During the summer, when it was warm and sunny there were hundreds of Helophorus beetles in our small barrel pond, and elsewhere, on my car one day! These were either H. grandis or H. aequalis and at the time I was struggling to identify these to species. The problem here was that the couplet in the key is about a small feature to do with shape. In the end I gave a specimen to LL who identified it for me as aequalis. Today I went back up to the trough where I'd found some Helophorus a couple of weeks ago, fortunately there were still some animals there. I took a specimen. Under the camera and microscope the id feature became clear, an evenly serrated edge to the final abdominal segement. And the reason I couldn't be sure of the id of aequalis in the summer, as I thought I couldn't see the feature.
Evenly serrated hind edge to the last abdominal segement = grandis, irregularly serrated hind edge = aequalis.
The beetles were only in the bath and not in the nearby concrete trough.
Loch of Harray
A sunny breezy day. Bit of an aurora going on now.