Friday, June 24, 2011

Revising my Views on Small Minnow Mayfly (Baetidae) Genera

I found this beautiful small minnow mayfly in the Doyles River this morning.  (The orange/red pigment is for real, by the way, those aren't eggs stuck to the body.)  I will show you some other good photos from today's trip later on.  But first,  I have to talk about something very important.

As you know, if you've been reading along in a regular way, I found a lot of small minnow mayflies this winter (February and March), all with two tails, and I argued that they were Baetis in terms of genus based on the presence of metathoracic wing pads (see the entries for 2/14, 3/2, and 3/16).  In the Peckarsky key (Freshwater Macroinvertebrates, pp. 34-36) the presence of metathoracic wing pads on a two-tailed Baetid distinguishes the genus Baetis from Acentrella.

Bad news.  I've now been told by a professional entomologist, a specialist on aquatic macroinvertebrates, that all Baetis small minnows in the eastern part of the country have three tails!  He also feels there is a very good chance that the nymphs I found in the winter were Acentrella in terms of the genus.  I worked a lot on this last night and this morning, and I now believe he was right:
the nymphs I found in the winter were probably genus Acentrella.  (So too is the nymph I found this morning -- the one in the photo at the top of the page.)

Let me make my case.  First of all, I'd refer you to a web publication entitled "Small Minnow Mayflies: Nymph key to genera" (go to,  where Acentrella as a genus is defined by a "Dense row of long fine setae on the back (or dorsum) of the legs."  It continues, "Everyone else [ i.e. all other genera have] rows of setae on the legs."  Here is a close-up of the legs of one of the nymphs from the winter that I had preserved.  (Please click on the photo to enlarge it.)

And, here is a close-up of the legs of the nymph at the top of the page.

That might seal the deal -- but I think it's important to see what the "Bible" -- in terms of dichotomous keys for aquatic macroinvertebrate entomologists -- has to say on this issue as well.  The "Bible" is:
R.W. Merritt, K.W. Cummins, and M.B. Berg, ed., An Introduction to the Aquatic Insects of North America, Fourth Edition (Dubuque, Iowa: Kendall/Hunt Publishing Company, 2008).  Having reduced our search to "two-tailed" Baetids, and eliminated the genus Heterocloeon from contention, we move to couplet 38:

38.  Hind wing pads present (although sometimes reduced to a small thread-like flap.... (go to) 40
38'. Hind wing pads absent.... (go to 39)

On the two-tailed small minnow nymphs I've been finding this summer, the hind (= metathoracic) wing pads have been "absent":  on the two-tailed small minnow nymphs that I found in the winter, they were small but "present".  So, let's refer to these as Summer Nymphs and Winter Nymphs and follow both directions above.

Summer Nymphs (hind wing pads absent) -- we go to couplet 39.

39. Femora, tibiae, and tarsi without a row of long setae on outer margin....Plauditus
39'. Femora, tibiae, and tarsi with a row of long setae on outer margins..... Acentrella (in part)

So, from the photo of the legs on the nymph that I found this morning, we'd have to go with Acentrella for the Summer group.

Winter Nymphs (hind wing pads present) -- we go to couplet 40.

40. Segment 2 of labial palps with well developed medially projecting corner....Baetis (in part)
40'. Segment 2 of labial palps without well developed medially projecting corner....Acentrella

Let me show you what "well developed medially projecting corners" look like.  This is a close-up of the labial palps of the "three-tailed" Baetis I found earlier this summer at Buck Mt. Creek.

That corner really sticks out like a shelf.  Now, here's a close-up of the labial palps from one of the "two-tailed" small minnows that I found this winter.

There is a corner -- but it's not well developed, no "shelf".  Our Winter Nymphs, I'd have to conclude at this point, were genus Acentrella, not genus Baetis as I have claimed in previous entries.  So, the two-tailed small minnows we found in the winter were apparently Acentrella, and the two-tailed small minnow nymphs I'm finding this summer are also Acentrella.  At this point in time, however, I feel strongly that I'm looking at two different species of Acentrella.  Remember, the "Winter" nymphs had wing pads showing; the Summer nymphs do not.  And, the nymphs I've found so far this summer are much smaller than the nymphs I found in the winter -- and some of the summer nymphs I've found have been pretty mature.

I hope I have this straight now.  But I am, after all, an amateur at this.  And given the complexities involved with genus ID in this particular insect family (Baetidae), this may be something we amateurs should leave to the pros.

Two other photos to look at from my trip to the Doyles this morning.  First is yet another beautiful Perlesta common stonefly.

But second -- and I'm really excited about this -- is a fully mature flatheaded nymph, genus Leucrocuta.
I've never seen one before.  Two photos:

And, if you look really closely at the second photo, you can actually see that there's fibrilliform behind gill number 6, but not on gill number 7.  Here's a microscope photo pointing that out.

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