Leptogaster was originally described as belonging to the Asilidae, but grass flies have been regarded as a distinct family, the Leptogastridae, by some authors (Martin 1968, Farr 1973, Papavero 1973) based on their morphological dissimilarity to other robber flies. However, this notion was not accepted by other contemporary authors (e.g., Oldroyd 1969). More recently, Bybee et al. (2004) placed the Leptogastrinae as the sister group to the remaining Asilidae in a molecular phylogenetic study (see results here) and therefore one could still rank them as a separate family. In contrast, Dikow (2009a, b) unequivocally showed with both morphological (see result here) and molecular data (see results here) and a comprehensive taxon sampling that Leptogastrinae is nested deeply within Asilidae. The sister-group of Leptogastrinae has not been established conclusively though as the morphological characters support a sister-group relationship of Leptogastrinae + Trigonomiminae (Dikow 2009a) while the combined morphological and molecular data-set supports a sister-group relationship of Leptogastrinae + (Asilinae + Ommatiinae) (Dikow 2009b).
The monophyly of Leptogastrinae, including the genus Acronyches that was placed within Leptogastrinae by Hull (1962) and Dikow (2009a), but not others, is unquestionable and there are many morphological and molecular autapomorphies unique to Leptogastrinae (Dikow 2009a, b, see also Martin 1968).
The phylogenetic relationships within Leptogastrinae have not been comprehensively studied with either morphological or molecular data. Dikow (2009a) included 6 genera and 7 species of Leptogastrinae in the morphological phylogeny of the entire Asilidae (see attached file dikow_2009_trees.png) while in the combined morphological and molecular study (Dikow 2009b) only Acronyches maya was excluded because no specimens for DNA extraction were available (see see attached file dikow_2009_trees.png). The phylogenetic relationships among the taxa between these two analyses differ only in the placement of Lasiocnemus lugens, which groups with the two included Leptogaster s. str. species (arida and cylindrica) or with Tipulogaster glabrata.
A preliminary molecular phylogenetic analysis based on 5 nuclear genes and 27 species (see attached file dikow_unpublished_2011_garli_tree.png, T. Dikow unpublished) illustrates the non-monophyly of Leptogaster, but support a clade of Leptogaster s. str. of several Nearctic and Palaearctic species. Two Nearctic Leptogaster species group far away from Leptogaster s. str. and Beameromyia and Psilonyx are closely related. Leptopteromyia and Lobus form the sister-group to all other included Leptogastrinae genera and both Apachekolos and Euscelidia are monophyletic.