Saturday, June 11, 2011
The Small Minnow Mayfly: Genus Centroptilum
I went to two streams this morning -- the Whippoorwill Branch of the Mechums and Buck Mt. Creek -- and when I came home, I thought the day had been pretty much of a bust. My microscope work proved me wrong. I'll note what I found at Whippoorwill Branch in a moment. First, on the exciting find at Buck Mt. Creek.
At Buck Mt. Creek, I found a lot of small minnow mayflies on the bottoms of rocks -- and I do mean a "lot" of them. Most were tiny, kind of yellow in color; they were genus Acentrella. But I saw two small minnows that stood out from the crowd. There was a tiny one with very dark wing pads.
Two tails. And when I looked under the primary wing pads there were metathoracic wing pads: it was genus Baetis! This is the first one I've seen here this summer; in the winter -- you may recall -- it was the only genus I found in this stream.
But it was the other "loner" that provided excitement. It's the larger of the two small minnows in the photo at the top of the page (the other one is a small Acentrella). Here's another look:
Note that this one has three tails. That eliminates two genera that we've already seen here this summer: Heterocloeon and Acentrella. So, we have to turn to the key for help with ID (Peckarsky, Freshwater Macroinvertebrates, pp. 35-36.) When our nymph has three tails (cerci), we move to the following couplet:
41a. Third segment of labial palps truncate
41b. Third segment of labial palps rounded
The labial palps on a small minnow mayfly are not easy to see. But my microscope was up to the challenge. The labial palps clearly are "truncate". (Click on the photo to enlarge.)
That sends us to the following couplet where the issue becomes: does the nymph have "metathoracic wing pads"? It does.
Mystery solved: this is a Baetidae (small minnow mayfly), genus Centroptilum. I've never seen one before. What a nice find.
Now, back to the Whippoorwill, which is where I started my day. I went there on a mission, and it's mission accomplished. I was looking for Limnephilids, Limnephilids that make two different cases; I've seen them here before. One of the cases is made out of pebbles and sand, the other has twigs for sides with wood chips and sand inbetween. Neither case is especially attractive. But the larvae construct their cases out of materials they find at hand, and I guess this is all they can find in this stream. Here are the two cases:
I virtually "begged" the larvae to stick their heads and legs out of their cases -- they wouldn't do it! Of course, as soon as I plopped them into vials for preservation, out they came. Here is a microscope view of the one in the case of pebbles and sand: he looked me right in the eye.
The head has a light colored background with dark spots -- one sign of the genus Pycnopsyche. And the other key feature -- the sclerite along the back edge of the lateral hump -- shows up very clearly in this microscope shot.
So again, our Limnephilids are genus Pycnopsyche.
The other thing I found of interest at the Whippoorwill Branch was a fingernet caddis, genus Dolophilodes, and this one posed nicely to give us a very good photo.
(Below -- the Whippoorwil Branch of the Mechums at the beginning of summer.)