Opomyzidae

Opomyzid Flies

DIPTERA > BRACHYCERA | Higher Diptera

MUSCOMORPHA > SCHIZOPHORA > Acalypratae > Opomyzoidea > Opomyzidae (16 species)

(subfamilies: none)

A small family of mainly slender-bodied grass flies with relatively narrow wings that have an apical spot and/or darkly marked cross-veins. There are just two genera, Geomyza and Opomyza; the former with ten small species with a body length of 2-4mm, that have moderately shiny black abdomens, and black, reddish or brown thoraxes; the latter with six slightly larger species which may reach 5mm, that are predominantly yellow or greyish-yellow. The general wing shape and venation also helps separate the genera; in Geomyza the wings are comparatively reduced compared with Opomyza and are without an anal vein, whereas in Opomyza the anal vein is well-developed. Similarly with the scutellum, where in Geomyza the disc is bare and the two bristles towards the base are much shorter than the two apical bristles, compared with Opomyza where it is hairy and with all four bristles of roughly equal length. And finally, the arista, which in Geomyza is relatively long bearing a number of longish hairs, compared with Opomyza where they are a little shorter with very short fine hairs.

The above combination of features makes it pretty straightforward to determine the genus, but it can be a bit more tricky from there to be sure of the species. The most important distinguishing feature for most species is the wing markings, the pattern and extent of the markings at the wing tips and, if present, the darkened cross-veins. Each species has its own unique pattern, although some are very similar and can sometimes be difficult to distinguish with certainty.

Three species are particularly common and well recorded which, in order, are Opomyza germinationis, Opomyza florum and Geomyza tripunctata. Next, with roughly half the number of confirmed sightings of Geomyza tripunctata, is Opomyza petrei, a species I've yet to see, and then with about a third of that number is Geomyza balachowskyi. The ranking of these top five species is pretty much the same locally, but with virtually no records for any of the other species.

Geomyza balachowskyi - this is a tricky one as there are two species that key out together. The distinguishing features are one presutural dorsocentral bristle and three postsutural, and cross-vein m-cu narrowly clouded - the large dark marking at the tip of the wings is immaterial. This description leads to Geomyza balachowski and the seemingly rare Geomyza hackmani, which has only been officially recorded a few times and has never been observed locally. That said, it has to be a consideration, which is why they are sometimes combined as Geomyza combinata agg. when the actual species can't be determined. This would always be the case with females as they can't be separated. The males can be identified by close examination of the genitalia and, I guess, that would be necessary for official recording purposes. However, Mike Hackston's key confirms that they can also be distinguished by the colour of the postnotum, which in Geomyza balachowskyi is reddish-brown and similar in colour to the rest of the thorax, whereas in Geomyza hackmani it is dark brown, contrasting with the rest of the thorax. So, on the basis that the individual featured below is indeed a male, as I believe it is from the shape of the abdomen, it should be Geomyza balachowskyi sensu stricto.

Geomyza tripunctata - whereas in the previous species, cross-vein m-cu is narrowly clouded, with Geomyza tripiculata both cross-veins r-m and m-cu are heavily clouded, which together with the large dark marking at the tip of the wings that extends into cell R3, give a distinctive pattern.

Opomyza florum - initially distinguishable from the following species by the wings, which have a relatively unmarked costal margin apart from the dark patch at the wing tip; there are only four distinct dark marks consisting of the apical patch, blotches across both cross-veins and a small spot midway along the last section of vein m. There may also be vague clouding around the last section of R4+5 and/or the tip of vein m. The general colouration of this species with the sides of the abdomen being entirely orange-brown to yellow, coupled with the described wing markings provide easy recognition. However, if the wings have additional rounded spots along the last section of vein R4+5, coupled with a dark central stripe on the thorax, the much scarcer Opomyza lineatopunctata should be considered. Or, if there are additional wing spots and the thorax doesn’t have a dark stripe, there's the possibility of Opomyza punctata.

Opomyza germinationis - whereas the wings of the previous species have a relatively clear costal margin, in both Opomyza germinationis and the very similar Opomyza petrei there is a distinct dark marking that runs along the costa beyond vein R1. From a dorsal view alone, the two species can't be separated with confidence as you need to have a side view of the abdomen to check the colour - dark, sometimes showing dull paler patches at the sides of each tergite as in Opomyza germinationis, or more of a dingy yellow with dark central and lateral stripes, the central stripe never spreading sideways along the posterior margins as in Opomyza petrei. They can also be separated by close scrutiny of any setulae on the propleuron (proepisternum), but you're unlikely to capture that level of detail when photographing a live individual. Opomyza petrei is probably under-recorded due to its similarity with Opomyza germinationis, but from the photos I've seen on the internet the colour and banding of the abdomen are obviously different so they should be separable. Hopefully, I'll be able to feature a photo of Opommyza petrei at some point in the future for comparison purposes, but for now I shall have to rely on this description.