Hey Big Head

In case you were wondering, the 2020 Challenge between myself and The Ghost is still ongoing. Neither of us has been furloughed and we've both been busy at work and otherwise. In fact, I've been substantially busier than usual at work ever since the Covid-enforced shutdown of the hotel. But I've still managed to get out and play from time to time.

Yesterday was my ninth consecutive day at work. I deliberately finished up by 3:30pm and hit the woods whilst the sun was shining (today, my one day off, it is raining. Exactly as was forecast...) Happily, I found a few interesting wee things yesterday afternoon including one fly that immediately grabbed my attention as it slowly floated beneath some bracken, legs dangling and wings catching the light very nicely indeed. One swipe of the net and it was captured.

Why Grandma, what big eyes you have! (And you appear to have a stinger too...)

This is one of the Big-headed Flies (Pipunculidae), closely related to the Hoverflies (Syrphidae), in fact these two families compose the superfamily Syrphoidea. I guess the term 'Big-headed Fly' is a bit non-PC these days, but hoverflies do hover and big-headed flies do have rather a big head, so....

Pipunculids have a bit of a reputation as being proper hard bastards to key correctly. A lot of this is due to earlier works not recognising species aggregates, something that is now far better understood, though work does still need to be done in some areas. Taxonomically there have been a good few changes since my copy of the RES Key was published in 1966. This key is still available to buy as a used hardcopy (mine has annotations in the margins) but this online version is far more readily available and is free to download. In this key there are 77 species described as British, a massive jump from Verrall's 1901 work which recognised 32 species. 

Dave Gibbs, possibly Britain's greatest pan-species lister, runs the Pipunculidae Study Group which has distribution maps for each British species and keys to genera. Nowadays there are 95 species of pipunculids on the British list. To quote Dave from his site -

...with the exception of a few very conspicuous species, all these records have been checked by me. Because of the considerable difficulty most people encounter with these flies, I have not accepted records unseen.

Dave is a professional dipterist, hence his maps can be considered overwhelmingly accurate. I currently have a handful of pipunculids on pins, all collected from here on Skye. Once their flight season is over I shall post them off to Dave for confirmation (or re-determination) and get a few more dots added to his atlas. Skye seems relatively devoid of 'pips' judging from the maps, with just 7 species recorded up here. I strongly suspect that this is entirely down to observer bias - all of the Skye records are clustered in the far south-eastern region, an area which was heavily frequented by a renowned entomologist whilst holidaying on Skye.  

Happy that I was looking at a pipunculid, I set about keying it to species with my battered 1966 RES key. First was the key to subfamily, which I'll run through in an abridged fashion.

Couplet 1 - are there ocellar bristles?

You can see the three small ocelli situated on a small bump between the rear edges of the eyes. No bristles visible from above or from the side. So far so good, go to Couplet 3...

Couplet 3 - is the hind margin of the eye deeply excised and do the thorax and scutellum have strong marginal bristles? 

Clearly there is no strong excision in the hind margin of the eyes and, although the scutellum has strong marginal bristles, the thorax does not. Which takes us to couplet 4 (Pipunculinae)

Couplet 4 - Coloured or colourless stigma?

The stigma definitely has a bit of brownish colour to it, which leads us to couplet 6.

Couplet 6 - Tricky one this, though you wouldn't think so from reading the key. Thorax universally (though minutely) hairy on top, or with two lines of (tiny) dorsocentral hairs and bare between? This is where I've gone wrong before when keying pipunculids. The hairs are almost invisible from most angles, but once you know what you're looking for they are fairly easy to see. Not so easy to photograph though! 

This was the best of a bad bunch of images! Hopefully you can see the two rows of pale hairs running back along the top of the thorax, just along the outer edge of each pale band. Helpfully, the microscope's ring light has illuminated the tips of these hairs making them easier to discern. Onwards to couplet 7.

Couplet 7 - Propleural fan of hairs present or absent? This is a bit of a bugger to see and all but impossible to photograph without removing the head, and I didn't want to remove the head. This may be a bit of a leap of faith for you, so just take my word and believe me when I say there's a propleural fan of hairs visible in this next image! 

So what you're looking at here is the back of an eye. Beneath the silver-coloured rim are lots of small downward/inward-pointing, essentially straight hairs that are directed into the cavity behind the huge eye and are facing towards the front of the thorax. These are easy enough to see. But, if you squint really hard, you should be able to make out a bunch of curved hairs coming from the depths of the shadow and curving from right to left. This is the propleural fan. In the image below I've highlighted this fan in coloured ink. A very young schoolkid could do better, I know, but at least it'll give you a head's up of what to look for in the image above. 

All of which has finally led us to a subfamily - Cephalops. Phew!

There are quite a few Cephalops in the 1966 RES Key. Thankfully, this specimen drops out to species very early in the couplets.

Couplet 1 - Vein M1+2 with or without an appendix of varying length?

No messing around there, that's a very definite appendix! Previously, I had a specimen with a tiny appendix along this vein (and on both wings too) which can occasionally happen in aberrant specimens. If I ignored the appendix it keyed through nicely to what I hope is the correct species. If I ran it through the keys as having an appendix - which is what I did first time around - it makes a nonsense and goes nowhere. Anyway, appendix present on this specimen - go to couplet 2.

Couplet 2 - a very long couplet filled with lots of detail which leads to one species, or alternatively lots of detail which leads to another species. Different set of details for males and females. This is a female (it has a wicked-looking ovipositor). Essentially it all comes down to colour of the tergites when viewed from above and the shape of the ovipositor. So...

"Tergites 2 to 4 are light grey, sometimes black or brown banded anteriorly, when this colour may also extend across the disc of the thorax".

"Base of ovipositor fairly deep, with an apical large brown protuberance above which appears rounded from side view"

All of which, miracle of miracles, would seem to lead to a straightforward identity! My fly is a female Cephalops furcatus. Unfortunately that name is no longer in use, my fly has now been redesignated as Cephalosphaera furcata. A quick glance at Dave's distribution map for this species shows it to occur northwards as far as central Scotland, making this is a notable record as it rather extends the currently known distribution. 

In other good news, I whacked the image up on the UK Diptera Facebook group and it was 'Liked' and commented upon by several decent dipterists, which is far better than the "I think you may have gone wrong with this one" type comment that I so often see.