Sunday, May 26, 2013

The flatheaded mayfly Epeorus fragilis: making the case

Yes: the flatheaded mayfly I found yesterday at South River was, indeed, Epeorus fragilis.  I feel safe in making this identification even without confirmation from Steven Beaty based on 1) more careful study, and 2) the matching photo of E. fragilis posted by Donald Chandler (  So let me go into a bit more detail.

I want to begin with what I hope is a clarification.  According to Beaty ("The Ephemeroptera of North Carolina," p. 17), E. fragilis is part of the "Epeorus pleuralis group."  Under "Epeorus" (the genus), here are the distinctions he makes:

I. Epeorus pleuralis group:
   1. E. fragilis
   2. E. pleuralis

II. Epeorus vitreus group:
   1. E. dispar
   2. E. subpallidus
   3. E. vitreus
   4. E. sp.1 

Personally, I think it is very confusing to use the same names (Epeorus pleuralis and Epeorus vitreus) for groups and species, and if I had the credentials I'd argue that that ought to be changed.  So, let me be clear: when I call something Epeorus pleuralis, I mean the species, and when I call something Epeorus vitreus, I mean the species.  I'd do away with naming the groups.

All right.  On to Beaty's description.

(E. fragilis) -- mature nymphs < 6.7 mm; lateral margins of head capsule with abrupt transition near outer anterior corners of eyes, leaving eyes to hide much of the posterolateral margin of the head; ratio of head width to distance between antennal pedicels 2.11 - 2.45 (median 2.21); head with anterior margin mostly pale with a distinct "V" medially: abdominal terga 3-7 without small, paired, dark submedial spots.

1. The nymph that I found measured almost exactly 6 mm.

2. The ratio of head width to the distance between the antennal pedicels measured 2.2 -- that's almost perfect.

3. The abrupt transition from the union of eye and head margin is marked in the following photo as is the space where the eye and head margin coincide at the rear of the head and the side of the head (posterolaterally), thus "hiding" the posterolateral margin.  Also clear is the "V" at the front of the head.

Much less of the eye touches the head margin on E. pleuralis and E. vitreus nymphs.

E. pleuralis:

E. vitreus:

4. Finally, there are no "small, paired, dark submedial spots" on any of the terga that I can see.

If you want to see "dark submedial spots," look at the terga of the small minnow mayfly Heterocloeon amplum, especially terga 5 and 8-10.

Our microscope photo of the abdominal terga of E. fragilis also reveals something that drew my attention at the stream yesterday: two of the terga -- 7 and 8 -- are darker than all the rest.  And, the rear edge of terga 5-8 (the hind 1/4 of each of those tergites) consists of a dark band.

I wish I knew more about this new species -- state distribution, specific habitat, etc. -- but I've not been able to find that information.  We can certainly say that it is found in some of the very same streams in which Epeorus pleuralis is found earlier in the year, and it seems to coexist in those streams with E. vitreus (i.e. late spring/early summer).  At least that's the case at South River.  But it's a much smaller nymph than those other species.

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