See Salmela and Vilkamaa (2005).

Ctenosciara exigua was described based on several specimens from mires in Central Finland. It was differentiated from Ctenosciara hyalipennis by the evenly broad gonostyli with lacking megasetae at the dorsal side of the apical tooth, the smaller size and less setose CuA₂. However, the most distinctive character, the shorter and roundish tegmen, was not mentioned. In Ct. hyalipennis the tegmen is much longer than wide, nearly triangular. In our material, the number of macrotrichia on CuA₂ varies from 0 to 18 and the tibial comb was also found to be undivided in some specimens. There were usually differences to the original description in every specimen studied. As seen in Fig. 4, showing one of the barcoded individuals, the gonostylus is more tapered and has apical megasetae. One might argue, that our specimens do not exactly match with Ct. exigua. But as they are in the same manner clearly and more different from Ct. hyalipennis, we confidently identify them as Ct. exigua.

Since the original description from Finland, the species was mentioned again by Heller et al. (2009) from Sweden and therefore it appears to have a Northern European distribution. As the identification of this species is only possible by careful microsopic analysis of the male genitalia and because the most similar species, Ctenosciara hyalipennis, is one of the most common European Sciaridae, it may have been overlooked.

The barcoding results coupled with the fact that Ctenosciara hyalipennis and Ct. exigua (in our understanding) are quite polymorphic raise the question " Is Ctenosciara exigua really a distinct species?" or "Is it only one variant of the former?". In Central Europe, Ct. hyalipennis shows two distinct morphs. The early spring form is larger and has clearly clavate gonostyles, whereas the summer variant is smaller, brighter and the shape of the gonostyles is just as parallel as in Ctenosciara exigua. The summer variant was treated as Ctenosciara thiedei in Thiede (1977), a nomen nudum, which was never officially published. The analysis of the COI did not show any significant differences between both seasonal morphs. The same situation could be present for the Ctenosciara exigua/hyalipennis complex. Recently Kurina et al. (2015) described a species of Mycetophilidae, which is not distinguishable genetically but only differs in the structure of male genitalia. Similarly is imaginable, that Ct. hyalipennis is a species that has only recently invaded from some other part of the world, then successfully occupied different ecological nices, but speciation has not yet progressed to a point where clear genetic differences have taken place. The bifid East-West distribution pattern in Canada (Fig. 6) might be be a reminiscence of two recent, parallel immigrations, which independently started from the eastern and western coasts. All those localities of Ct. hyalipennis are in the vicinity of typical entry points like harbors, airports and bigger cities with massive human activity or spreading already upstream. Geographic distribution of haplotypes underpin this assumption as Neartic population is gentically less diverse than European (Table 1). Keeping in mind, that most of the sciarid sequences (96%) discussed here originated from the Global Malaise Trap Program and thus sequences are only single strand generated, so some of the singletons may reflect in fact sequencing artefacts. Also earlier faunistic studies from North America (Johannsen 1912, Pettey 1918) do not mention this species and it was found neither in historical collections nor in younger material until 2000 (Mohrig pers. comm.). Further morphological, ecological and genetic analyses are needed to shed light on species concepts of Ctenosciara hyalipennis sensu latu. For the moment we propose to continue treating Ctenosciara exigua as a distinct species.