Thursday, July 26, 2012
Summer Insects in the Rivanna at Crofton: The "Usual" and the "Unusual"
Two of the nymphs I was expecting to see in the Rivanna at Crofton this morning: a broad-winged damselfly, Haeterina americana, and a small minnow mayfly, Heterocloeon curiosum. Here is a better look at both of these insects.
While I can find more small minnow mayflies in the Rivanna at Darden Towe Park, when I'm after damselflies and dragonflies, I go to Crofton. The damselfly I often see here is the narrow-winged damsel (Coenagrionideae) -- but I only saw small ones today; the most common dragonfly that I see is the Corduliade, Neurocordulia obsoleta (the "Umber Shadowdragon") -- they were all over the river!
But on to new things. There are three taxa that I have only seen in the Rivanna at Crofton, and to those we turn our attention. Number one, the "little stout crawler" mayfly: the "trico" to fly fishermen, but in entomological terms, family: Leptohyphidae, genus Tricorythodes. (I've seen the family and genus names reversed, by the way, with the "genus" being Leptohyphus. I guess the pros can't make up their minds.)
Some photos, which are not terribly good for two reasons: 1) these little nymphs are almost always covered in silt, and 2) they're little, 3-4 mm.
Tricos are easy to ID with minimal magnification. They have large, triangular, operculate gills that reach back from segment 2. These (to me they look like "chaps"!):
In both of the photos above, you can see them sticking out from the sides of the abdomen, and in the following photo, I've pointed them out.
The "trico hatch" is not a favorite of fly fishermen -- the adults, as you might expect, are tiny! But when the trout are on them, you'd better have a good imitation. It's the only thing the trout will go after.
The second taxon that I found this morning that, to date, I have only seen at this spot in this river is the common stonefly, genus Agnetina. And, to date, I have only seen nymphs that are immature and therefore quite small. This one measured 3.5 mm.
Agnetina nymphs differ from Paragnetina nymphs (on which see the last entry) by having subanal gills. Both genera have a complete setal row on the occipital ridge (back of the head). The row of setae is easy to see on this nymph; the subanal gills are not (the nymph is too immature). In the photo below, the subanal gills are the light gray patch between the cerci.
I'd like to ID this nymph to the level of species, but it's too immature to make that determination.
Head pattern is a crucial factor in species ID, and the pattern here is not yet fully developed. That being said, I think there's a good chance that this is A. flavescens. Clearly, I need to return to this site in the winter and find a specimen that's fully mature.
Finally, there is another taxon that I have only seen at this location: the gilled snail Hydrobiid. Here's a bunch that ended up on my tray.
If you look closely, you can see the tentacles protruding from almost every one of these shells. This is a tiny, tiny snail: these shells were 1-3 mm high. If you don't know what you're looking for, you might think you had picked up some very small pebbles -- until you see the tentacles sticking out as the snail moves across the tray.