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Megachilidae (mason bees, leaf-cutter bees, carder bees; order Hymenoptera, suborder Apocrita) Large, cosmopolitan family of solitary bees, in which the pollen scopa consists entirely of tracts of stiff hairs on the abdominal sterna (see STERNUM). The labrum is longer than it is broad, and basitibial and pygidial (see PYGIDIUM) plates are absent. The family includes the mason bees and leaf-cutter bees, and also such genera as Anthidium and Callanthidium, the females of which tease out (card) plant hairs to use as nest-lining material–hence the common name ‘carder bees’. Nests are usually made in pre-existing cavities, e.g. in beetle borings in dead wood or pithy stems. Some species specialize in using old snail shells; others build exposed nests on rocks, woody branches, etc. One species, Megachile rotundata (alfalfa leaf-cutter bee), is cultured commercially in the western USA as a pollinator of Medicago sativa (alfalfa or lucerne).