Sunday, September 1, 2013
Rhyacophila Brunnea-Vemna Group: Working out the ID of our Montana freeliving caddisfly larva
For the data that follows, I have relied on two sources: 1) the very thorough study of Rhyacophilids -- "Annotated list of Rhyacophila of North America with larval key and descriptions," compiled by Joe Giersch and Robert Wisseman, and 2) the "BugLab Western Rhyacophila Distribution Summary," available on line at: http://usu.edu/buglab/Content/Western%20Rhyacophila%20Distributions.pdf. Many thanks to Roger Rohrbeck and Dave Ruiter for bringing these sources to my attention.
The Rhyacophilid that I found in Grant Creek in Missoula, MT one week ago belongs to the Rhyacophila Brunnea-Vemna Group. That means it is one of seven possible species: R. brunnea, R. inculta, R. starki, R. vao (R. brunnea Gr.), R. gemona, R. robusta, or R. vemna (R. vemna Gr.). However, if we rely on the "BugLab Distribution Summary," we can narrow that down to two possible species: R. brunnea and R. vao. Those are the only species from either group that are attested in the state of Montana. (But we must note that Giersch and Wisseman in their more recent study -- published in 2012 -- attest R. gemona, R. robusta, and R. vemna in Montana as well, complicating the picture.)
In any event, Giersch and Wisseman note that "Larvae of the Brunnea and Vemna Groups cannot be separated at this time" (p. 110), adding that "species of the Brunnea Group are exceedingly common in western montane streams, from small streams to hard-bottomed rivers," and "Final instar larvae of the Vemna Group are significantly larger than Brunnea Group species," and "Species of the Vemna Group are found in colder, higher elevation, small to mid-size streams in mountains of the Pacific Northwest." ("Annotated list of Rhyacophila of North America," p. 47) Our larva is a member of the Rhyacophila Brunnea-Vemna Group -- that's as far as we can take it.
What is it that R. Brunnea gr. and R. Vemna gr. larvae have in common? All species in both of these groups have "Three pairs of gills on abdominal segments II-VII." (Giersch and Wisseman, p. 10) The gills are in clusters -- or multi-branched -- and in the following photo I've tried to point out one set of 3 on segment 5 of the larva I found.
R. Brunnea gr. and R. Vemna gr. larvae have only one pair of gills on abdominal segment I and two pairs of gills on segment 8. In the following microscope photo I've done my best to show those gills on segments 7 and 8.
Giersch and Wisseman note two other diagnostic features that we can also see in this photo: 1) "Basoventral hook: Present, but not strongly developed," and 2) Anal claw, ventral teeth: One large and one small ventral tooth." (p. 47)
Rhyacophila Brunnea-Vemna Group -- but this is a small one, 10 mm. Final instar larvae of R. Brunnea can be as much as 21 mm (p. 47) while R. Vemna larvae tend to be slightly longer, 25 mm. (p. 110)