An Early Season Muscid

It was nice and sunny today and, out of the cool wind, it was modestly warm too. I quickly headed into Uig Wood in search of flies sunning themselves on tree trunks and within ten minutes I'd potted up four sizeable muscids, all of which looked to be the same species. A brief bout of sweeping low vegetation added several very small flies to my tally, all of which I brought indoors to pin and identify. But first the four muscids.

They were all from the genus Phaonia and I used this key to take them to species level. There are almost 50 species of Phaonia in Britain, though not all of them occur this far north, whilst others only fly later in the year. This particular key is very user-friendly and is well illustrated throughout, indeed it was almost a joy to work through.

The key is in two parts, the first for males and the second for female flies. I had three males and one female and they all keyed through very nicely. 

Male Phaonia have eyes which almost meet in the middle of the head, females have eyes that are widely separated from each other, as shown here

The underside of the abdomen differs between the sexes too. Males exhibit a horseshoe-shaped feature which houses the genitalia and the end of the abdomen is broadly rounded. In the females this feature is narrower and far more elongated and the abdomen itself has a more pointed profile

Right, so now we are ready to identify our first fly. I chose a male and began keying. 

Couplet 1 - is the underside of the scutellum bare or is there a patch of fine hairs?

No hairs under there - it's completely bare

Couplet 2 regards the length of the hairs on the arista, are they short, very short, or longer than the diameter of the widest part of the arista? This was easy to see but a complete pig to photograph! 

You're looking at the antenna which has a thick "bristle" projecting from it. This "bristle" is the arista and you'll see small hairs radiating from it, almost creating the impression of a feather. It's the length of these small hairs that we need to be checking. Clearly they are wider than the diameter of the arista at its widest part, which takes us to...

Couplet 3 - which asks whether the radio-cubital node has bristles or not. I forgot to take a photo of this, but it was bare. You'll just have to take my word for that. That leads us on to...

Couplet 5 -  now we need to look at certain bristles on the top of the fly's thorax. Bristles and hairs cover much of a fly's body and each 'tract' of bristles has a term unique to it. Sometimes even an individual bristle has its own name, as does each part of the body where they occur. It's quite boggling, but entirely necessary in order to determine that you are looking at the correct set of bristles. Anyway...

Does the area in front of the suture have 1+ pairs of acrostichals, or none? I've added coloured dots to the image below. In this instance the acrostichals are coloured BLUE. The suture is the thin line that runs across the width of the fly's back, BLUE dots on one side of it and RED dots on the other.

Clearly there are acrostichals in the area before the suture, which leads us to...

Couplet 31 (that's a pleasing jump, we're really getting somewhere now!) - there are a few parts to this couplet leading to just two choices, but essentially it breaks down to this:  

1) is the underside of the abdomen entirely translucent yellowish or is it black?  

2) Are there 4 dorsocentrals or not?

3) Are the sides of the scutellum hairy or bare beneath the strong bristles?

1) We can see from the images above that the abdomen is yellowish not black - easy.

2) The dorsocentrals are a series of bristles that form a line running along the top of the thorax, approximately halfway between the midline and the sides. In the image above I've marked them with RED dots. Clearly there are four of them.

3) Check the images above and below. The scutellum is the rounded pad between the wing bases. It's attached to the fly's back but has its own set of named bristles and hairs. In this instance we're interested in the strong marginal bristles, each of which I've marked with a RED dot in the image below

No hairs below those bristles - the sides are completely bare

So we have a yellow underside to the abdomen, 4 dorsocentrals and bare sides to the scutellum, which is the option that takes us to....

Couplet 32 - 1 pair of presutural acrostichals and an entirely reddish-yellow scutellum OR 2-3 pairs of presutural acrostichals and the scutellum broadly darkened at the base. The presutural acrostichals are marked with BLUE dots (see two images above), the scutellum is the image directly above. The second option fits our fly which takes us onwards to....

Couplet 33 - Another multi-part couplet leading to just two options 

1) arista scarcely as wide as antennal segment or distinctly wider? We've already looked at the arista back at Couplet 2 and you can see that, including the fine outstanding hairs, it is not quite as wide as the antennal segment it arises from. Definitely not "distinctly wider", at least.  

2) Middle tibia with 2 or with 3 posterior bristles? All flies have bristles on their legs and, being flies, all of these bristles have different names and positions. The posterior ones are obviously the ones on the back of the leg facing the direction behind the fly.

Two posterior bristles on the middle tibia

The third and final part of this couplet is a bit confusing and it took me a minute to figure out exactly what it was that the key was asking me to look at. But it's very obvious once the penny drops! 

3) Seen from behind, the median pair of presutural dark thoracic stripes, level with the first dorsocentral, is quite two thirds of the width of the grey dusted stripe separating them, OR not more than one third the width of the grey dusted stripe separating them. Clear???

Go back up the screen three images, to the one where I've added the coloured dots. This time look at the ORANGE dots. These mark the first dorsocentral bristle (edit - I've just noticed that the upper orange dot is at the wrong bristle - oops!) Between these orange dots is a series of black and grey stripes. The couplet wants to know if the middle two black stripes are two thirds the width of the central grey stripe or less than one third the width of the grey stripe. I reckon they are definitely over half the width, thus I favour the two thirds option over the barely one third option. 

All of which...wait for it...drops us out at a species: "Phaonia subventa - Very common and generally distributed; often emerges during the winter in dwellings". Very common it may well be, but it's the first time I've identified it! The other two males ran through the exact same route and dropped out at Phaonia subventa. The female fly too, with all couplets being identical to those in the male key apart from the last one where the black stripes are narrower, half the width rather than two thirds the width. It fitted and it too is Phaonia subventa.  

What a beauty!