Tuesday, April 17, 2012

"Drunella" Day at Buck Mt. Creek

After telling you how rarely we see this genus -- Drunella (remember the "muscular" fore femorae with turbercles on the leading edges) -- of spiny crawler mayfly, I found three of them today right off the bat!  And, my guess was that they were three different species.   Note how the nymph in the photo above is basically brown with no banding on the leg segments.  Now look at the second.

Quite different, wouldn't you agree?  Prominent banding on the femorae and tibiae, with patterns of colors on the wing pads and abdominal segments.

The third?  This one we've already seen in the Doyles River (look back to the 4/9 entry) -- Drunella walkeri.  Note the anterolateral projections that stick out on either side of the head.

So back to the first two nymphs which differ dramatically in appearance.  I've just looked at them with the microscope, and I can't see a single anatomical difference in terms of the features used for identification.
Both have paired dorsal tubercles that are very clear on segments 5-7.  Both have long occipital tubercles.
And both have a "long posteromedian tubercle on [the] mesothorax" (Beaty, "The Ephemeroptera of North Carolina," p. 25).

That essentially brings us to a choice between D. allegheniensis and D. tuberculata.  Let's see how Beaty describes each of these species.

D. allegheniensis -- nymphs 8-9 mm; frons with a dark horizontal band; head with long occipital tubercles that diverge apically; long posteromedian tubercle on mesothorax; abdomen with paired dorsal tubercles always well developed on segments 5-7.  Common in the summer in the mountains and foothills.

D. tuberculata -- nymphs 7-9 mm; head with long occipital tubercles not divergent apically; frons without dark transverse band; abdomen with paired dorsal tubercles always developed on segments 5-7.  Collected in spring and summer from rivers.  Uncommon. 

I'm undecided about the ID of our two nymphs.  On the one hand, I'd be surprised if there's something "uncommon" in Buck Mt. Creek, and the "tubercle on the mesothorax" is only mentioned with D. allegheniensis.  But, I cannot see a dark horizontal band at the front edge of the head (frons).  Nor can I say for sure that the "occipital tubercles diverge apically".   So, let me sit on the fence for the moment and get back to you with the details when I work this one out.  Difference in colors and patterns?  Is it a male and a female?


Other photos today.

1. An Epeorus vitreus flatheaded mayfly.  They're now all over the bottoms of rocks in this stream.  (Same insect; different lighting.)

2. A tiny Plauditus dubius small minnow mayfly -- a female.  (Not the best of photos).  On key features of P. dubius nymphs, see the entry posted on 4/10.

3. And a very small -- and I do mean small! -- Baetis intercalaris small minnow mayfly, the first I've seen this season.  Note: three tails with prominent banding.

When it grows up it will look something like this and the "parentheses" marks (  ) on the abdominal segments will be easier for us to see.

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