Hoverfly for ID - Sphegina Sp.

[MattPrince]

Hello,

Found this hoverfly at Umbellifers in a shaded spot near a stream through an old woodland on Dartmoor (earlier this month - June 2011).

It keyed out (Stubbs) as a Sphegina species - rounded upper outer cross vein and double stripes on the hind tibia, very waisted.

However after that I got into the species key and straight into the incomplete/complete bridge over the rear coxae. I think its got a complete bridge (see photo of rear leg structure below - dark area appears continuous at least in this extreme leg position) but not at all sure of this character.

It was a female, very waisted and with a yellow lower face, rest dark, dark humeri - all in all suggesting clunipes - however the wing venation (to my amateur eye) suggest verrucunda - with respect to the sc point vs r-m cross vein.

Help!

Photos that follow:-
Left - wing veins (both wings slightly tattered, the rounded corner to the upper outer cross vein is just visible, sc appears to be about level with r-m cross vein)
Mid - rear leg structure, insect inverted, head to left, abdomen to right.
Right - moribund insect in silhouette

[Geoff F]

Just a quick thought, Matt, but looking at the abdomen shape, have you checked out Baccha elongata?

I have found this to be a good year for them in S. Devon.

[MattPrince]

Thanks for the suggestion Geoff, but its definately not baccha elongata, I had that on the weekend, it doesn't have the massively inflated rear femurs of my hoverfly (amongst other characters).

[Geoff F]

In that case, Matt, going by the RM cross vein position I suppose it must be S. verecunda. Unless it is S. elegans, which would better fit that long slender abdomen and the humeri are unusually dark.

I'm not familiar with the wing veins of that species though.

[MattPrince]

Hence the post Geoff - I'm well out of my depth, so I suspect I've made a mistake somewhere along the line, but at the moment, apart from it being Sphegina, nothing seems to quite fit with the key.

I did wonder if I'd messed up with the bridge point - a bit of an awkward character for a beginner!

Then I wondered if the sc vs r-m x vein character was subject to the angle of the wings and could be misinterpreted (Though you're agreement suggests that I've read this character correctly)

A verrucunda female should be quite 'dumpy' for a sphegina, but then this is my first one, so I'm not a good judge of that.

I hadn't considered that an anomolous elegans might have dark humerei. I guess melanism / partial melanism should always be considered when dealing with problem hovers.

Thanks for your help anyway, for the moment I'll leave it as sphegina sp.

[Geoff F]

There have been a number of discussions and identifications of Sphegina at the Hoverfly Recording Scheme forum Hoverfly Recording Scheme - Search

And it includes S. sibirica although that appears to be rather localised in Scotland and a few areas of Wales.

That apparent very long abdomen on your specimen is what made me wonder if it could have been mistaken for Baccha elongata. But the 'bottom corner' of the outer cross vein in yours is definitely well rounded compared with the 'sharp' B. elongata.

[Roger Morris]

All

This is definitely a Sphegina

I fear that a certain ID will be tricky but it seems you have a specimen so send it to me.

From the photos, which I fear are not quite adequate, I think there is evidence that the chitinous bridge is present - see my note in the Hoverfly Newsletter - but I'd want to check the specimen.

There is no reason to dismiss S. verecunda - I would not describe it as dumpy but I would say it is generally not as elongate as S. clunipes or S. siberica. I'm pretty certain it is not S. clunipes, going by the wing venation.

S. sibirica has reached the south-west and has been recorded from Dorset.

Roger

[MattPrince]

Hello Roger,

Not quite adequate Not surprising as I was juggling camera, microscope and a little LED light torch - set-up is very Heath Robinson at the moment & I omitted the front face picture as it was even worse than those attached!

Yes I did retain the specimen, though she's pretty butchered - not technically very good at this malarky and I detached one wing for the photo, subsequently lost when I coughed

Regards,
Matt

[MattPrince]

Very fortuitiously some of the countries most experienced dipterists are in Exeter this week, on their annual residential session, and I had the good fortune to accompany some of them around local habitat yesterday and to pop by on the lab sessions and see them sifting through the days catch.

A friendly, down to earth, and helpful bunch, Roger (Morris) even took the time to show me my Sphegy under a microscope and it became immediately obvious where I had gone wrong...

In fact Geoff you were very close - it turns out the insect in question is Sphegina Elegans, my problem was my poor lighting on the microscope set-up and my inexperience leading me to expect bright yellow, rather than washed out, muddy, yellow humeri.

I shall console myself with getting it to the right genus and getting to grips with the chitinous bridge character for the first time (thus ruling out e. sibirica).

[Geoff F]

This thread, plus seeing a Baccha elongata today has reminded me that I have a couple of photos of a 'short abdomened Baccha' form May in my unidentified files.

On a second glance it is definitely Sphegina and the cross vein seems similar to your specimen.

I will try to have a more serious think about it soon (when it rains and I catch up with my ID backlog), but I'm not sure if I have enough details showing.