Saturday, July 12, 2014
Heterocloeon curiosum (small minnow mayfly) at the Rivanna: summer for sure
When I found this tiny nymph at Darden Towe this morning, I rushed out to take a few photos thinking I might have found the uncommon Heterocloeon petersi -- this one, which I found two years ago in the Rivanna at Crofton.
But I'm pretty sure I was wrong. The little guy -- and it is a "guy" (male) -- in the first photo is probably a small, immature, Heterocloeon curiosum, an insect which is common at this site every summer. When they mature, they look more like this.
Alas! The Rivanna at Darden Towe is still a little bit high, but it's now perfectly safe to wade and scrounge up some bugs, and there were plenty of them to be found. Lots of small flatheaded mayflies, a fair number of common netspinners -- even a whirligig beetle! But let's stick with the H. curiosums since they too were around in good numbers. Here is one of the females I found.
The females, you may recall, are always bigger than the males -- but less colorful. But on the larger female, one of the key features of H. curiosum shows up very well: there is a dark splotch of pigment in the center of each of the gills. Still, to be sure you have the small minnow mayfly H. curiosum, you have to see the little white gills that stick out at the base of the forecoxae. These.
These nymphs are not easy to photograph: they don't stay in one place very long. But, I'll get plenty of shots at good photos from now through September.
1. The common netspinner, Hydropsyche rossi/venularis. I think H. rossi and H. venularis are the only netspinners I see in the Rivanna, and they're fairly tolerant larvae (4.8 and 5.1 respectively). I took photos of two of these insects this morning: one was brown, the other one green.
In the end I wasn't sure if they were H. rossi or H. venularis -- or possibly one of each. The head pattern is much the same on both species: "Frontoclypeus with two pairs of yellow spots anterolaterally" (Schuster and Etnier, "a Manual for the Identification of the Larvae of the Caddisfly Genera Hydropsyche pictet and Symphitopsyche ulmer.., p. 96). We can see those spots in this head shot of the brown nymph.
But to tell the two species apart, we must look at the rows of muscle scars on the sides of the head. Do they, or do they not, "curve dorsad posteriorly" (Beaty, "The Trichoptera of North Carolina," p. 76). Here are two microscope photos. You decide. I'm inclined to say they were H. rossi, but the evidence just isn't certain.
Best to exercise caution: H. rossi/venularis.
2. A whirligig beetle larva, the first of the year: also a difficult photo to get. They wiggle and wiggle and wiggle!
Nasty looking pincers. This one was determined to get ahold of the H. curiosums.
3. And last but not least, a Maccaffertium ithaca flatheaded mayfly: I found two.
It's great to get back into the Rivanna. Every summer I find new things in this river.