Ephemerella invaria: Another Spiny Crawler to Add to Our List

In the entry I posted on Thursday (3/1), I said that the spiny crawler in the photo above, found in the Doyles River on February the 6th, might well be Ephemerella invaria -- not Ephemerella dorothea, the spiny species that we commonly see in the spring.  I presented some evidence in support of that view.  Today I can make the case even stronger.

To clearly distinguish E. invaria from E. dorothea, and from E. subvaria, you have to look closely at the abdominal tergites, and the questions to answer are-- 1) are there "paired submedian tubercles" on the tergites? which tergites? and what do the tubercles look like?

This is an E. subvaria nymph that I found a few weeks ago at the Rapidan River.

Steven Beaty ("The Ephemeroptera of North Carolina," p. 28) says the following in his description of E. subvaria nymphs: "...abdominal terga with moderately long, sharp paired submedian tubercles on segments 2-9."  And those "long, sharp" tubercles are very clear in this microscope photo: look at the black, sharp spines at the rear of segments 4, 5, and 6.

Now here is an E. dorothea nymph that I found on February the 6th at the Doyles River.  (The pale, medial stripe running the length of the body is quite common on E. dorothea. )

On E. dorothea nymphs, in contrast with E. subvaria, there are no tubercles on the terga, though they "may have slightly developed spiculate protuberances."  (Beaty, p. 27)  If you look closely at the following photo of E. dorothea, you can see tiny "nipples" (i.e. "spiculate protuberances") on tergites 5 and 6.  (Click on the photo to enlarge it.)

Now for the problem.  I did not preserve the nymph at the top of the page that I found on 2/6, so I can't look at the terga!  But, I did preserve the following nymph that I found at the Rapidan River on 3/24 last year, and I think you'll agree that it must be the very same species.

Note the visual match: in both cases, there is a dark stripe on the leading edge of the pronotum, and, there is a dark pattern on the thorax at the start of the wing pads, and the tips of the wing pads are dark.  This nymph I did preserve, so for this nymph we can get a microscope view of the abdominal segments.

What should we see?  "...abdominal terga with short, sharp, paired sumedian tubercles on segments 3-8 (rarely, small tubercles on 2), small on 4-7, sometimes barely discernible on segments 3 and 8." (Beaty, p. 27)  So, we focus on segments 4-7 and look for "short, sharp, paired submedian tubercles."

And sure enough, there they are.  (Enlarge the photo for the best view.)  Our nymph is Ephemerella invaria.

I found one other photo in my pix from last year that might also be of an E. invaria nymph.  This one is more mature and was found at the Doyles River on April 11th.

So we can add E. invaria to the list of spiny crawler mayflies that we find in our streams.   With a tolerance value of 2.6, E. invaria nymphs are slightly less tolerant than E. dorothea (3.3) which might explain why, to date, I've only found them in two of our streams -- the Doyles and the Rapidan rivers.   E. dorothea nymphs, by contrast,  show up in huge numbers in almost all of our streams in the spring.

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E. dorothea and E. invaria nymphs swimming together.  Doyles River, 4/11/11.

And, E. dorothea together with E. subvaria: Rapidan River, January 18, 2012.