Hairy eyes and yellow knees: the Broad Centurion

Lately, aside from the wildlife-friendly gardening and farming (more of that soon), I've been taking an interest in diagnostic morphology - using the fine detail of invertebrate structures to determine why they are what they are. This isn't needed for some species if they are distinctive in some way at a macro level, but for a lot of species, a close-up is needed for identification, often including dissection. Personally I find that the best way to learn how to do this is to have ready-identified specimens so you can work through a key with the beastie under the microscope and see if you get to the known right answer. If not, work bask to see where you went wrong and try again. Once you've conquered this, it's time to try specimens you're not sure about.

Before I start with today's species, one minor caveat - the images here, although I'm pleased with them as they do the job they are required to, are not necessarily what might be considered professional quality. However, my aim is to show what can be done by an amateur, or (like me) freelance professional at home, without the latest laboratory/museum set-up. All my work here is done with a good quality, but old, binocular microscope (acquired for free during a university clear-out from a lab tech who didn't want it to go into a skip), a new but inexpensive high-power monocular microscope, and a point-and-shoot compact digital camera.

So, enough background - today's beastie is the common Broad Centurion (Chloromyia formosa), a soldierfly (family Stratiomyidae) known from umbellifers in a range of habitats throughout lowland Britain, and found from May to August, and occasionally September. This one was found dead in our back garden and although we have no umbellifers there are plenty nearby.

Dorsal view of C. formosa showing hairy eyes, green pronotum & scutellum, and bronze abdomen, colours seen to varying extents in many soldierflies.

Ventral view of C. formosa showing the darker underside (gold tufts on the abdomen) and dark legs with yellow 'knees' and tarsi.

So, the first stage is to identify the family the fly is in. It is about 9mm long (excluding wings/appendages) and superficially not unlike many hoverflies (family Syrphidae). The key here is to look at the wing.

Wing with the front edge at the top. The discal cell (small and shaped as a rough pentagon) is near the centre of the wing. There are several gently curved veins pointing to the rear edge and these become quite faint. The two long veins nearest the body converge and join just before the rear edge of the wing and hence enclose the 'anal cell'. In hoverflies there would be two cross-veins running parallel to the rear edge of the wing.

The haltere (the upper 'drumstick') is a modified wing found near the wing in all true flies (which is why they have only one pair of wings). The hairy paddle-shaped structure below is a flap near the base of the wing.

There are other clues on the head and legs.

The head showing large eyes which meet on top, indicating that this is a male. The antennae are short and clearly show the banded 'flagellum' and hair-like 'arista' (hence the front of the head is at the top of the photo). On top of the head (just below the centre of the photo), the shiny ocelli (simple eyes) can be seen. The hairiness of the eyes is also clearly visible.

The leg (femur at the top, tibia to the side) showing the yellow 'knee'. The tibia has small yellowish hairs but no large bristles.

This antennal structure is typical of the soldierflies - also, the legs with hairy (but not stoutly bristly) tibiae, along with the wing features above means that it is certainly in the family Stratiomyidae. To work out the genus and species, we need to look again at the dorsal view.

The scutellum, a rounded flap just behind the pronotum has long pale hairs and clear punctures, but the key feature is that the rear edge (towards the top) has no protruding spines. The presence (and then number) of spines would indicate a different genus of soldierfly. The slightly out-of-focus second green structure is the post-scutellum which lies just under and behind the scutellum itself.

The abdomen detached in dorsal view - note the bronze colour (bluish and wider in females) and short stocky shape.

This combination of features - overall length, scutellum without spines, hairy eyes, and stocky abdomen bring us to the species-level identification, Chloromyia formosa. As far as identification goes, in Britain at least, this is all that is required - however, it can be useful (or simply interesting) to investigate the organism more thoroughly, so here are a few more images of different structures.

The underside of the head showing the groove where the mouthparts lie. These are short which is why they feed on umbellifers which have small flowers and easy available nectar.

A close-up of the abdomen showing punctures and hairs.

The tip of the abdomen showing the hair-fringed reproductive opening.

I hope that was a worthwhile overview of soldierfly morphology (for one species in any case) - more to come and the next instalment is likely to be either a moth or a ground beetle.

Reference

Stubbs, A. & Drake, M. (2001). British Soldierflies and Their Allies. BENHS, Reading.
An excellent book and essential for the study of soldierflies in Britain - it also includes the Acroceridae (hunchback flies), Asilidae (robber flies), Athericidae (water snipeflies), Bombylidae (bee flies), Rhagionidae (snipe flies), Scenopinidae (window flies), Tabanidae (horse flies), Therevidae (stiletto flies), Xylomyidae (wood soldierflies) and Xylophagidae (awl flies).