Stiched up by a Suture!

Last week I swept a small fly from a patch of Ramsons and, despite twice keying it to Tachinidae, it fell down almost at the first hurdle seeing as it lacked an unbroken suture across the thorax. I put it aside for another time, intending come at it with fresh eyes at a later date. 

Today I keyed it again and once more it dropped out at Tachinidae. That's three different keys all telling me the same thing, hmmm... It was a bristly wee thing with an inflated sub-scutellum, which fits Tachinidae perfectly, but with no suture (unbroken or otherwise) whatsoever - which clearly means it isn't one. 

I bit the bullet and keyed it through as if there were a suture. 

The fly in question

Highly inflated sub-scutellum (arrowed) - diagnostic of tachinid flies

Any suture should cross the thorax somewhere between the fly's head and the pin

I used Belshaw to key this fly and seeing as I'm very unfamiliar with the vast majority of tachinids I ran it through the key to genera first. Below is a highly abbreviated version of the route I followed

1) Very large black fly with a yellow head (no) >>2

2) Bright metallic (no) >>4

4) Arista with long hairs (no) >>15

15) Eyes with hairs longer than width of arista (no) >>74

74) Vein R1 with hairs (no) >>82

82) Vein R4+5 with a line of hairs extending beyond half-way to crossvein r-m (yes) >>83

The line of hairs extending over half-way from the node to the cross-vein

83) Parafacial area with a line of bristles not extending beyond middle of antennae 3 (yes) >>84

84) Excavation on tergite 1+2 extending all the way to rear of segment (no) >>86

Excavation only extends about half the distance to end of segment

86) Abdomen 3 and 4 without median discal bristles (yes) >>87

87) Proboscis extremely elongated (yes) >> Siphona

That's one heck of a proboscis!

You'll just have to take my word for it that the full version of my route through Belshaw's Key to Genera was far more complex than the above few lines would imply. I left out lots of detail! 

So, despite not having a suture (and hence not being a tachinid...) it keyed very well to genus Siphona (a tachinid). Online pics showed that I was in the right ball park for size and overall looks, and that they do occur in this part of the world and that they fly in the spring. Excellent! Now to figure out which Siphona it was that I had.

There are ten members of the family known to occur in Britain (or there were in 1993 when the book was published) which didn't seem too daunting. Into the next key...

1) Thorax with 3 or 4 post-sutural dorso-central bristles? You're having a laugh! I looked at my fly and tried to conjure up a suture. I saw a vague depression running from a side margin and extending maybe a fifth of the way across the thorax. The opposite side held no clues whatsoever. Taking this as my 'suture' (the book did state that sutures may fizzle out a bit towards the middle...) I counted three dorso-centrals behind it  >>2

2) Tarsal claws as long as the tarsal segment they are attached to (no) >> 3

Claws are shorter than the length of the tarsal segment they are attached to

3) Abdomen segment 1+2 with a marginal bristle at each side of the segment (yes) >>5

5) Femur of front leg with four postero-ventral bristles (yes) >> Siphona (mesnili) confusa

The leg accidentally came off, but certainly helps with the angle of photography!

These four bristles, amongst the British Siphona, are unique to S.ingerae (tarsal claws as long as the segment) and S.confusa, so that seems a pretty certain ID. I turned to the species account to see what that had to tell me about my fly. 

Buggerall really. 16 records, all from late April and into May, and only discovered new to Britain in 1993 by the author of the book! Hosts unknown. Happily the Tachinid Recording Scheme have a bit more to say about it, though even now there are still only a handful of records (21 on the NBN map).

One feature that I struggled to understand is the shape of sternite 5. Supposedly "the apex of its lobes are abruptly turned in at 90 degrees", a feature that is unique to Siphona confusa. Mine was a male, which is meant to make things easier. Here's my pic of the relevant part, you're looking at the underside of the fly's abdomen. 

And the same image but I've added red lines to show what I've interpreted as being the two possibilities that refer to the "apex of the lobes turned abruptly inwards forming a 90 degree angle" 

If you've stuck with it this far, chances are you're probably properly into your flies, so feel free to tell me what it is that I'm supposed to be checking! 

Anyway, it's a nice fly and one which I've now spent quite a bit of time with. It seems that the flight season is pretty short, so I need the temperatures to raise a bit higher than the two degrees it is at the moment if I want to find any more of these. It was snowing two days back, it didn't settle but was followed by sideways hail. Always a joy when you're approaching mid-May. 

One last pic of the beast, all labelled up and ready to go into the relevant storebox. I can see myself needing to split my Brachycera collection into more boxes before too much longer. Currently I have "Diptera Nematocera", "Diptera Syrphidae" and "Diptera Non-Syrphidae" and the latter is becoming just a tad full. Clearly I shall need to split the Eremoneura, the Acalyptratae and the Calyptratae into their own boxes at some point. 

Not too much space taken up by this one - each label is 12x7mm