The Strepsiptera-Odyssey: the history of the systematic placement of an enigmatic parasitic insect order

The history of the phylogenetic placement of the parasitic insect order Strepsiptera is outlined. The first species was described in 1793 by P. Rossi and assigned to the hymenopteran family Ichneumonidae. A position close to the cucujiform beetle family Rhipiphoridae was suggested by several earlier authors. Others proposed a close relationship with Diptera or even a group Pupariata including Diptera, Strepsiptera and Coccoidea. A subordinate placement within the polyphagan series Cucujiformia close to the wood-associated Lymexylidae was favored by the coleopterist R.A. Crowson. W. Hennig considered a sistergroup relationship with Coleoptera as the most likely hypothesis but emphasized the uncertainty. Cladistic analyses of morphological data sets yielded very different placements, alternatively as sistergroup of Coleoptera, Antliophora, or all other holometabolan orders. Results based on ribosomal genes suggested a sistergroup relationship with Diptera (Halteria concept). A clade Coleopterida (Strepsiptera and Coleoptera) was supported in two studies based on different combinations of protein coding nuclear genes. Analyses of data sets comprising seven or nine genes (7 single copy nuclear genes), respectively, yielded either a subordinate placement within Coleoptera or a sistergroup relationship with Neuropterida. Several early hypotheses based on a typological approach − affinities with Diptera, Coleoptera, a coleopteran subgroup, or Neuropterida − were revived using either a Hennigian approach or formal analyses of morphological characters or different molecular data sets. A phylogenomic approach finally supported a sistergroup relationship with monophyletic Coleoptera.