Seen one before in 2007 but didn't have a camera so I scrubbed that record because I didn't at the time confirm the yellow halteres [remains of the diptera hind-wings (they used to have 4 wings but now evolved into only two) which now acts as an organ of balance. There are three species of Didea but only Didea fasciata has yellow/pale halteres, while intermedia and alneti (last recorded in 1989) have a black haltere and neither has been recorded in Mid or South Wales. To identify a Didea you first have to confirm the loop in the wing of the R4+5 vein which is unusual amongst yellow and black hoverflies. Secondly, the oblique abdominal yellow markings gives Didea a very distinct and aggressive appearance. Another useful feature is that Didea has all black hind legs. Now we know it is a Didea we can check the halteres which in the case below can clearly be seen to be yellow/pale making it Didea fasciata.