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

This was found by suction and was another female. There was some microsculpture visible on the first visible sternite but this was very weak. Another point in favour of aceris rather than impressus was the wings which were full - apparently impressus is usually short-winged, although "usually" implies some are full-winged. I don't have any references that show the spermathacae for most Stenus species so I don't know how helpful this might be, but I photographed it anyway in case it might be useful. I'd done so for the impressus I found previously and that looked quite different - but both were apparently different from a very poor photo of impressus spermatheca at coleonet.de.

female Stenus aceris showing hind leg, hind tarsomere lengths, hind tarsomere 4 lobes, tergites, basal tergites, folded wings and spermatheca (2 views), Wendling Beck Environment Project (Norfolk, UK), 26th September 2025