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25 March 2021 I collected some Potamogeton polygonifolius with mined leaves. Three ephydrids (I think) emerged in May, two females and a male. Here is the male.

The site is a shallow weedy pond at Moelyci Farm, Tregarth, North Wales.

Martin Drake's unpublished key mentions two species associated with Potamogeton: Hydrellia flavicornis and H. fulviceps. The genitalia of this fly are somewhat like those of H. flavicornis. But the fly doesn't key to Hydrellia- the lunule doesn't extend up to near the ocelli. And H. flavicornis is supposed to have yellow antennae and no strong presutural dorso-central.

So, does anyone recognise it, please?

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This shot is in alcohol so colour darker.

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Dry this time.

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Dry.

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In alcohol. Genitalia stowed.

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Genitalia prised out, from between a couple of shoebrush-like structures.

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Last photo. I thought I had pictures of the leaf mines but I don't. They will have to follow next year.

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John - check the mid femora for small pv bristles. If they're absent, I would say this is Hydrellia maura (though the genitalia are hardly like flavicornis). Don't worry too much about the low lunule - normally it would appear to be much higher, but it does vary a bit. H. maura is known to mine Potamogeton. If you are still concerned, I'd be happy to have a look at the male for you.

Sorry - wrote this before seeing the genitalia pics. Will look at them closely now.

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Thank you very much. I'll get back in a couple of days.

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The mid-femora do have postero-ventral bristles, three in the middle and four at the base. See photo. So I guess that rules out maura. Also the genitalia don't match maura in Martin Drake's key. Or should I say your key? He credits you with the keys and he added comments.

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Here is another photo of the ?aedeagus. That is what I'd call it in a beetle. Not sure if it is the same term in a fly. Anyway, it is quite a good fit for fuscus and the fly will key to fuscus without too much forcing. Is that a species associated with Potamogeton?

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Thanks for that John - I agree this does look like fusca. One of the characters for this species is the swollen mid tibia, which I think I can see in one of your pics.
The species is known to mine Potamogeton.

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Thanks for the agreement. Yes the mid-tibia are slightly swollen in the middle but not so you would notice if you hadn't been told to look. I wasn't sure how much swelling to expect.