The First Twisted-Wing Parasitoids (Insecta: Strepsiptera) from the Early Eocene Green River Formation of Colorado

Abstract Strepsiptera is a clade of entomophagous parasitoid insects with fewer than 30 previously reported fossils. Two new species of Caenocholax (Strepsiptera: Myrmecolacidae) described here represent the first reported adult strepsipterans preserved as organic compression fossils. Their occurrence in the Early Eocene (about 50 Ma) Green River Formation (Colorado, USA) is the northernmost New World record of Myrmecolacidae and the oldest record of Caenocholax. Caenocholax barkleyi sp. nov. and Caenocholax palusaxus sp. nov. are each known from one adult male. The aedeagus of C. barkleyi lacks a median projection and terminates in two hooks, an apomorphy of the species. Caenocholax palusaxus has distinctly intermediate wing vein density and a larger ratio of antennomere 6 to antennomere 7 than any other species of Caenocholax. The fossils reported here expand the known insect biota of the formation in taxonomic richness as well as the fossil record of Strepsiptera in space and time. Moreover, the Eocene specimens hint at an unappreciated fossil diversity of endoparasitic insects.