Thursday, August 17, 2017
Macrostemum carolina: no doubt about it
It's the weirdest common netspinner (Hydropsychidae) we see in our streams and I've long wondered about the species ID.
Macrostemum is a netspinner that I've found a number of times in the North Fork of the Rivanna and just last year, I found one in the Rivanna at crofton. I've posted entries on this one in July, 2011 and August 2012, where I've discussed the genus ID, but to review, there are two features that are fairly dramatic: 1) "the flattened head with a sharp U-shaped carina," and 2) the "dense fringe of setae on the fore tibia and tarsus." (Glenn B. Wiggins, Larvae of the North American Caddisfly Genera, p. 110 in the 1977 edition) These.
In the past, using Beaty's "The Trichoptera of North Carolina" (p. 77) I've resisted speculating on species ID, divided between M. carolina and M. zebratum. The distinction is small: on carolina the tubercles near the eyes are large; on zebratum they're little. But, our new key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- seems to resolve the issue in a definitive way.
115 Anterior tubercles above each eye conspicuous ........ Macrostemum carolina
115' Tubercles above each eye inconspicuous .......... Macrostemum zebratum
On the larvae I've found, those tubercles are very, very conspicuous.
Actually, no microscope photo is needed.
I'd say that mystery's solved. Macrostemum carolina for sure.