First Lifer of the Year

Another glorious day demanded my attention, so I took the sieve and tray up into Uig Wood in search of spiders in the leaflitter and tussocks. I had high hopes of a good few specimens running around in the bottom of the tray and I wondered how many species I'd identify from the hoped for haul. 

I'm ready! 

Just look at that lovely clean tray, it hasn't seen any action for months! That soon changed once I started adding handfuls of Rhytidiadelphus and leaf litter into the sieve and giving the whole lot a decent bout of shaking.

The arachnologists' moss of choice, or so I've heard

I took a quick pic of the year's first sievings for posterity. I shouldn't have bothered, there was bugger all in there!

Lifeless and barren

I had more luck shaking tussocks over the sieve (forgot to bring up a knife) than from sieving the leaf litter and mosses, resulting in a few fungus gnats from the family Mycetophilidae, plus a couple of tiny chironomids which I studiously ignored. There were a handful of linyphilid spiders, only one of which was an adult. It keyed through to one of about nine species and the palps matched Tenuiphantes zimmermanni perfectly. Damnit, I keep finding T.zimmermanni, a bit of variation would be nice.  

I also sieved a couple of sphaerocerid flies, the so-called Lesser Dungflies. Well, these were each around 2mm long, so I certainly won't argue with the 'lesser' part of their name! I tried keying one, but kept going wrong somewhere. I've put it into the Backlog Box for now, I shall try it again soon. The second fly I had more success with.

As I mentioned, at 2mm body length this was not a large fly!

There was one thing in particular about this fly which struck me as a little unusual. Can you see it?

The thing that struck me was the short wing length. Surely it couldn't actually fly with those tiny things?? I have a copy of the RES Key to Sphaeroceridae kindly passed to me by Murdo of the HBRG in the hope that I can put it to good use. Well I did today! Looking through the introductory chapters, I discovered that several species of sphaerocerid exhibit some degree of brachyptery so I was heartened to learn that I probably didn't just have a runt mutant on my hands. 

There are somewhat over a hundred species of Sphaeroceridae and, as always, where unfamiliar with a group it's a good idea to key the specimen to the correct family for starters. With flies this very often requires a check of the wing veins and their precise distribution across the membrane.

Reduced number of veins, no anal vein, the basal and discal cells are merged into one and vein M1+2 fails to reach the wing edge. That in itself is enough to place this fly in the family Limosininae, but we can add an extra confirmatory character simply by checking the hind tibia, which lacks a strong ventral seta. Squint your eyes and you should be able to see the lack of such a bristle in the following pic. 

I know it's rubbish - just remember the entire fly is only 2mm long!

So we're now in the key to Limosininae, which is by far the largest family in the British Sphaeroceridae with over 90 species. Gulp. A massively abbreviated version of the key to species now follows:

1 - Scutellum with discal setae in addition to the marginal bristles >>2

   - Scutellum lacking discal setae, it just has the marginal bristles >>10

The scutellum is the padded area at the rear of the thorax. I've marked it with a red dot in the image above, plus I've marked the bases of the four marginal seta in orange. You can see that the disc of the scutellum is bare, no hairs at all. 

10 - Scutellum with more than 2 pairs of marginal setae >>11

    - Scutellum with just the 2 pairs of marginal setae >>22 

Well we just covered that in the image above, there is a basal pair of setae and an apical pair, no others.

22 - Mid first tarsomere with long ventral seta >>23

     - Mid first tarsomere lacking a long ventral seta >>26

The couplet is asking us to look at the first segment of the tarsus on the middle leg. Nowadays this is usually called the basitarsus, tarsomere is the same thing though. I've spared you what would have undoubtedly have been a truly awful image, because I've just realised that I didn't take a photo of that feature (or lack of) at all! Don't worry though, the next image is a corker in terms of being completely shite. Anyway, there was no long seta on the ventral side of the first tarsomere.

26 - Mid tibia with a pre-apical ventral seta (there is more to this part of the couplet) >> P.fenestralis

    - Mid tibia without a pre-apical ventral seta >>27

Right, so here's your shocker of a pic. I've highlighted the relevant bristle for you. 

I mean c'mon...who else would go to this much trouble for you?

Well, that seems to drop us out at Pteremis fenestralis, better just check the rest of the first part of that couplet makes sense when applied to our fly. 

The entirety of it reads as follows: Mid tibia with a pre-apical ventral seta. Wings either reduced or not. Vein R4+5 curved forwards and distinctly overpassed by costa. Mid tibia with pair of dorsal setae in basal third. Plus some nonsense about the genitalia which I hadn't hooked out or placed beneath a microscope slide. 

Breaking it down, we know the pre-apical ventral seta is present on the mid tibia (the big red arrows are pointing at it!) We know the wings are reduced, I doubt it could even take off never mind actually fly. Vein R4+5 is curved forwards and the costa does extend beyond its tip. Pic below to show just that with vein R4+5 in red dots and the costa in golden dots.

This is Pteremis fenestralis, a new fly for me and only the second sphaerocerid I've managed to key through. The blurb reads that this is a common fly across Britain with females being commoner than males. Usually found amongst grass tufts and in mouse runs (no disputing that!) and that all populations in northern Britain comprise the brachypterous form. Sweet, I was chuffed with that. 

What I wasn't quite so chuffed about was the comment that Ali left after seeing this unaltered image

"...I was gonna say Tomocerus because I can never remember the new name. Is that Pteremis fenestralis, perhaps?" Bugger me, but he's good is that chap!

I found a nice article that goes on to describe the varying degrees of brachyptery in some detail. Here's a pic which shows how wing venation slowly reduces with decreasing wing size, all very interesting stuff.