Adding to the previous post, I do have an old Horse Chestnut tree which has a sap run.
(Skipping Eumerus until I sort my second one out.)
Helophilus pendulus, males and females.
Male, earliest record this year 11th May, latest 24th October 2009. Males have a gap between the eyes but it is narrower than females, and the top section has more or less parallel edges.
Two other features to separate males from females are:
1) Look at the last abdominal tail section, females have two small stripes where males have a rounded tail end without stripes.
2) On males I have noticed the 3rd abdominal section has a smaller black space between the yellow, on females the black extends along the top edge of the yellow as a general rule but there may be variations.
This one should be quite easy but seems to present doubts. The only other Helophilus which is not rare or near non-existant here are H. trivittatus and H. hybridus. Be aware also of Parahelophilus.
I found a Helophilus key (from the French site with the Eristalis key) which should help.
Google Translate
H. trivittatus is much different on the abdominal pattern. The difference to look for between H. pendulus and H. hybridus is on the hind legs with H. pendulus having the apex 1/3 approx. of the femora and top part of the tibia by at least half or more yellow. H. hybridus has only the very end of the femora and approx. 1/3 of top part of tibia yellow.
These can be variable in colour. I have also noticed males don't look to have as much yellow (or red-yellow) on the femora as females as can be seen in the copulation pic.
