The bees and the sphecoid wasps have long been regarded as allied groups (Comstock, 1924) and are united as the superfamily Apoidea. As indicated in Section 2, a character traditionally used to demonstrate this relationship is the pronotal lobe, which in these groups is differentiated but rather small and usually well separated from the tegula, whereas in other aculeate Hymenoptera the pos-terolateral part of the pronotum reaches the tegula.
Brothers (1975) emphasized this character (Brothers' character 21.2), and Lanham (1981) stated it as enlargement ofthe anterolateral parts of the mesoscutum, which results in the prothoracic lobe feature (see also Brothers' character 27.1). Brothers also listed two other strong synapomorphies that unite sphecoids and bees: (1) a ven-trolateral extension of the pronotum to encircle or nearly encircle the thorax behind the front coxae (Brothers' 23.2), and (2) an enlargement of the metapostnotum (propodeal triangle), carrying the third phragma posteriorly (Brothers' 35.3; see also Brothers, 1976). Other, less impressive characters uniting bees and sphecids are the shortened pronotum, its posterior margin commonly broadly concave (Brothers' 18.2 and 22.l); and the fusion of the mesopleural suture with the intersegmental suture (Brothers' 33.l.l). Most of these characters also exist in other aculeate groups; they are therefore weaker than the strong characters listed above. The manner of cleaning the thoracic dorsum is also an interesting character (Jan-der, 1976). Most Hymenoptera use forward scraping by the front tarsi for this purpose; many sphecoids and most bees use the middle tarsi. The noncorbiculate Apidae appear to have reverted to wasplike cleaning behavior; otherwise the use of the middle tarsi seems to be a feature showing the relationship of sphecoid wasps and bees.
To me the evidence for close relationship between sphecoids and bees is highly convincing, in spite of Lan-ham's (1981) contrary views suggesting that the characters listed by Brothers (1975), Lomholdt (1982), and others as uniting sphecoid wasps and bees are convergent rather than synapomorphous.
Additional material on the origin of bees will be found in Sections 14, 22, and 23, on fossil bees and their antiquity.
This is as appropriate a place as any to explain briefly some terms used here and in subsequent sections that are in common use by systematists, yet are perhaps little understood by others. The terms monophyletic (holo-phyletic) and paraphyletic are explained at the end of Section 2. A clade is a monophyletic group, i.e., the organisms subsumed by a branch of a cladogram.
A cladogram is a treelike diagram representing a hypothesis of phylogenetic relationships. It is in contrast to a phenogram, which is a tree based on numbers of differences or similarities rather than on phylogeny. The term dendrogram is used for any type of treelike diagram, or one made without a clear indication of the methodology used in its preparation. The term cladistic is said of an analysis that seeks to infer the branching sequence of a phylogeny, or of a classification based on such an analysis. A cladistic analysis or classification is the same as a phylogenetic analysis or classification. The term phenetic is said of a classification or analysis based on degrees of difference among taxa, as distinguished from a phyloge-netic or cladistic classification or analysis based on genealogy, i.e., the sequence of branching in a cladogram. An apomorphy is a character state that is derived (not ancestral) relative to other states of the same character; a synapomorphy is an apomorphy shared by two or more taxa and inferred to have been present in their common ancestor. A plesiomorphy is a character state that is ancestral relative to all other states of the same character. A symplesiomorphy is a plesiomorphy shared by two or more taxa because of ancestral relationships. The word polyphyletic (or diphyletic) is said of a taxon whose distinctive features arose independently (nonhomolo-gously) in several (or two) clades. Such a taxon must be divided into several (or two) taxa that have shared, but nonhomologous, features. In a dichotomous cladogram, any two branches arising from a single point are sisters; each is the sister group to the other. The word taxon (pl. taxa) is used for any named systematic unit at any classi-ficatory level.
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