Friday, September 27, 2013

The Rivanna flatheaded mayfly of the moment: Maccaffertium modestum

When I went to the Rivanna River at Crofton last week I found a flatheaded mayfly that looked much the same as the one in the photo above, but I failed to get any useable photos.  So yesterday, I went to the Rivanna at Darden Towe Park to see if I could find the same insect -- and I did.  What's the species?  It appears to be Maccaffertium modestum, which, says Steven Beaty, is the "most common [Maccaffertium] species in North Carolina."  ("The Ephemeroptera of North Carolina," p. 20)

That probably means that it's common in Virginia as well and that I've seen it before, but I've been hesitant to attempt this species ID.   I'll tell you why.  Let's look at Beaty's description.

M. modestum -- nymphs 8-11 mm; has 15-50 (usually 20-40) hairs, 4-7 spine-like setae on maxillary crown; claws with denticles (some specimens have one foreclaw with and the other foreclaw without denticles) highly variable dorsal and ventral color patterns.

Now, that's not a whole lot to go on for the "most common" species we see.   First of all, as I've shown in previous entries, it is not easy to count the hairs and the spines on the maxillary crown of a Mac, especially when the nymph is a small one, and while I can usually see if there are denticles on the foreclaws, that trait does little to narrow down the possible species ID.  That leaves us with a "highly variable dorsal and ventral color pattern."  I.e. it leaves us with nothing to go on.

However, this description has been expanded in the second version of Beaty's Ephemeroptera manual.  (Go to:   Much improved.

modestum -- nymphs 8-11 mm; has 15-50 (usually 20-40) hairs, 4-7 spine-like setae on maxillary crown; claws with denticles (some specimens may have one protarsal claw with and the other without denticles); posterolateral projections absent anterior to segment 6; highly variable dorsal and ventral color patterns; two common ventral patterns: (1) presence of anteromedial bars on 7-9 with bar on 7 possibly obscure and that on 9 extended posterolaterally to form an inverted "U" or (2) sterna 2-8 with two pair of small dots; one pair anteromedially placed and the second pair posterior and lateral to the first pair; may be faint on anterior segments.

Is this a good description of the nymph that I found yesterday?

1) maxillary crown

While this is not the best of photos, I have no trouble seeing at least 15 hairs on the crown, and through the microscope, I could count 6-7 spine-like setae.

2) denticles are present on the tarsal claws

3) posterolateral projections

They are very clear on segments 7-9 -- that's about it.  Certainly nothing there before segment 6.

4) So we're back to the issue of ventral pattern.

Pattern 1, I'd say, is out: there are no bars on sterna 7-9.  Pattern 2?  Possibly.  Two pairs of medial dots are clearly visible on segment 6, though the posterior pair does not appear to be "lateral" to the anterior pair.

So, can we call our nymph M. modestum?  I think we can, but we need to turn once again to Don Chandler's photos posted on "Discover Life."  (Go to:  I think his photos of M. modestum -- both dorsal and ventral -- confirm this ID.  We just need to add this to Beaty's description.

Maccaffertium modestum

By the way, this is the most tolerant Maccaffertium species we find, with a TV of 5.7.

As I did when I explored the Rivanna at Crofton, I found lots of stoneflies at Darden Towe, including the very same A. abnormis -- the one that lacks the "M" pattern on the head and has an abdomen that's entirely brown.  It's a big nymph.  This one -- still immature -- measured 13 mm.

And as we can see in this second photo, Agnetina annulipes was also present.  Note the difference in species size: A. annulipes is small.

Finally, I again saw quite a few Giant stoneflies, Pteronarcys dorsata.

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