Dexiosoma caninum vs Dexia vacua

I always find that one of the hardest identifications to make from photos is the split between Dexiosoma caninum and the much rarer Dexia vacua. Dexia vacua is so rare that many experts thought that it was extinct in Western Europe so getting good, reliable records from photos is very important because it seems that the UK is its last stronghold.

I have been examining the few specimens I have for Dexia vacua and under the microscope they are very easy to split because Dexia all have a very pronounced central facial ridge and Dexiosoma don’t, but this is often very difficult to see in photos. Instead you have to look for other features, in what seems to be quite a variable fly. I should also add that male vacua are fairly easy to identify because they usually have orange abdominal side patches but the females are grey-brown and can look virtually identical to Dexiosoma.

So, here is a tentative list of features that I think are fairly stable. The median discal bristles aren’t always easy to see unless you get a good lateral shot but combined with the other features you should be able to get a good ID:

FeatureDexiosoma caninumDexia vacua
Central facial ridgeabsentpresent
Median discals on T3 & T4absentpresent
Median vein appendixlongshort
Median vein from m-cu to the bend lengthless than or equal to the distance from the marginlonger than the distance to the margin
Pre-sutural acrostichalsabsentpresent

These are photos that I have decided are Dexiosoma caninum:

These are photos that I have decided are Dexia vacua:

All images are (c) copyright Graham Watkeys