Sphecid Wasps and the Origins of the Bees
The "bees" comprise approximately 16,000 species of pollen-feeding aculeate Hymenoptera (Michener 2000). Bees are clearly a monophyletic group united by numerous morphological and behavioral synapomorphies. Some of the synapomorphies of the bees are related to the switch from preying on other insects to collecting pollen (e.g., the branched, plumose hairs and the expanded hind basitarsus), while many others seem to have no direct relationship to the transition to pollen collecting (e.g., some mouthpart characters, the articulation of the mid-coxa, several wing vein characters, modifications of the male apical sterna [S7 and S8], and modifications of the sclerites of the female sting apparatus [division of T7 into two lateral hemitergites]). Alexander & Michener (1995) listed 13 characters that support bee monophyly, a result congruent with previous studies (Brothers 1975, Lomholdt 1982, Brothers and Carpenter 1993). Michener (2000) describes in detail the morphological characters that support bee monophyly. Characters described by Michener (2000) supporting bee monophyly are listed below:
- Character a: branched or plumose hairs
- Character b: pollen feeding larvae
- Character c: broadened hind basitarsus
- Character d: larval maxilla with one apical papilla
- Character e: T7 in females divided medially into two lateral hemitergites
- Character f: absence of strigil on hind tibia (present in spheciform wasps)
- Character g: one sperm cell develops from each spermatocyte
- Character h: foreleg is cleaned by drawing it through flexed middle leg
- Character i: male S7 and S8 are modified relative to anterior sterna
- Character j: bristles on outer surfaces of tibia absent or weak
- Character k: basitibial plate present
- Character l: simple larval mandible
- Character m: cleft claws in many bee taxa may be derived
- Character n: location of the flexion lines in forewing
The most recent morphological analysis of bee/sphecid relationships (Melo 1999) shows bees as sister to a large group of sphecid wasps in the family Crabronidae, a result congruent with Lomholdt’s (1982) earlier findings (although he referred to the group as the Larridae). Melo’s (1999) overall conclusions regarding higher-level phylogenetic relationships within the sphecid wasp + bee clade (= Superfamily Apoidea) were as follows:
While Melo chose to refer to the bees as a single family (Apidae) this was rejected by Michener (2000) as potentially creating chaos for bee systematists. Currently the best designation for the monophyletic bees is Apiformes and for the paraphyletic sphecid wasps (including Melo’s four families) is Spheciformes (Michener 2000). We adopt this terminology throughout this site.
Jon Seger (University of Utah) and Michael Ohl (Museum fur Naturkunde der Humboldt-Universitat, Berlin) are developing additional molecular data sets for resolving higher-level spheciform relationships.