Wednesday, March 2, 2011

Buck Mt. Creek: Questions, Observations, and Photos

             (A "humpless case-maker" -- Brachycentridae -- and a midge face off in the petri dish!)

Question:  Why am I seeing so many black fly larvae in leaf packs?  This is something I don't understand.  Normally, they've got their bottoms plastered to rocks, bodies and heads standing up in the current, so they can catch their food in their whiskers.  How can they do that when they're inside of leaves?  My only thought is that with the leaves in the water, they're standing up on the edge, but as soon as I take the leaves out of the water, they collapse and roll up into balls.  If someone has a better idea, please let me know.

1. There are still a lot of genus Strophopteryx large winter stoneflies around, most of them very large now -- clearly close to hatching.   What I've noticed is -- and this really is an "observation," nothing more than that -- that when the green/yellow striped and mottled large winters (Strophopteryx) get close to hatching, their wingpads lighten in color, turning tan; when the brown/orange striped and mottled large winters (Strophopteryx) get close to hatching their wingpads darken in color, turning dark brown or black.  Pictures below.

2. My second observation is that no matter where I run into small minnow mayflies right now -- and there were a lot of them crawling the rocks this morning -- the genus is always the same: Baetis.   I think the seasonal sequence in all of our streams might prove to be: Baetis (winter/early spring), Heterocloeon (late spring and summer), and Accentrella (fall).   But this may need fine tuning.  We did find genus Baetis in mid-summer last year in Meadowcreek Park -- but they were the "three tailed" Baetis; those I'm finding right now have only two tails.  I would not be surprised to learn that the Baetis small minnows present in Meadowcreek have a higher tolerance value than the Baetis small minnows I'm finding in Buck Mt. Creek and the Doyles River.  Tolerance values are thought to vary by family, genus, and species.  In some lists, for example, Baetis tricaudatus is estimated to have a TV of 1.8; Baetis frondalis, by contrast, has a TV of 8.

I don't think that all genus changes, by the way, vary by season: some depend on the stream.  Thus, in most of our streams the common stonefly we find is, by genus, Acroneuria: but in some other streams (and I'm thinking of Powells Creek in Crozet) the most common genus I see is Eccoptura (and the occasional Neoperla).  One common stone genus that may be determined by season is Perlesta: I've only seen it in streams (Buck Mt. Creek, the Rivanna in Darden Towe Park) in the summer.

Photos:  It was a good day for photos: bright sunlight and, for a change, some cooperative insects who held still for the camera.  So...

1. A water penny beetle, both dorsal and ventral sides (this is the first time I've thought to do a photo shoot of a live one.)

2. A few of small minnow mayflies -- genus Baetis.

3. A midge (Chironomid) who slowed down long enough to get a photo of its front and back prolegs.

4. And finally, a couple of shots of a very pretty Nemourid stonefly, again genus Nemoura.

No comments:

Post a Comment