Friday, September 15, 2017
Flatheaded mayfly: Heptagenia marginalis
The Rivanna River in Charlottesville has been high and muddy most of the summer. Some of the time that could be blamed on heavy rain -- but not all of the time. Somebody was doing something upstream, not sure what. In any event, it now seems to be dropping and clearing so I went to Darden Towe Park this morning to see what I could find.
There were a number of insects to photograph, but these pictures turned out the best. Flaheaded mayfly: Heptagenia marginalis. While I found this nymph once in the Moormans, I've seen it more often in the Rivanna at Darden Towe and always at this time of year. Heptagenia nymphs have fibrilliform behind gill number 7 (it's absent on 7 on Leucrocuta and Nixe). For the species ID, all we have to see are the dark sublateral streaks on the terga.
Tolerance value is 2.2 -- not all that bad for the Rivanna. Found two.
I'm not sure what explains the bright yellow marks on one of the nymphs (terga 1, 8 and 9) but not the other. Guess it could be a gender distinction.