Mayfly - Acerpenna macdunnoughi

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Roger, thank you and give a big thanks to Lloyd! I've got the specimen saved for Don Chandler, and if you want to see a larger more detailed picture, check on my website.
It looks like the last gill might be bluntly pointed.

… tom murray, 10 May, 2010 - 5:15pm

Baetid nymph, likely Acerpenna macdunnoughi

Tom, Initially, I believed this to be a Baetis flavistriga nymph. However, I just received the following communication from Lloyd Gonzoles:

I noticed Tom’s nymph (BG #394204+5) and your ID. My first impression was that it looked like Baetis intercalaris, based on the three pale spots on the rear of the darkest tergites (as opposed to the two, often kidney-shaped spots usually found in flavistriga). However, closer inspection leads me to believe that it is probably Acerpenna macdunnoughi for these reasons:

1. The photo appears to be a better match for the illustration of the dorsal habitus of Baetis macdunnoughi (fig. 31b in “The Baetis Larvae of North America,” Morihara and McCafferty, 1979) than that of flavistriga (fig. 25e) or intercalaris (fig. 24e, though this is the closer of the two).

2. The dark tergites have “anteriorly converging dashes” (see the species description in Morihara and McCafferty). These marks can also be seen in Dr. Chandler’s female A. macdunnoughi specimen on his Discover Life page.

3. Tom and Dr. Chandler’s female specimens have distinctive pale markings on the mesonotum that I would describe as a fat “M” with a line through it. In his original 1937 description, Ide described this marking as “halberd-shaped,” though I assume he was referring to a symmetrical shape like some of the Chinese halberds rather than the asymmetrical beaked Swiss halberds. (smiley face)

4. There is a distinct lateral darkening of the 9th tergite in Tom’s specimen and both of Dr. Chandler’s specimens.

5. The upward attitude of the head of Tom’s specimen appears to show a distinct “raised keel” between the antennae bases. This is one of the key traits used to distinguish Acerpenna from Baetis in the “Photographic Key to the Baetidae of EPA Region Three” (2006).

I would suggest that Tom submit the specimen to Dr. Chandler for inspection. If the 7th gill turns out to be “bluntly pointed,” it is surely A. macdunnoughi.

Lloyd Gonzales

… Roger Rohrbeck, 10 May, 2010 - 9:47am

The gill

is a little more rounded than usual, but it is a nice specimen of Acer. macdunnoughi.

… Donald S. Chandler, 19 June, 2010 - 7:19pm