Sunday, June 3, 2012
The Perlesta Problem
This is the only insect for which I was looking in the Doyles River this morning: Common stonefly (Perlid), genus Perlesta. At the moment this is the "only show in town," in a lot of our streams, in terms of stoneflies, very small streams in the mountains excepted. In the small streams, we're still finding Perlodids: Isoperla holochlora and Remenus bilobatus. Perlestas, generally speaking, don't inhabit such waters.
When I was collecting some nymphs for photos, I also saw a lot of Perlestas that had already hatched, sunning themselves on the rocks in mid-stream. Of course, my camera was back in the car! When I returned to the stream with my camera, the adults were all airborne. Alas! I'm sure I'll see them again.
We might begin by reviewing the traits that we use to identify this Perlid genus. As I so often do, let me use Steven Beaty's description ("The Ephemeroptera of North Carolina," p. 19).
Nymphs 8-12 mm; setal row on occiput complete, sinuate, and with irregular gaps; abdomen with numerous short, stout intercalary setae, often with pigmented bases giving abdomen a speckled appearance; anal gills present; body covered with fine, dark clothing hairs.
(Note that Beaty italicizes the critical features.) We should add -- as many keys do -- that the subanal gills are "branched," bunched into two distinct clusters. Here's an overall view of a nymph on which most of those features have been pointed out. (Click to enlarge.)
Here's a better look at the branched subanal gills.
And for a dramatic look at just how "speckled" these nymphs can be, and a closer look at the occipital setae and the irregular gaps that appear in this sinuate row -- let's look at the nymph I found in the Doyles on 4/27.
The "Perlesta problem" is, that as much as I'd like to ID the nymphs that I find to the level of species, it just can't be done. Most Perlesta nymphs remain "undescribed". In fact, very few Perlesta nymphs have been "associated" with the adults they produce. Remember that species names are initially given to the adult, terrestrial insects. Before we can name and describe a nymph, we have to know which nymph goes with which adult. This "association" of nymph with adult requires raising a nymph in the lab until it hatches. Fortunately, there are people doing this work, including Steven Beaty and his colleagues at the NC DWQ. They are currently working on five different Perlesta species.
As for Perlesta species that we might find in Virginia -- In 2002, Stewart and Stark (Nymphs of North American Stonefly Genera, p. 356) listed three: P. frisoni, P. placida, and P. teaysia. Three more species were noted in 2006 by Kondratieff, Zuellig, Kirchner, and Lenat ("Three New Species of Perlesta from Eastern North America and Notes on New State Records," Illesia 2(5): 31-38): P. leathermani, P. bjostadi, and P. cranshawi. Then just last year, 2011, yet another species was found, at least in North Carolina. It has been named P. beatyi to honor Steven Beaty for the work he is doing in associating nymphs and adults. (See, Kondratieff, Zuellig, and Lenat, "A New Species of Perlesta from North Carolina, With Additional Records from North Carolina and Virginia," Illesia (7(27): 297-301). Beaty himself, in "The Plecoptera of North Carolina, p. 19," has found the following species in North Carolina: P. cranshawi, P. decipiens, P. placida, P. shubuta, and P. teaysia.
Apparently, nymphs of four of these species have been described -- P. frisoni, P. decipiens, P. shubuta, and P. teaysia -- but those descriptions are in articles that I've not yet seen. (See Bill P. Stark and Brian J. Armitage, Editors, Stoneflies (Plecoptera) of Eastern North America, Volume II, pp. 88-95.)
So, we will leave our ID at the level of genus!
Here are some more shots of the two nymphs I photographed this morning: we'll be seeing a lot more Perlestas this month.