I have quite a large number of all black, flying insects. They are on the fringes of ancient woodland among white poplar saplings bordering a field. There is a near dry ditch, but it seems to be the poplar that are attracting them. The insects are all airborn, appear to be entirely black, perhaps 25mm long and to have a black appendage dangling diagonally beneath the body by perhaps 10mm. They are moving too fast for close study or (my) photography and appear not to be settling other than fleetingly.
Not been able to find them in a seach.

Normally I'd say we can't attempt an insect ID without a photo and some knowledge of where you are (country, plus area). However, assuming you are in the UK then I would put money on this being Bibio marci, commonly called St Mark's fly due to its appearance around April 25th every year. What appears to be a long dangling "appendage"  when they are flying is actually their legs.

More about them plus photos to see if I am correct here:

http://www.nhm.ac.uk/nature-online/spec … index.html