Description & Statistics
This large family contains the leaf-cutting bees and the mason bees, which provision their cells with honey and pollen. However, among them are several genera and species that are inquilines in the cells of other members of the family. The behavior of Coelioxys rufitarsis Smith, a natural enemy of Megachile spp. in North America, was studied by Graenicher (1905b). The female bee utilizes the sharp, rigid end of the abdomen in tearing open the inner layer of leaves, from which the cell of the host is constructed, and oviposits through the aperture. The egg's anterior end remains in the opening, with the opposite end being embedded in the beebread. Oviposition may occur either before the host egg is laid or after the larva has begun development. The 1st instar larva possesses gigantic mandibles, a very evident adaptation for combat. Feeding occurs on the provisions in the cell, but the larva gradually works its way to the surface, where it finds the Megachile larva and destroys it, despite the stage of development it has attained.
These are long-tongued bees with their labrum being longer than wide (Finnamore & Michener 1993). The mandibles of females and many males are usually wide apically, with the apical margin forming 3 or more teeth. The forewing has 2 submarginal cells. The scopa are absent on hind legs, when present they are restricted to metasomal sterna. The basitibial plate is missing and the pygidial plate of females is absent or represented by a spine and the pygidial plate of males is generally absent.
This is a large, cosmopolitan family with many genera and several thousand species. There are more than 615 species in North America . The family has two subfamilies: Megachilinae an Lithurginae. The large and somewhat angular head, especially of females, together with the wide mandibles, makes most species recognizable on sight as belonging to this family. The small subfamily Lithurginae, represented in North America only by the genus Lithurge, contains a few species the generally resemble Megachile. Megachilinae contains the usually yellow and black species of the tribe Anthidiini as well as black (sometimes with metasoma red) to brilliantly metallic blue or green Megachilini. In both tribes cleptoparasitic as well as nest-making genera are known.
Nests are either in soil, in pithy stems, or they occur in formed cavities such as the haunts of Coleoptera in wood. They may also be deposited on rocks, stems, or even leaves. Unlike cells of other bees, cells of Megachilidae are made of foreign materials brought into the nest or at least used to divide a burrow into segments that function as cells. The foreign materials may be leaf pieces (used by Megachile species), chewed leaf pulp, plant hairs (used by Anthidium species), resin, or mud or sometimes mixtures of the preceding and sometimes supplemented with pebbles.
This family of solitary bees transport pollen in a scopa instead on on the hind legs. Most genera are known as mason bees and leafcutter bees, because they build their nest cells from soil or leaves. Some species also collect plant or animal fibers, and are known as carder bees. All species utilize nectar and pollen, but a few are cleptoparasitoids (= cuckoo bees). They feed on pollen collected by other bees in the same family. Parasitic species do not have a scopa. The family ranks among the world's most efficient pollinators because of their great activity among flowers. They are frequent visitors of many plants, and they are quite inefficient in obtaining the pollen when compared to other families in the Apoidea. They may have to make five or more visits to flowers to gather enough pollen for one brood cell.
There are many species in North America, but some Megachile spp have been introduced to serve as pollinators. One species, Osmia lignaria is deployed in orchard crop pollination.
Nests are typically divided into cells, each cell receives a supply of food (pollen or a pollen/nectar mix) and an egg; after finding a suitable spot (often near where she emerged), a female starts building a first cell, stocks it, and oviposits. Then she builds a wall that separates the completed cell from the next one. The larva hatches from the egg and consumes the food supply. After moulting a few times, it spins a cocoon and pupates. Then it emerges from the nest as an adult. Males die shortly after mating, but females survive for another few weeks, during which they build new nests.
Nests are frequently built in cavities. Some species place individual cells in clay or resin that is affixed to surfaces. Nest cavities are usually linear, as in a hollow plant stem, but the shells of snailss are used by some Osmia, and some species will use other cavities as well. On the other hand, some genera are brood parasites that have no ventral scopa (e.g. Stelis, Coelioxys).. They may enter an unsealed nest to lay eggs in a cell. The young parasitoid larva then kills the host larva if it is still alive.
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