So Rare it was Considered Extinct

I have a bit of a mammoth backlog of posts to compile, but this one stands out as what I'd consider a decent stroke of luck and well worth blogging about. 

I managed to sneak away for a few days last week and, amongst other places, dropped in to Abernethy in Speyside. The amount of campervans and cars slung into every roadside layby, car park and flat bit of verge was horrifying to see. The two large dumpster-style bins with piles of plastic bags heaped up inside and all around them was also pretty yucky to witness. Not as yucky as the "mountains of human excrement" supposedly everywhere you step in the Glenmore beach area, but I didn't slow down as I hurried on by that particular scene. I was off to explore Abernethy!

My cunning plan was thwarted, the tracks through the forest were 'closed to public access'. Well, that was a bit crap. Undeterred I wandered up a private road (the Scottish rights to roam laws are particularly generous) which terminated at a cottage to my right, woodland to my left and a shallow river ahead of me. I spied a bit of boardwalk and was suddenly surrounded by a flock of crossbills which I'm convinced are Parrot Crossbills. At least 40 of them! Just listen to this relentless bi-chuu begging call, have you ever heard anything like that from a Common Crossbill? And just listen to the tone of the calls from the flock, these are Parrots all the way for me! I know it's a shoddy bit of video, but look at the big heads and thick beaks on these birds. Sadly I'd left my binoculars in the car else I'd have digi-binned some images too.

Rather a large crop from the vid above. Parrot Crossbill. Utter beast!

But this is a beetle and fly blog, I hear you cry. Yeah, I'm getting there, I'm just setting the scene is all...

A while later I began ambling back down the road towards my parked car, where I caught a huge Tabanus sudeticus bouncing off the bonnet and windscreen. I briefly worried that I'd need to call Autoglass to replace the screen, but luckily it grew tired of vandalising my car and slowly veered off across the tree tops and was gone. I spied an open patch of heathy bog through the trees and went off in search of White-faced Darter (no luck, I may in fact be destined to never see one). I pushed through a strip of Bracken and spied the rather smart tachinid Dexiosoma caninum sat on a tall frond. One swipe of the net and it was secured, along with my first ever Leptogaster guttiventris which is an incredibly thin species of robberfly and a fantastic bycatch - I hadn't even seen it when I swung the net!

Fast forward six days to this morning where I've been keying through yet more flies. I reached the Dexiosoma and, although I was pretty happy with the ID, I sat down and ran it through the keys. 

The fly is 9mm long and its wings are 7mm long each

I used Belshaw to ID this fly. An abridged version of the key to genera follows

1) Very large black fly with a yellow head. No >> 2

2) Body bright metallic colour, usually green or blue/violet etc. No >>4

4) Arista with hairs longer than its maximum width. Yes >>5

The long hairs of the arista are easily visible in this image

5) Proboscis extremely elongated. No >> 6

6) Face with a ridge between the antennae. Yes >>7

Very definitely has a sharp ridge between the antennae

7) Excavation on tergite 1+2 extending to rear margin or only two thirds towards the rear margin?

The 'excavation' is a small hollow on the fly's back. I have no idea why it's there or what purpose it performs, but it is used as a feature in the key. Here's a pic from the key showing the excavation reaching the rear edge of the tergite (16) and only reaching two thirds of the way (17)

And this is a pic of my fly. The excavation is the big pitchy-black coloured area

So that extends to the rear edge of the tergite. >> DEXIA

So not Dexiosoma at all, but the very similar Dexia. There are only two species of Dexia on the British list, Dexia vacua and Dexia rustica. The key to species couldn't have been much easier to navigate.

Abdomen 4 with a black stripe along its posterior margin. 6-9mm in length. Thorax with 2 katepisternal bristles >> vacua

Abdomen 4 without such markings. 7-12mm in length. Thorax with 3 katepisternal bristles >> rustica

Let's have a look then....

Big, thick black band at posterior edge of abdomen 4 - check! 

The katepisternal bristles are a bit tricky to decipher from a 2D image, though they're easy enough to see whilst slowly turning the fly back and forth beneath a microscope. The katepisternum itself is the large body 'panel' situated above the middle and hind legs, hence the katepisternal bristles are the  bristles to be found on this area of the body. Weird terminology, I know. And they keep changing it too! So much jargon to learn.

Anyway, I've added three red dots where the katepisternal bristles sit. You'll notice that one dot has no big thick bristle.

All of which means this fly is Dexia vacua. Cool, an unexpected lifer. I better have a quick Google and find out a bit about it, y'know just in case it's a goodun....

One of the first sites I looked at was an ID article written by Chris Raper, an expert dipterist with a penchant for tachinids. He's written papers and keys on tachinids, not just British species but on a worldwide basis. He runs the Tachinid Recording Scheme and is one of Britain's most respected authorities on these bristly beasties. "It's a bristle thing" is his tagline, in fact. These are the opening lines from his ID article regards telling Dexiosoma and Dexia apart:

I always find that one of the hardest identifications to make from photos is the split between Dexiosoma caninum and the much rarer Dexia vacua. Dexia vacua is so rare that many experts thought that it was extinct in Western Europe...

Say what?!?!?!?

I looked at my fly, dead on a pin. Killed with mine own hands. Oh shite.

Still dead. And getting rarer...

Belshaw gives the flight period as late July and August. He also suggests it's relatively widely spread across mainland Britain. Mind you, it was written the best part of thirty years ago and things don't stay static out there as years roll by. In fact, further Googling took me to various sites where I learned that several European countries have recently declared Dexia vacua as no longer occuring in that country. Indeed, it now seems that Britain is the stronghold of the species in western Europe and it's pretty flippin' rare here too.  

With a small degree of trepidation I popped a few pics up on the UK Diptera Facebook page and tagged Chris Raper in the post. Within minutes he responded - 

Wow, very nice indeed - a very rare fly. They're almost extinct in Western Europe

Confirmation from The Master, phew I hadn't ballsed it up and I hadn't gotten in trouble either (ha!) For future reference, if I net a Dexiosoma all I need to do is check for a ridge between the plumose arista and a black band at the rear edge of the 4th abdominal tergite. If it has those it's another Dexia, and no need to kill it, I'll just take lots of really good pics instead!