Family Megachilidae

(2 subfamilies, 7 tribes, 74 genera, ~3170 species)

The family Megachilidae includes the familiar leaf-cutter bees, which line their cells with precisely cut leaf disks. Megachilid bees are easily distinguished from other bees by the presense of a dense, metasomal scopa (present in all females except in parasitic species and Pararhophitini), a broad, rectangular labrum, and two submarginal cells in the forewing (except in Fideliini). Roig-Alsina & Michener (1993) list six adult characters that unite the Megachilidae (including Fideliinae), including mouthpart, pronotal, and other thoracic characters. McGinley & Rozen (1987) described the behavioral and life-history characters that unite Fideliinae (previously considered a distinct family) with the Megachilidae. There are many cleptoparasitic lineages of Megachilinae and cleptoparasitism has apparently arisen multiple times within the subfamily (Rozen 2000). Megachilid bees are extraordinarily diverse in nesting habits with leaf-cutters, masons, stem-nesters, ground-nesters, and wood nesters. They are also important managed pollinators (such as the alfalfa leaf-cutter bee, Megachile rotundata).

The biogeographic distribution of the Megachilidae was discussed by Michener (1979). Among the five tribes of Megachilinae, Lithurgini is widespread but is most diverse in South America, where it is thought to have originated. The Anthidiini is most diverse in the Palearctic and African regions. Michener (1979) hypothesized that this tribe originated in the Palearctic region. Dioxyini is also thought to have had Old World origins. The tribe Osmiini is primarly Holarctic and African while the Megachilini is widespread (present on all non-glaciated landmasses except New Zealand!). The broad distribution of many megachilid groups is presumably due to their habit of nesting in wood and plant matter. Nests containing larvae and perhaps adults may float in water for long periods of time. More recently human-mediated transport has lead to trans-continental dispersal of wood nesting species (e.g., Ascher 2001).

Among the most interesting of all the megachilid groups is the Fideliini which has an apparently ancient biogeographic distribution with only two, widely separated genera: Fidelia living in arid parts of southern Africa and Morocco (Whitehead & Eardley 2003) and Neofidelia living in arid central Chile (Engel 2002). Engel’s (2002) cladistic analysis of the generic and subgeneric relationships indicates that the Fidelia and Neofidelia are sister groups. Presence of sister groups on Africa and South America suggests an ancient vicariant event at roughly the time Africa and South America separated (~100 mya). The presence of one species in Morocco suggests the Fideliini may have had a much wider distribution in the past (Engel 2002).

Relationships among the subfamilies and tribes, based on Roig-Alsina & Michener (1993) are as follows:

Molecular data may help resolved the tribal level relationships within the Megachilinae. All tribes appear monophyletic except perhaps the Osmiini, which may be rendered paraphyletic by Megachilini (Michener 2000). Questions also remain about the phylogenetic affinities of the Dioxyini.

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Data sets:

none published