Assuming this to be Episyrphus balteatus, I'm intrigued by the pair of red lines on its abdomen. This is the first instance of this that I've seen. Does anyone else have knowledge of this?
(Worthing's Heene Cemetery, 14th June 2022.)
A quick glance at on-line photos shows that the same red lines are regularly seen in both male and female balteatus. So we can deduce that they are not associated with the reproductive organs. Dissection is probably the best way of determining what they are. Malpighian tubules are a possibility. From what I've read, at least some flies have substantial paired tubules, but I've no idea what they actually look like.
Interesting. I assume that we should see signs of these anatomical internals in other hoverflies, at least those with sufficiently pale bands on the abdomen for them to be visible. Would you agree?
I'm not sure how much variation there is in syrphid internal anatomy. I did have a look at on-line images of Syrphus, and couldn't spot any that had the red lines, so either their yellow patches are not as translucent, or they have different internal anatomy. Bearing in mind that there are lots of Episyrphus without the red lines, I think detailed anatomical investigation is the only way to understand what's going on. Could be an interesting project if you have the time and a steady hand.