Friday, September 16, 2011
E. subvaria and E. dorothea: Two Spiny Crawlers of Great Interest to Fly Fishermen
I've decided to try my hand at species identification of some of the spiny crawler mayflies (family: Ephemerellidae) that we find in our streams, and I've identified two that are of great interest and importance to fly fishermen. One -- pictured above -- is Ephemerella subvaria, which provides us with the "Hendrickson" hatch in spring, the other -- and pictures of that in a moment -- is Ephemerella dorothea which gives us the "Pale Evening Dun (PED" hatch in early summer (mostly in June).
When I lived in Vermont, the Hendrickson hatch was one of the big events of late spring -- well, for fly fishers that is -- often occurring the last two weeks in May. It's a big fly, easy to recognize, and provides great dry fly fishing (one of the reasons we like it so much!). In our area (Virginia. that is), hatches are apt to occur earlier in the year, a fact that is noted by Knopp and Cormier (Mayflies, p. 195). They point out that in the the southern Appalachians, the Hendrickson hatch "typically peaks in mid-April." That very clearly accords with my findings.
My first sighting of this nymph -- and I have only seen it in that Virigina "Mecca" of trout fishing, the Rapidan River -- was on January 19th.
And I saw it again on February 23rd: note how the wing pads have lengthened.
And my last sighting was on March 24th, the nymph that's featured at the top of this page. That one was close to hatching -- it almost looks like the wings are partly revealed.
How do we identify this as E. subvaria? First of all, a word of caution: not all E. subvaria nymphs will have these same colors and patterns. As always, ID requires matching of key anatomical features. That being said, one of the features we use for ID is the markings on the tibiae and tarsi: there's banding that is dark brown or black. No need to use the microscope; we can see that in our photos. But the two other features do require microscope photos. They are: 1) sharp, paired "tubercles" on the back edges of abdominal segments 2-9.
The tubercles on 2 and 9 may be hard to see in this photo -- the rest are clear as can be. And the other feature that we need to see -- posterolateral projections (sharp edges) on terga 4-9.
And there we have it: E. subvaria, the "Hendrickson" to fly fishermen.
And now, for E. dorothea, the "Pale Evening Dun".
This is the most common spiny crawler we see in our streams, showing up in the hundreds and thousands in the spring -- March, April, and May -- with peak emergence occurring from mid-May to mid-June. On E. dorothea, Knopp and Cormier (Mayflies, p. 207) note that they are "reliable June emergers," but that in the "warmer southerly trout waters [they] may produce hatches as early as the first weeks of May."
One of the first sizeable E. dorothea nymphs that I saw was at the Whippoorwill branch of the Mechums: that was on March 18th. (The one in the photo above is from April 14th, Buck Mt. Creek.)
And the last E. dorothea I saw was in a small, clean and cold, tributary to the Moormans on May 18th, and you can tell from the rich colors and patterns that it's fully mature.
What are diagnostic traits for this spiny species? Some are really obvious from our photos: the abdominal terga are brown, covered with light freckles and pale dots, and, there are no tubercles on the terga. Also, there is a visible, pale stripe running from the head to the tail of the nymph. Again, for the other key features -- there are two -- we have to use microscope photos.
1) As with E. subvaria, there are "posterolateral projections" on abdominal segments (terga) 4-9.
2) But, unique to this species, are the "stout setae" -- short hairs -- on the edges of those posterolateral projections.
Voila! E. dorothea, or the "Pale Evening Dun."
Two final notes. 1) For tolerance values -- the North Carolina Division of Water Quality assigns a 3.3 to E. dorothea, but feels it does not have enough evidence to decide a TV for E. subvaria. And 2) for good photos of the "adult" terrestrial forms of these insects, I suggest you look at troutnut.com.
E. subvaria (Hendrickson): http://www.troutnut.com/specimen/494
E. dorothea (PED): http://www.troutnut.com/specimen/743
(Below, E. dorothea nymph posing next to a caddis larva case (Lepidostomatid) -- tributary to the Moormans River, photo taken on May 6th.)