Tuesday, July 10, 2012

The Humpless Case-makers: Brachycentrus and Micrasema

The "Humpless" case-maker caddisflies that I find in the Rapidan River are all Brachycentrus in terms of the genus.  But my good friend who sifts through the waters that flow through Sugar Hollow recently found a pair of Micrasema Brachycentrids.  Since I have her permission to post her photos, I thought I might say something more about these two genera.

Humpless case-makers (family: Brachycentridae), you may recall from the general entry I posted a long time ago (1/11/11), are so called because they lack humps.  Almost all case-making caddisfly larvae have humps (fleshy protrusions) on either side of the first abdominal segment, and some have one on top of that segment as well (lateral and dorsal humps).  The larva uses its humps to anchor itself on three sides of its case and increase the flow of water through its case by abdominal undulation.  Faster water means higher levels of dissolved oxygen.  Since Brachycentrids do not have these humps, they must fasten their cases to rocks over which the flow of the water is constant with the front of the cases facing upstream.  A case attached to a rock looks like this.

Almost every rock that I lifted on Saturday at the Rapidan River had 20-40 cases attached to it, and I was careful to have the cases facing upstream when I put the rocks back into the water.

The humpless caddisfly larvae I'm finding right now apparently represent a second generation to show up this year (see Thomas Ames, Caddisflies, pp. 175-176.)  The first generation hatched in March and April.  I have found, it is true, some humpless cases during the winter -- both last year and this year.  Still, the cases I typically see in the summer (June and July) far outnumber those I've seen at other times of the year.

I. Brachycentrus (The American grannom).  The Brachycentrus genus ID is established by two different features.  Quoting Beaty ("The Trichoptera of North Carolina"): "...mid and hind tibia with process and spine; sa 1 sclerite always absent."  The process -- a wide "ledge" at the front of the tibia -- and spine are very clear in this microscope photo.

The sclerites at sa 2 (a position on the pronotum) are clear in the following photo of a live insect: there are no sclerites at sa 1 -- the top portion of the pronotum.

The sa 2 sclerites are clear in these photos as well.

But we can also establish the species ID for these Rapidan Brachycentrids: they're Brachycentrus appalachia larvae, for which the tolerance value assigned in North Carolina is 1.0.

"B. appalachia -- larvae up to 12 mm; mid- and hind tibia with 1 large basomesal seta and fuscous [brownish gray color] for basal third; head with 5 black stripes.  Mountains.  Relatively common."

In the photo above of the mid and hind legs, I've pointed out the large baseomesal seta on the hind tibia.  For the 5 black stripes, we can use the following photo.  (It's difficult to clearly see all 5 from any angle.)


II. Micrasema (Tiny black grannom).  Just look at this wonderful photo my friend took last week of a pair of Micrasema larvae she found in the Moormans.  She uses a high-powered macro lens with which she can get extreme close-ups.

While the larvae look large in this photo -- they're not.  Ames notes (Caddisflies, p. 182) that Micrasema larvae are typically no more than 6 mm in length (about 1/4").  Ames also has a good description of the cases these larvae make: "Their silk cases are tapering and cylindrical, straight in some species and curved in others, and often permeated with sand or plant material."  These larvae were in a bed of moss on top of a rock over which the water plunged down, and note that they have used pieces of moss to form the ribbons at the tops of their cases.  The bottoms of the cases are made out of sand.  They look quite different than the 4-sided "log cabin" cases in which we find Brachycentrus.

Beaty's genus description for Micrasema reads as follows: "Larvae up to 8 mm; first abdominal segment without dorsal or lateral humps; ventral apotome wider than long; transverse ridge on pronotum extends to anterior margin. Micrasema is almost always associated with living vascular plants (moss, Podostemum, etc.) in fast water."  ("The Trichoptera of North Carolina")

It would take a microscope photo to check on the "ventral apotome" (the underside of the head) -- and we don't have one -- but that the transverse ridge on the pronotum extends to the anterior margin" is actually easy to see in the photo posted above.  (Note: that "transverse ridge," or "divot," or "notch" on the pronotum is something we find on all Brachycentrids.)

The species ID depends a lot on the pattern at the front of the head.  We can see the front of the head in the following photo...

but I don't think there's enough detail in this photo to make a definitive identification.  I'm tempted to say they're Micrasema wataga -- a species that's "common and widespread" (Beaty) -- but I certainly don't know that for sure.

There are only two genera of Brachycentrids in the southeast -- Brachycentrus and Micrasema -- and we find both of them in local streams.

One more photo of our Micrasema humpless caddisfly larvae.  These should be around for awhile, and I hope to get some pictures of my own before the summer is over.

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