Wednesday, 22 May 2019

Turns out that pigs do fly

Yesterday was spent out and about in the woods, swishing my net at anything that looked promising or interesting. By contrast, today I've sat at my desk for several hours, working through the specimens collected, running them through the keys and have slowly racked up a few more additions to my PSL. It was all going quite well, no real issues with any of them - and then I picked up a small hoverfly. Suddenly everything went pear-shaped.

Fairly innocuous looking chap, shouldn't prove too tricky to ID
I didn't recognise the family, but that's not very surprising seeing as I'm pretty poor when it comes to hoverfly ID. I ran it through the family key and went wrong. Ok, so I'll just flick through the pictures shall I? That enlarged hind femur looks pretty darn distinctive...

I stuck with the keys and this time dropped out at Neoascia. Obvious once you know it, I mean just look at those right-angled veins in the wing membrane! Turns out there are only six species of Neoascia in Britain, this was going to be a doddle. Or so I thought. An hour came and went, I still hadn't nailed the bugger to species. 

There are a whole bunch of features to help get you to species in this small genus, loads in fact. But no matter which route I took through the keys, I couldn't get a conclusive ID, one that ticked all the right couplets in the key.

So...let's do this together. Maybe you can see where/if I'm going wrong.

Firstly, two subgenera have been proposed, separated by the presence or absence of a band of chitin  running side to side across the underside of the body, just behind the rear legs. Two species have this complete band, 4 have a broken band with a membrane present along the midline of the abdomen. It's a complete arse to view and all but impossible to photograph - but here's my effort

Paler membrane separating the two dark plates of chitin

These are the two options, pics taken from the key. The black area is the chitinous band, the white the membrane. The pair of curved shapes at the top of the black patches are the hindmost coxa

Continuous band of chitin running side to side just behind the rear legs

The band of chitin doesn't meet in the middle - wide membrane running in between

So the six species of Neoascia in Britain are geniculata, interrupta, meticulosa, obliqua, podagrica and tenur. Two of these, podagrica and tenur (proposed subgenus Neoascia) have a continuous band of chitin, so we can lose those two straight away. The remaining four (proposed subgenus Neoasciella) shouldn't take too much effort. He said.

To the key, at last (abridged version)

1 - upper and lower marginal cross-veins infuscated (darkened) - 2
These veins clear - 5

They were clear (first pic) So I went to 5.

5 - Face projection blunt. Front and mid femora and tibia extensively yellow - [N.annexa]*
Face conical, apex pointed. Front/mid femora often dark, or at least dark shading in the middle - 6
* - not yet known from Britain but included in the key as a likely coloniser in the future

Conical face with a pointed apex, bit of dark shading visible on front/mid femora - go to 6
6 - Third antennal segment scarcely longer than wide - N.geniculata
Third antennal segment more elongate, almost twice as long as wide - 7

Clearly the third antennal segment (the big, oval part) is about twice as long as wide, so on to 7.

7 - Hind femur narrowly yellow at tip - N.meticulosa
Hind femur entirely black at tip - N.tenur

Quite definitely entirely black at tip
So there we have it, Neoascia tenur, I turned to the species account and read "The chitin bridge across the hind coxa is always well-developed" and that it occurs in "marshes, fens, is particularly abundant beside eutrophic lakes on southern heaths...frequently associated with beds of bulrush and common reed". Arse, well that sure ain't right! 

I'm very glad I checked the presence/absence of the chitin bridge, it's not taken into account in the main key, just mentioned beforehand and in the species accounts themselves. So, where had I gone wrong?

Back to the start of the key

1 - darkened cross-veins or clear? At this point I notice some further text warning that the common N.podagrica may have these darker areas faint. I looked at the wing again - yeah! Faint darkening along the cross-veins! I'd missed that the first time around, this must be Neoascia podagrica - I turned to the species account which said, "The infuscated outer cross-veins are normally apparent but could be overlooked in pale specimens. Thus it should be noted that the third antennal segment is elongate (at least twice as long as wide) and the male normally has bands on tergites 2 and 3" - aah, that makes a lot more sense, I had a pale specimen is all. Furthermore, "whilst it can occur at marshes, compared with other Neoascia it is perhaps more typical of hedgerows, wood edges and even gardens". Perfect, mine was in open woodland.

Then I recalled that N.podagrica has that unbroken band of chitin behind the rear coxa. Oh FFS! Arse, what the heck is wrong with me tonight??? HOW do I keep messing up this damn fly?

Back to the start. Again.

1 - veins darkened or clear? Ok, vaguely darkened. More dark than clear. Go to 2.
2 - third antennal segment short or elongate? Elongate, definitely elongate. Go to 3.
3 - Tergite 2 black or with pale marking? It has markings. Go to 4 (new ground...ooh!)
4 - Tergite 2 with transverse yellow band. Tergite 2 little more than twice as long as wide, alula particularly narrow (narrower than longest fringe hairs) - N.podagrica
- Tergite 2 with oblique yellow bars. Tergite 2 three times longer than wide. Alula not as narrow (width wider than longest fringe hairs) - N.obliqua

Alright, so there's a few more options for us to check through. Tergite two - does it exhibit a transverse band or oblique bars?

Oblique bars, no two ways about that!
So that in itself should make it Neoascia obliqua. Let's continue checking the other features though.

Alula particularly narrow or not as narrow? Here's a pic of the fly's alula, which I've highlighted in red dots

That's a very narrow profile for an alula!
But is it wider than the length of fringe hairs? No, it really isn't. Well that's just bloody great, that puts us right back at Neoascia podagrica - which it can't be because of that bloody chitin bridge behind the rear legs. Arse!!!

Oh, and tergite 2 is approximately twice as long as it is wide, which also makes it Neoascia podagrica

In desperation, I whupped its nads out in case there was a determining spine or curve or teeth or anything. I was reaching desperation point!

Neoascia nads. Marvellous.
I was seemingly going around in circles. Maybe the key was wrong (yeah right, what do Stubbs and Falk know about flies...) I needed more literature. Happily, I found some online which threw more spanners in the works. 

I managed to access a paper by Martin Speight entitled Neoascia podagrica in Ireland, with a Key to Distinguish it from Related European Species. Well, that was handy. It readily informed me that the mighty, trusty, weighty British Hoverflies (Stubbs and Falk) wasn't actually giving all the facts regards variation in abdominal markings and the chitin bridge. There are exceptions which display the wrong set of characteristics, leading, the author insists, to commonplace misidentifications! 

Variation in T2 markings - obliqua 1st row, podagrica 2nd row
It also states that N.podagrica occasionally has the chitin bridge interrupted, in which case the membrane width is less than the width of the base of the rear femur. N.obliqua always has the membrane present and it's always far greater than the width of the base of the hind femur. Looking at the image of the chitin bridge in my specimen, I'm undecided - seems to fall somewhere between the two options. 

Likewise the patterning on T2 doesn't quite match the options in the key or any of the variations noted in the online paper. 

Maybe it's new to Britain. To top it all, I knocked it's head clean off whilst manipulating the fly in an attempt to photograph some part or another. I didn't see where the head went. Hopefully I won't need to rely on frons dusting to clinch that new to Britain. 


7 comments:

  1. For what it's worth I'd say the x-veins are clearly infuscated in pic 1. Not that I have an answer. I've already mis-ID'd a Neoascia this year, but Roger re-detted it on iRecord mercifully!

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    1. Yes they are, I agree. It's just that the key shows them to be very obviously darkened, to a far greater extent than exhibited on my fly, hence I thought 'clear' at first glance.

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  2. Changed my mind - I'd say why not obliqua? From this angle the alula is at a different angle from the hairs so it may be more or less the same. Either way I plump for obliqua. I'll check what Van Veen says tonight if I remember. Might grab it at lunchtime

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    1. Alula is definitely narrower than the length of the longest fringe hairs, I have the benefit of seeing it in 3D rather than from a dodgy image ;)

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  3. Key per Van Veen: 1. no bridge -> 5. x-veins brown -> 6. terg4 black -> 7. femur 1 yellow -> obliqua. No mention of alulas, but also "tergite two basally parallel-sided", which seems fair enough

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    1. That's all seems very simple! :D I've convinced myself that it's an aberrant podagrica rather than obliqua. Distribution and habitat seem to fit a lot better. I've still got it, obvs. I'll have another look tonight. Cheers for all this, buddy!

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    2. NO worries. If you put your images on iRecord maybe Roger will shed some light on it

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