David you absolutely nailed it - well done. Lovely details great DOF, the image is very clean. You may wish to check the brightness of some of the brightest areas - for example the first abdominal segment, I suspect a luminosity adjustment would help.
If the image is a crop I would suggest a slightly looser one if not a crop maybe extend the canvas just a little bit.
I have had the darters in tandem around my pond but have not been able to get a decent shot of them this year, there is still another month so fingers crossed.