Monday, January 7, 2013

Wrapping things up on the Rapidan findings: Brachycentrids and Apataniids

In the entry I posted on Friday (1/4), I made very few comments on the insects I had found at the Rapidan River choosing to let photos tell the story.  But let me add some notes on the caddisfly larvae I found -- Apatania incerta and Brachycentrus appalachia -- drawing again on the work of Ames (Caddisflies, pp. 173-181 and 229-231).

I. Apataniidae, Apatania incerta.  For the ID of this insect, see the post of 12/9.  You'll recall that I photographed two larvae at the Rapidan River -- though I saw 4-5, all, by the way, on the same rock.
Here are the photos:

larva 1, dorsal and ventral:

larva 2, dorsal and ventral:

Beaty, you might recall, describes the Apataniid cases as "cornucopia-shaped" -- very fitting for larva 1; Ames (p. 230) goes with "tear-drop shaped," which works very well for larva 2.   What's important to note is that these larvae are small: the case of larva 1 was 6 mm long, the larva itself 5; with larva 2, the case was 5 mm while the larva was 4.   You can find them on the same rocks as you find the Uenoids, but the shape and the size, and the smaller pebbles used in their cases, should help you distinguish Apataniids from the Uenoids.

In describing the larvae, Ames notes what we can clearly see in our photos: "The soft parts are bright yellow, the hard parts are dark brown to black." (p. 229) He adds that they pupate in early spring and that "by mid-April the last of them are sealing their cases." (p. 230)

Ames makes an interesting point on the function of the top of the case which is shaped like a hood.  "In the final instar the case is curved so that when the insect is ready to pupate the opening at the front is concealed underneath a hood. ... This design offers some protection as the larva grazes on the surfaces of streambed rocks.  Some biologists believe that the case structure protects the larva from freezing, thus ensuring survival at high latitudes." (p. 230)  Note that the "hood" of larva 1 completely conceals the larva's head and legs, but you can see a bit of the legs sticking out from case 2.

One other point: Ames uses "Little mountain casemaker" as the common name for Apataniidae.  The Apatania incerta adult -- for the fly fishermen -- is an "Early smoky-winged sedge."

II. Brachycentridae: Brachycentrus appalachia.  

Love those photos!  Some of the best I've taken of the Brachycentrids that abound in this stream.  If you look back to the entry posted on 7/10/12, you'll see how we ID the Brachycentrids we find in the Rapidan River as Brachycentrus appalachia.   This is just a reminder that we find these "humpless" casemakers at two different times of the year since it's a "bi-brooded" insect producing two generations each year.  So, if you go to the Rapidan River where it comes out of the national park, you'll see these cases at this time of the year, but you'll see them again in July.

To fly fishermen, Brachycentrus adults are called the American Grannom, and, notes Ames, "The grannoms are among the first wave of cadddisflies that appears on the mountain streams and tailwaters of the South in early March."  Time to get ready!

One other thing.  I also picked up some common netspinners on Friday -- Ceratopsyche alhedra.

The tolerance value of C. alhedra is 0.0; that of Apatania incerta is 0.6; that of Brachycentrus appalachia is 1.0.  These are very intolerant caddisflies.  Says something nice about the Rapidan River and the quality of the streams that flow out of the Blue Ridge Mountains.

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