Small but mighty – A Phylogeny of Spilomenina (Hymenoptera: Apoidea: Crabronidae)

With about 10,000 known species, the paraphyletic digger wasps (Ampulicidae + Crabronidae + Heterogynaidae + Sphecidae sensu stricto) are one of the most diverse taxa within the aculeate Hymenoptera. With the bees (Anthophila), they form a well-supported clade, the Apoidea. As a possible sister group of the bees, the Crabronidae or one of its subgroups (Pemphredoninae, Philanthinae) are discussed. The Pemphredoninae include the morphologically highly diverse Spilomenina, which is distributed worldwide but mainly occurs in the southern hemisphere.

The Spilomenina are characterized by the reduced palpal formula and wing venation as well as by the presence of silk glands in the female sixth metasomal tergit. Hitherto, the group contains four genera: Arpactophilus Smith, 1863, Microstigmus Duke, 1907, Spilomena Shuckard, 1837 and Xysma Pate, 1937. With a body size of 2-9 mm, all species are relatively small and morphologically strongly derived, which can be seen especially in the reduced wing venation. Species of the spilomenin genus Microstigmus are the only known digger wasps which show eusocial behavior.

Although the presence of silk glands implies monophyly of the Spilomenina, the phylogeny of this group has not been comprehensively studied yet. The aim of this project is to conduct a detailed cladistic analysis of the Spilomenina based on morphological characters of recent and fossil taxa.

The phylogenetic analysis confirms the monophyly of the Spilomenina as well as the monophyly of Arpactophilus and Microstigmus. Xysma is not supported by apomorphies and is treated as a synonym of Spilomena. The species Spilomena subterranea McCorquoadale & Naumann, 1988 is the sister of the remaining Spilomenina and will be placed in a new genus.

As a remarkable novelty, we discovered that the males of the New Caledonian species of Arpactophilus have only 12 antennomeres. In the majority of male digger wasps, the antennae have 13 antennomeres, whereas females have 12. Although a reduction of the antennomere number in males is known from a few other digger wasp genera, it is obviously an unusual modification.