Saturday, April 7, 2018
An odd pair of spiny crawlers at the Doyles River yesterday
It's the time of year when we see a lot of spiny crawlers in all of our streams, for the most part, genus Ephemerella. E. invaria nymphs are present in appreciable numbers from February to April; E. dorothea nymphs show up in HUGE numbers from April to May to June. I saw a lot of them yesterday at the Doyles River including this striking pair, male and female.
It wasn't until I got home and downloaded my photos that I noticed just how striking they were. What's unusual is the orange spots on the sides of terga 5 and 6 and at the top edge of the mesonota.
Thinking I might have found a species I'd not seen before, I did the microscope work, using Beaty's "The Ephemeroptera of North Carolina" as my guide. In the end, they keyed out to E. invaria, or rather, "E. invaria group". Let me review the critical features.
Nymphs 6-13 mm; pale transverse stripe between eyes, may be interrupted medially; tarsal claws with 5-10 denticles; abdominal terga with short, sharp, paired submedian tubercles on segments 2-9, rarely on 2, sometimes barely discernible on segments 3 and 8, small on 4-7, rarely on 9; posterolateral projections on abdominal segments 3 or 4-9 (variable to absent on 3); may have dot-dash pattern on pale ventral surface and speckling on last few segments. ("The Ephemeroptera of North Carolina," p. 52)
1) Our male nymph measured 6 mm, the female -- the larger and lighter of the two -- measured 8.
2) The pale stripe between the eyes is clearly visible on both of our nymphs; they don't seem to be "medially interrupted."
3) I could see 5 denticles on the tarsal claws, with a dark spot behind them (more denticles?).
4) On the female nymph (didn't check the male), there are paired, sharp pointed tubercles on segments 3-9. In this photo, I've pointed out those on segments 4 and 5. (Note, the "tubercle" is just that part of the pale spot that projects beyond the rear edge of the terga.)
5) There are posterolateral projections on terga 4-9, and yes, the last few ventral segments are speckled, and the dot-dash ventral pattern shows up very clearly.
So, Ephemerella invaria nymphs for sure. But two things to keep in mind. Beaty notes that "This is the most variable Ephemerella species in terms of size, color pattern, and size of tubercles," number one, and two, E. invaria is now considered to be a "group," which includes the species E. inconstans and E. rotunda. No way to know which member of the "group" I found without finding some adults.
The other taxa that is crowding our streams at the moment is Isoperla montana, the nymphs that hatch as "Yellow Sallies" for you fly fishermen. They were also prominent yesterday.