DIPTERA > NEMATOCERA | Lower Diptera

BIBIONOMORPHA > Sciaroidea > Sciaridae (267 species)

(subfamilies: none)

Although these species are usually referred to as Dark-winged Fungus Gnats, they're also known as Black Fungus Gnats, which is arguably a more meaningful collective name for the family given that the body colour of most adult sciarids is predominantly black. The alternative name is probably more distinctive, but it does infer that this particular group are distinguished from other fungus gnats by having dark wings, which is misleading as the majority of sciarids have clear or smoky wings - only a few of the larger species have truly dark wings.  

Similarly with the larvae, as it's important to note that while some will develop in fungus, it's certainly not to the extent of the Mycetophilidae with which these species have a much closer association than with any of the other related fungus gnat families. They also feed on various decaying plant matter, rotting wood and animal excrement, as well as being found in soil, greenhouses or on commercial mushroom farms where, not surprisingly, they are regarded as economic pests. Sciarid larvae are characterised by having a shiny black head and a 12-segmented, white translucent body.

Sciarids were originally treated as a subgroup of the Mycetophilidae with relatively few known species at the time, but following a great amount of detailed work, and new discoveries, they were formally separated. Subsequent research over many years has seen the total number of catalogued species slowly increasing with every publicised update of the British List to the point that Sciaridae is now one the largest dipteran families.

The latest version of the checklist confirms that we currently have nearly 270 recorded species, separated into 24 genera. At present these genera are all lumped together as there are no recognised subfamilies, although I have read that there are proposals to split the family into Sciarinae, Chaetosciarinae and Cratyinae.

In early works most sciarids were placed in the Sciara genus, which dates back to Meigan (1803), but that situation has changed greatly over time as species have been gradually reassigned to other genera. I'm not sure of the exact figure, but I believe that there are now only about a dozen described species of Sciara in Europe with just seven occurring in Britain. The largest genus today is Bradysia with 75 species, followed by Corynoptera with 57 species (in various subgenera), then Scatopsciara and Cratyna with 22 and 17 species respectively.

Despite the fact that most sciarids are small and inconspicuous, they are easily recognised from a combination of characters especially in respect of their simple, pretty much uniform wing venation, where the median vein has a distinctive curved fork that starts well down, leaving a long stalk and forming a large bell-shaped cell. That feature alone should confirm the family, but it's also worth noting the small head and long antennae.

In more general terms, they are described as minute to medium-sized (1-6mm, rarely larger) gnats that are not dissimilar to the Mycetophilidae true fungus gnats, but are typically more compact with a proportionally smaller head that is deeper than it is broad; compound eyes that are pretty much united above the antennal bases by a characteristic eye- bridge of variable width; three ocelli present; antennae with 16 simple segments (14 flagellomeres); wings that are rather broad and rounded at the base; usually clear despite the common name, although in some cases tinged; generally simple and uniform as previously noted, lacking cross-veins apart from the short r-m, Sc weak and free-ending, M and CuA forked, the former as described above with the proportion of the length of the base to the stem aiding keyed identification;  leading edge wing veins well-marked, others rather faint; legs relatively long; tibiae with apical bristles or spurs; abdomen cylindrical, normally strongly tapered posteriorly in females; male terminalia exposed, often broader than the rest of the abdomen. Although subtle variations in the wing venation can be useful in determining the genus, some females, such as Epidapus sp., have either greatly reduced (brachypterous) or no (apterous) wings.

Identification to species is primarily based on males, which have to be meticulously prepared and mounted to enable proper microscopic examination. It is, therefore, accepted that with the exception of Sciara hemerobiodes, which has a bright yellow abdomen, the individual members of this family cannot be identified from photos. Although I believe that all other species are predominantly black or dark brown, I have seen a reference to other Sciara species having some yellow colouration. I know that's the case with Sciara analis, but that species does not occur in Britain. I've also read that it's only females that have the yellow abdomen, but I've not been able to confirm that's correct, although I suspect it is. Sciara hemerobiodes is by far the most frequently recorded member of the family and, although there are six other recognised species in the genus, the NBN Atlas database only lists Sciara humeralis with more than a small handful of records.

Despite the previous comment about identification, it's interesting to look at the available distribution records. I could be wrong, but I would have thought that there are only a few individuals around the country who will have the knowledge to collect, prepare, examine, identify and record specimens, so it's rather surprising to see so many confirmed observations of defined species. Admittedly, quite a lot of these have only a few records, but there are others that are worth noting, such as Schwenckfeldina carbonaria and Phytosciara flavipes, which both have a respectable number that suggests that they must be visually distinguishable unless all of the submissions are from microscopic examination, which I doubt. Similarly, with Leptosciarella trochanterata, Bradysia placida and Trichosia caudata. There are others that I could mention, but I shall hold back on those until I have further information that could help with identification. I understand that the most useful document in that respect is Menzel and Mohrig (2000), but the only copy I've managed to access is in German, which obviously needs translation in part.

In addition to fully confirmed species, I couldn't help noting that Sciara is pretty much the only listed genus where further identification to species has not been possible. With other difficult families where full identification to species is mainly the domain of experts, there are usually numerous records at a higher taxon level, but that's not the case here. I suspect that there would have been more records of this type if subfamilies had been an option, but they're not, which partially explains why only two other genera feature; Bradysia and Epidapus, each with just a small number of entries. Bradysia though, does have twenty or so records at species level but, with 75 in the genus, it does beg another question as to why so many of these species appear to have never been officially recorded.

Obviously, all this examination of records achieves very little, but personally I find it useful when trying to obtain a better understanding of a family.

Updated, Dec.24 (v.4)