on this strange behaviour, points out that the host can have no idea why the inquiline haunts her nest. “Why then should the Andrena feel alarm? If the species of Nomada attack the species of Andrena too much, it brings about the destruction of its own species more certainly than that of the Andrena.”
More violent in its methods is the larva of a Stelis, whose operations in the nest of Osmia leucomelana have been studied by Verhoeff. The female Stelis lays her eggs earlier than the Osmia, and towards the bottom of the food-mass; the egg of the Osmia is laid later, and on the surface of the food. Hence the two eggs are at opposite ends of the food, and both larvae feed for a time without conflict, but the Stelis, being the older, is the larger of the two. Finally the parasitic larva attacks the Osmia, and digging its mandibles into its victim’s head kills and eats it, taking from one to two days for the completion of the repast.
| Fig. 4.—Under Side of Worker, carrying Wax Scales. |
| (From Cheshire’s Bees and Bee-keeping.) |
Social Bees.—The bees hitherto described are “solitary,” all the individuals being either males or unmodified females. The most highly developed of the long-tongued bees are “social” species, in which the females are differentiated into egg-laying queens and (usually) infertile “workers” (fig. 6). Verhoeff has discussed the rise of the “social” from the “solitary” condition, and points out that for the formation of an insect community three conditions are necessary—a nest large enough for a number of individuals, a close grouping of the cells, and an association between mother and daughters in the winged state. For the fulfilment of this last condition, the older insects of the new generation must emerge from the cells while the mother is still occupied with the younger eggs or larvae. One species of Halictus nearly reaches the desired stage; but the first young bees to appear in the perfect state are males, and when the females emerge the mother dies.
| Fig. 5.—Abdominal Plate (worker of Apis), under side, third segment. W, wax-yielding surface, covering true gland; s, septem, or carina; wh, webbed hairs. |
| (From Cheshire’s Bees and Bee-keeping.) |
Among the social bees the mother and daughter-insects co-operate, and they differ from the “solitary” groups in the nature of their nest, the cells (fig. 25) of which are formed of wax secreted by special glands (fig. 5) in the bee’s abdomen, the wax being pressed out between the segmental sclerites in the form of plates (fig. 4), which are worked by the legs (fig. 7) and jaws into the requisite shape. In our well-known hive-bee (Apis) and humble-bees (Bombus) the wax glands are ventral in position, but in the “stingless” bees of the tropics (Trigona and Melipona) they are dorsal. A colony of humble-bees is started in spring by a female “queen” which has survived the winter. She starts her nest underground or in a surface depression, forming a number of waxen cells, roughly globular in shape and arranged irregularly. The young females (“workers”) that develop from the eggs laid in these early cells assist the queen by building fresh cells and gathering food for storage therein. The queen may be altogether relieved of the work of the nest as the season advances, so that she can devote all her energies to egg-laying, and the colony grows rapidly. The distinction between queen and worker is not always clear among humble-bees, the female insects varying in size and in the development of their ovaries. If any mishap befall the queen, the workers can sometimes keep the community from dying out. In autumn males are produced, as well as young queens. The community is broken up on the approach of winter, the males and workers perish, and the young queens after hibernation start fresh nests in the succeeding year.
 | |
| Fig. 6.—Ovaries of Queen and Workers (Apis). | |
A, Abdomen of queen, under side. P, Petiole. o, o, Ovaries. hs, Position filled by honey-sack. ds, Position through which digestive system passes. od, Oviduct. co.d, Vagina. E, Egg-passing oviduct. s, Spermatheca. | i. Intestine. pb, Poison bag. pg, Poison gland. st, Sting. p, “Palps” or “feelers” of sting. B, Rudimentary ovaries of ordinary worker. sp, Rudimentary spermatheca. C, Partially developed ovaries of fertile worker. sp, Rudimentary spermatheca. |
| (From Cheshire’s Bees and Bee-keeping.) | |
The appearance of the heavy-bodied hairy Bombi is well known. They are closely “mimicked” by bees of the genus Psithyrus, which often share their nests. These Psithyri have no pollen-carrying structures on the legs and their grubs are dependent for their food-supply on the labours of the Bombi, though, according to E. Hoffer’s observations, it seems that the female Psithyrus builds her own cells. The colonies of Bombus illustrate the rise of the inquiline habit. Many of the species are very variable and have been differentiated into races or varieties. F.W.L. Sladen states that a queen belonging to the virginalis form of Bombus terrestris often invades a nest belonging to the lucorum form, kills the rightful queen, and takes possession of the nest, getting the lucorum workers to rear her young. In the nests of Bombi are found various beetle larvae that live as inquilines or parasites, and also maggots of drone-flies (Volucella), which act as scavengers; the Volucella-fly is usually a “mimic” of the Bombus, whose nest she invades.