Stenus aceris

This came to light in my garden. I used Lott & Anderson to key this at the same time as examining a very similar insect that keyed the same way right up to the final couplet, between impressus and aceris. I would have liked to have checked both identities with reference to the genitalia but unfortunately both were female, however I am fairly confident they were not the same species. On the other one, which I believe to be impressus, I was able to see, under high magnification, reticulate microsculpture on tergite 3 whereas on this one I could not make this out clearly. There was a suggestion of microsculpture, and the pattern seemed to be rounded but not fully reticulate. Lott & Anderson also refer to a possible difference in the colour of the knees. I'm not sure if there "knee" extends to the apex of the femora but on the impressus the femora (on all legs) was only slightly and narrowly darkened at the apex, whereas this one had much more obvious and extensively darkened mid and hind femora at the apex. I was unable to find the spermatheca in this one. Another probably unimportant difference was that the fourth tarsal segment of the front legs was more widely and obviously lobed on the impressus. This one was also fractionally larger, but both were within range for both species.

female Stenus aceris showing head & pronotum, antannae & palps, basal abdominal tergites, hind leg and hind tarsi, North Elmham (Norfolk, UK), 23rd March 2022