Aquatic Insects of Central Virginia: Searching for Small Minnow nymphs: A Look at Powells Creek

(Please note that Blogger has been down for almost 24 hours, and my post from yesterday was lost and apparently cannot be restored. So, I'll reconstruct it the best that I can. My trip to the Powells was on 5/12/11.)

No. This is not a "small minnow mayfly" (Baetidae). But let me clean up some unfinished business before I move on to my mission. If you've been following along in a regular way, you'll recall that I found three of these nymphs -- the one in the photo above -- at the Rapidan River last week. This is a Perlodid stonefly, and in terms of genus, it keyed out to either Clioperla or Isoperla; I was quite sure that it was the latter. I now have confirmation that that is correct. Roger Rohrbeck with Bugguide.net informed me this morning (5/12/11) that this appears to be Isoperla holochlora, and told me to take a look at the photo by Donald S. Chandler at http://www.discoverlife.org/mp/20p?see=I_DSC520&res=640. So, that mystery is solved. I found a number of these today in Powells Creek. (And I can add that I found a "ton" of them this morning -- 5/13/11 -- in the Doyles River at Doylesville.)

Now on to the main matter at hand. I came to the Powells to look for small minnow mayflies since they are showing up again in Buck Mt. Creek. I only found two -- but they're interesting samples for us to look at.

Notice that both nymphs have three tails. Every nymph I found in the winter had just two tails. Here's a look at a nymph from Buck Mt. Creek in the winter to jog everyone's memory.

Every nymph I found in the winter was genus Baetis. By contrast, the "new" small minnows I found at Buck Mt. Creek were Heterocloeon (pigmented gills, but still two tails) and Acentrella (two tails, but a very broad upper thorax).

In the version of this current entry that I posted yesterday, I claimed that the two nymphs I just found in the Powells were genus Baetis -- just the form with three tails instead of two (there are two kinds). Let me take advantage of this "re-post" to qualify the claim that I made. I think they were both genus Baetis: I cannot really be sure. Why? Because I stupidly forgot to bring the second nymph pictured above home with me to my lab! And, with the first nymph above, I saw "metathoracic wing pads," which normally means genus Baetis, then discarded my sample, only to remember that with the "three-tailed" Baetis you need to check the shape of the labial palps to be sure of the genus ID (see Peckarsky, Freshwater Macroinvertebrates, pp. 35-36 on this point.) So, I'll have to return to the Powells for more samples. In any event, the critical point is that the nymphs I found in the Powells were not 1) genus Heterocloeon, 2) genus Acentrella, nor were they 3) the same type of Baetis that I found in this stream in the winter.

Why is this important -- or at least interesting? Because small minnow mayflies can produce more than one generation a year. The nymphs that hatch in late winter can lay their eggs, which can grow and mature through the spring and summer to hatch in late summer or fall. Will we see a "second generation" of the bugs that we saw in the winter? Possibly, and I'm looking. But I don't think I've seen that so far.

Oh. One other thing on the one small minnow nymph that I found yesterday. Unfortunately, I put it in the tray along with a Perlodid stonefly (genus Diploperla). Big mistake. As I looked on, the stonefly started eating the small minnow mayfly! At first I thought -- "This will be neat -- to snap photos of the progress made by the stonefly as he devours the mayfly." Then I remembered that I needed the mayfly for purposes of genus ID, so I quickly put a stop to the action! (I was already too late to save the tails and the very end of the abdomen.)