I found this small fly in July on some thistles. It was so beautiful and looked as if it was made of porcelain. I've been unable to find out what it is by looking in my limited ID books and would be very grateful if someone could identify it.I would also very much like to know the function of the blue projections from its abdomen.
I've cropped the photos tightly to help ID.
I've been searching the web since I posted and think it might be a male Terellia; longicauda or serratulae? I'm still stuck on finding one with the blue projections. They looks like a natural part of its body, but could they be caused by some kind of pathogen?
I'd be grateful for confirmation or denial of these wild guesses
ID is correct. I do not know the name of the sacks but they most definitely play a role in mating / mate 'selection'. Whether it is for physical or chemical signalling I cannot say off-hand.
Many thanks Paul. I didn't know what to call the sacks either, which has made it hard to find search terms. I haven't noticed anything like that on any other flies I've seen so I'm going to keep looking to satisfy my curiosity. It was worth letting thistles grow in the garden just to attract such a lovely creature