Sunday, January 13, 2013

The Ephemerella (spiny crawler) sequence: entering the E. invaria phase

There are three spiny crawler mayflies that are of great interest to fly fishermen, and we find all three of them in our streams.  The first to appear -- in terms of the nymphs -- is Ephemerella subvaria, the one that, to date,  I've only seen at the Rapidan River.  It's a real beauty.  This is the first one that I found this season -- on 11/5 -- but you may recall that I saw some last year in September.

And this is the latest one that I saw, on my trip up on 1/4.

E. subvaria hatches as the "Eastern Hendrickson," and it's a big mayfly.  Knopp and Cormier (Mayflies, p. 195) put the hatch in the northeast from mid-April through early June.

Next in the sequence comes E. invaria, the nymph that I'm now just starting to see.  That's an E. invaria at the top of the page, one that I found at South River last week (1/10).  But I found a number of them at the Rapidan on 1/4, and I found a tiny one at the Doyles -- you may recall -- on 12/21.  This one.

E. invarias hatch as "Sulphur Duns," from mid-May to late June (Knopp and Cormier, p. 203).  Though these can vary a lot in color and pattern, some are spectacular when they mature.  This one was found in the Doyles River on April 8th last spring.


The third Ephemerella we find in our streams in E. dorothea, a species that I normally see by early March, if not before.  This is the species we see in HUGE numbers in April and May, the one that can dominate "samples" when you monitor streams in the spring.   While not all E. dorotheas can be recognized by color and pattern, some are very distinct.  If your spiny crawler nymph has a "mediodorsal stripe" (Beaty, "The Ephemeroptera of North Carolina," p. 27), chances are good that it's E. dorothea.

Other E. dorotheas tend to be uniform in color, showing no pattern at all.

E. dorotheas hatch as the "Eastern Pale Evening Dun" from late May into early July: best time to fish them -- warm nights in June (Knopp and Cormier, p. 207).

How do we identify these spiny crawlers as Ephemerella in terms of the genus, and how do we distinguish them in terms of the species?

Ephemerella is the most common spiny genus we see, and all of its species have two things in common.  Let me quote this from Beaty (p. 26): 1) "lamellate gills present on abdominal terga 3-7," and 2) "caudal filaments with heavy intersegmental setae and without whorls of spines at apex."  The gills will look like this (and note that gill 6 covers all but the tip of gill 7):

And here is a close-up of the tails (caudal filaments).

Now, how do we tell our three species apart?   Well, the seasons are different -- but they do overlap.  It is possible to find all three species in a stream at the same time.  Size will help: mature E. subvaria nymphs are always big (8.5-9.5 mm); mature E. dorotheas are 6-8 mm while E. invarias run 6-13.  And then there's the matter of color and pattern:  there is no mistaking E. subvaria, but E. invaria and E. dorothea can be confused.

The best key to use?  You have to look at the abdominal segments -- posterior edges.  Do you see "tubercles" on the terga, and if you do, where do you find them and what is there size?

E. subvaria nymphs have "moderately long, sharp paired submedian tubercles on segments 2-9" (Beaty, p. 28).  They're very clear in this picture.

E. invaria nymphs have "short, sharp, paired submedian tubercles on segments 3-8" (Beaty, p. 27).

Finally, the terga on E. dorothea nymphs are "without dorsal tubercles but may have slightly developed spiculate protuberances."

Oh.  There is one other thing you can look for on E. invaria nymphs, the species that is showing up now.  They have a "pale transverse stripe between [the] eyes [that] may be interrupted medially."  That's visible on all of the E. invarias I've found so far this year.

12/21, Doyles River:

1/4, Rapidan River, number 1

1/4, Rapidan River, number 2

And 1/10, South River:


So it's Ephemerella time here in Virginia, and the fishing is bound to be good.  My favorite, I have to confess, is E. subvaria: the nymphs are gorgeous, and the Hendrickson hatch, when I lived in Vermont, was one of the best of the year.

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