Categorise those tiny flies

After a bit of a break looking at bees and ladybirds among other things, it's time to return to the topic of taxonomic morphology - how the careful observation of structure allows the identification of species, even if they initially appear difficult or are in unfamiliar groups. The true flies (Diptera) are one group that can be tricky as there are many whole families that are unfamiliar to non-specialists and consist of small unpatterned species. Having collected one such specimen recently, I thought I'd use it as an example of how something that initially looks difficult might not be - if you have the right literature and observe carefully.

Working through the useful (if imperfect - it's not an easy key to write) Unwin (1981), the fly below, which is largely yellow-brown, bristly and about 3mm long (excluding legs & wings) turns out to be a member of the Lonchopteridae. The antennae consist of short broad segments (the 3rd one almost spherical) with a long terminal bristle (arista), and the wings are somewhat elongate and pointed, the veins running largely longitudinally and also being more or less equally thick and evenly separated.

Specimen of Lonchoptera

Now, the family has been determined, it's time to move onto the species-level key of Smith (1969) - there are only a few species of this family in Britain and they all belong to the genus Lonchoptera. Although it's not especially clear here, the middle tibia lacks a 'distal anteroventral' bristle (i.e. a bristle near the base of the tibia, on the front of the underside). Moving on, it is clear that the scutellum (the semicircular structure behind the thorax) is not an intense black colour.

Thorax and scutellum of Lonchoptera. The thorax has two bristles (the bases shown by red arrows) while the wings only have cross-veins near the base (e.g. as inidicated by the black arrow)

From this point, the identification is straightforward - the antennae are dark, indicating this this is L. lutea, a common species across Britain which flies between April and October.

Head of Lonchoptera lutea showing dark antennae (the near-spherical 3rd segment bearing a long arista) and the triangle of orange simple eyes (ocelli) on the top of the head.

So, we have a species-level identification of a small fly - though common it is likely to be missed by many non-specialist recorders and is a good example of a small and initially unremarkable-looking species that can be confidently identified without dissection.


References

Smith, K.G.V. (1969). Diptera: Lonchopteridae. Handbooks for the Identification of British Insects 10(2ai): 1-9.
Unwin, D.M. (1981). A key to the families of British Diptera. Field Studies 5: 513-553. Available for download here.