Friday, June 29, 2012
Catching Up and Looking Ahead
I've been out two days in a row, and both days I've well come back essentially empty-handed. Well, I did find this beautiful Baetis pluto small minnow mayfly in Buck Mt. Creek yesterday. Actually, I found a lot of insects in Buck Mt. Creek -- but no Acentrella nadineae. And this morning, at the Moormans in Sugar Hollow, I thought I finally had one: it turned out to be a female Plauditus dubius! I'm not sure why I didn't notice the banding at the back of the tails.
Still, there are some things to report. First -- on the "unknown" small minnow mayflies I found at the Moormans on Tuesday, this gorgeous nymph turned out to be another H. curiosum, just one that was still immature. I saw that right away when I got home, where, using the microscope, I could plainly see the procoxal gills at the base of the front legs.
And there's another way we can make this ID, by noting the abdominal pattern: segments 3-5 are pale in color, so too are segments 9 and 10.
The very small nymph that I found on Tuesday -- this one (bottom)...
appears to have been a mature Acentrella turbida small minnow mayfly. That's the way it keys out: there is a dense row of setae on the dorsal margins of the femora, the tibia and tarsi (see Beaty, "The Ephemeroptera of North Carolina," p. 4). I have just one hesitation on this decision: this nymph measured only 3 mm in length, mature A. turbida should be 4-6 mm. Still, I can't find anything else it might be.
Now the "looking ahead" part of this entry. At Buck Mt. Creek yesterday, the other thing of interest I found was an immature "broad-winged damselfly" nymph -- family, Calopterygidae.
Since we'll be seeing a lot of broad-winged damsels in the Rivanna River this summer, I thought I might try my hand at species ID using the great key that I just found on line, "The Odonata larvae of Michigan," for which go to: http://insects.ummz.lsa.umich.edu/michodo/mol/Home.htm. The key page you'll want to visit is: http://insects.ummz.lsa.umich.edu/michodo/mol/Hetaerna.htm.
That the broadwinged damselflies in our local streams are Hetaerina in genus, we can tell by looking at the prementum.
How far is the prementum "cleft" from the "base of the prementum"? If it is almost halfway to the base, the genus is "Calopteryx," if it's closer to the base of the palpal lobes, it's "Haeterina." So, we're clearly looking at a Haeterina nymph. (For genus ID, see Barbara Peckarsky, et.al., Freshwater Macroinvertebrates, p. 46.)
Now using our "Key to Mature Larvae of Michigan Hetaerina" -- our choices are H. titia or H. americana. On H. titia nymphs there are "prominent sharp tubercles on [the] postero-lateral margin of [the head]: on H. americana, the "tubercles on postero-lateral margin of [the] head [are] low and blunt." Let's have a look:
I'd call those low and blunt. There's another key test: on H. titia nymphs, the "gills [are] distinctly banded"; on H. americana, they "are not banded as above, except perhaps on [the] margins." On the nymphs I've collected, there is only marginal banding.
Our "Key" adds that H. americana is "fairly common in the LP [Lower Penninsula of Michigan]." Looks to me like they're fairly common in central Virginia as well. Still, this is something I'll continue to look at as the summer proceeds.
Below: another look at that beautiful Baetis pluto.