Penthetria Meigen, 1803

Skartveit, John & Nel, André, 2017, Revision of fossil Bibionidae (Insecta: Diptera) from French Oligocene deposits, Zootaxa 4225 (1), pp. 1-83 : 8

Genus Penthetria Meigen, 1803

While it is straightforward to distinguish the genera Penthetria and Plecia in recent species, this is not always so with fossils. In the recent species, it is most convenient to identify the genera based on the shape of the vein R2+3, which is long and horizontal in Penthetria , short and steep in Plecia . However, in fossil species the shape of R2+3 (sometimes named R3+4) varies continuously ( Rice 1959) and it is not possible to draw a non-arbitrary line between Penthetria and Plecia wing venation. Handlirsch (1908) chose to put all fossil bibionids with forked Radial sector in the genus Penthetria , this may be warranted given the limited information available on the morphology of most of the fossil species. However, some fossil species are quite typical of the genus Plecia . In recent species, male terminalia in Plecia spp., though highly variable, is almost always decidedly different from the plesiomorphic pattern invariably found in Penthetria species. However, many fossil species that habitually conform to Plecia have plesiomorphic terminalia, with digitiform gonostyli which are attached distally on the gonapophysis and are directed mesally (e.g., Skartveit 2009). Typical Penthetria males can still be recognized on the very robust, pilose, curved gonostyli, but this is apparent only in very well preserved fossils, whereas a large proportion of the named species have been described from females or poorly-preserved males. One often has to resort to general habitus to assign these species to genus since characters of the terminalia are not available. Penthetria species are generally larger (most species more than 10 mm), more gracile with markedly longer legs, slender male abdomens, the wings are longer, slender in the males and notably large in the female, usually fumose. Plecia species are generally smaller (most species smaller than 10 mm), more robust with shorter legs, male abdomen stockier, and shorter wings which are often hyaline.