Monday, November 17, 2014
A third strong-case maker (Odontoceridae) larva: Psilotreta rufa
We appear to have found a third stong-case maker species: Psilotreta rufa. (Well, there's no "we" involved: the larva was found by my friend in Sugar Hollow, the one who lives next to this superb little stream.)
We've collaborated on the ID, but all of the photos of rufa were taken by her.
Almost all of the Odontocerids I see are Psilotreta labida, a larva with a broad dark stripe extending from the head through the mesonotum and long, pointed anterolateral pronotal projections.
You'll recall that I found hundreds of them just a few weeks ago in the Rapidan River. The only other species I've seen -- and I've only found it in one little stream -- is Psilotreta frontalis.
We see that same dark band on P. frontalis -- but the pronotal projections are short.
Clearly, the larva my friend found last week is something entirely different. There is no dark band on the head and the nota: rather, the head and pronotum are a dark, reddish brown while the mesonotum is almost entirely yellow.
Our new larva keys out to Psilotreta rufa which Beaty describes in the following way.
(P. rufa) -- larvae up to 11 mm; head and pronotum uniformly reddish brown without stripes but may have darker pigmentation along frontoclypeal and coronal sutures...head longer than wide and relatively flat between carinae; seta 17 about half the length of seta 15; pronotum darker laterally; anterolateral pronotal projections short. ("The Trichoptera of North Carolina, p. 97)
The color of the head and pronotum, the shape of the head (longer than wide), and the short pronotal projections all show up very well in the following photo.
We might see darker pigmentation along the frontoclypeal suture in the following photo -- but the color distinction is admittedly slight.
But this very nice lateral photo clearly shows that the pronotum is dark laterally, and, I think, that the head between the carinae is "relatively flat." (The carinae are the raised ridges that run from front to back on either side of the head.)
That leaves us with the question of the relative lengths of seta 17 and 15. I confess that I had no idea where to look for these setae until we found the following article, one that is referenced by Beaty: C.R. Parker and G.B. Wiggins, "Revision of the caddisfly Genus Psilotreta (Tichoptera: Odontoceridae)," ROM (Royal Ontario Museum): Life Sciences Contributions, No. 144 (1987). In that article, on p. 29, the head and nota of P. rufa are illustrated in detail, and setae 17 and 15 are clearly marked: 17 is behind the frontoclypeal suture, and 15 is along the carina, the ridge. Unfortunaely, despite the very fine photos my friend has provided, I can't be sure I can pick out those hairs!
Nonetheless, Parker and Wiggins describe P. rufa in a way that supports our identification.
p. 10 "head uniformly reddish brown or with pale areas laterally; thoracic nota uniformly reddish brown to pale yellowish." Yes.
p. 21 "Head longer than wide, and in lateral aspect posterodorsal angle more or less squared; dorsum relatively flat between carinae; seta 17 thinner than, and usually about one-half as long as, seta 15. Pronotum with projection of anterolateral corner short, about 0.2X middorsal length of pronotumn. Mesonotum with black marking along posterior ridge extending laterad to posterior inflection."
The point of inflection is where the thin band we can see above the thicker black band rises up at the side. If you look closely you can see it in the following photo.
I'm not sure we really need to see the head setae to make this identification given the preponderance of evidence that we have. Psilotreta rufa.
Two final points of interest. 1) The habitat for P. rufa (Parker and Wiggins, p. 21) is a perfect match for our stream: "P. rufa larvae occur in small spring seeps and spring-fed streams." And 2) Parker and Wiggins include the following site among the locations where rufa is found: Albemarle County in Virginia, south fork of the Moormans River above the Charlottesville Reservoir. (p. 22) Bingo! Not at all far from the small stream in which this larva is living.