Soldiering on with identification

I was down on our local community farm the other day and took the opportunity to have a look at the large pond which is used for both wildlife and irrigation. Aside from some familiar dragonflies and water-bugs, I noticed something a little odder twisting and turning in the water. Around 50mm long and spindle-shaped, it was certainly a larva of some sort, but what type?

Mystery larva, around 50mm long

Firstly, it seemed clearly aquatic - though not an efficient swimmer like, say, water beetle larvae, it was making slow progress and was not acting like a terrestrial larva that had fallen in. It's overall form is that of a fly (Diptera) larva, but a closer look is needed to narrow it down further. Being aquatic, dorso-ventrally flattened, large, leathery and having an elongate, blunt-ended last abdominal segment (bottom right here, indicated by a red bar), it is relatively straightforward to determine that it is a soldier-fly (Stratiomyidae) of the genus Stratiomys. which feeds on microscopic organisms (Smith 1989). However, identifying it to species is not so easy.

In the next photo, the ring or tuft of specialised floating hairs is just visible (the thin white structure indicated by a black arrow), and is used to keep the last abdominal segment with its spiracle (breathing hole) close to the surface. Indeed, as the larva moved, the tuft was always at or near the surface. Some of the abdominal segments have small pale triangular tooth-like projections at the front corners (indicated by white arrows), while the head (red arrow) bears yellowish bristles on the underside. The lack of ventral hooks beneath the hind edges of segments 9 & 10 separate it from Odontomyia ornata even though the pale side stripes make it appear superficially very similar.

Stratiomys sp. larva - oblique dorsal view.

Stratiomys sp. larva - lateral view.

The abdominal features are important for identifying different larval Stratiomys, but using the key in Stubbs & Drake (2001) presents a problem as the authors note that there are no known features which are reliable for British specimens. The abdominal projections indicate either S. longicornis or S. singularior but both of these are associated with brackish waters and this is a freshwater pond. However, whereas S. longicornis specialises in saline conditions, S. singularior is occasionally known from waters which are not saline, but which have some similar water quality characteristics such as brick-pits. This is not a brick-pit, but it does have edges made largely of sandy gravels, some of which is spoil from elsewhere, so the water may be suitable for this species. Also, the large size of the larva is another feature in its favour (it can be up to 65mm long). So, although the habitat suggests S. potamida or the rare S. chamaeleon, the abdominal projections discount these and hence I have tentatively identified this as S. singularior, albeit in a slightly unusual location.

References

Smith, K.G.V. (1989). An introduction to the immature stages of British flies. Diptera larvae with notes on eggs, puparia and pupae. Handbooks for the Identification of British Insects 10(14): 1-280.
Stubbs, A. & Drake, M. (2001). British Soldierflies and Their Allies. BENHS, Hurst.