The spider fly genus Eulonchus Gerstaecker is found throughout the Nearctic Region. Six species are recognized and intraspecific morphological variation is documented in several species. A phylogeny of Eulonchus based on DNA sequence data of three molecular markers (COI, CAD, and 16S) is presented and relationships of species are discussed in the light of biogeography and host usage. All six species of Eulonchus are redescribed using natural language descriptions exported from a character matrix, and a key to species is presented. Lectotypes are designated for Eulonchus sapphirinus Osten Sacken, Eulonchus smaragdinus Gerstaecker, and Eulonchus tristis Loew.
Keywords: Antrodiaetidae; Euctenizidae; Lucid; biodiversity; cybertaxonomy; phylogeny; small-headed fly; spider parasitoid; tarantula.