Tuesday, January 11, 2011
The "Humpless Case Maker" Caddisfly (family: Brachycentridae)
The Humpless Case Maker Caddis (live photo above) is so named for one simple reason: it has no humps. All other case makers have fleshy humps or bumps on the first abdominal segment; two on each side (lateral humps) and one on the back (a dorsal hump) -- though the Lepidostomatid case maker has only the lateral humps. Below are pictures of, first, a Limnephilid (Northern Case Maker) caddisfly, and second, a Uenoid caddis: these photos show off these humps very well.
If we look at the Brachycentrid, on the other hand, it should be clear that these humps are missing.
Case making caddisflies use their humps to stabilize themselves inside their cases, then they undulate their abdomens, in this way creating or increasing the flow of water through the case. This is how they provide themselves with an adequate supply of oxygen. With this ability, a caddis can adapt to life in slow moving waters, or waters that have warmed up, thus having a diminished supply of dissolved oxygen. The "humpless" caddis, lacking this ability, MUST live where the currents are consistently swift: they're normally found glued to rocks at the heads of riffles. In the Rapidan River, a stream that is rocky and swift, with few long pools where it flows out of the National Park, I have seen Brachycentrids literally covering rocks at certain times of the year (spring and summer). There were easily 30-40 cases on some of the rocks, all of them facing upstream -- head first -- to receive the water as it flows through the case.
In the streams in our watershed (Rivanna River) we do not normally see large numbers of these in our samples, and they are only found in a select number of streams -- Buck Mt. Creek seems to have quite a few. But we also find them in the main stem of the Rivanna. Nonetheless, the TV of Brachycentridae is 1 (scale of 0-10).
There are two Brachycentrid genera that we find in our streams: Brachycentrus and Micrasema. It is Brachycentrus that we see most often, and it's easily recognized by its case: it's the case we see in the photo at the head of this entry. It's four-sided, with neatly stacked rows of vegetation. As Voshell points out, the sides of the case look like a log cabin (J. Reese Voshell, Jr., A Guide to Common Freshwater Invertebrates, p. 153). If the larva has slipped out of its case and must be identified in a lab, that too can be easily done: Brachycentrids have a distinctive "notch" or "divot" that horizontally slices through the pronotum. In the photo below, the color change between dark brown and tan on the pronotum marks this divot's location.
The other Brachycentrid genus that we find in our streams (Micrasema) is much smaller in size, and lives in one of two types of cases. They make a case of thin ribbons of vegetation wrapped into a tube, or, they live in a cylinder composed of small grains of sand. Here are photos of each.
One last photo will give the reader a good indication of the size difference between these two genera and the cases in which they live. For the sake of comparison, the actual size of the Brachycentrus case (log cabin) is about 1/2". (Micrasema Brachycentrids are small.)
Humpless caddisflies normally hatch from April through June -- though this varies with location and elevation. I've seen lots of larvae firmly in place, not in pupation, in June and July in the Rapidan River. And according to Thomas Ames (Caddisflies: A Guide to Eastern Species for Anglers and Other Naturalists), the Brachycentrid hatch (the "American Grannom") usually occurs in August and September out in the Rockies.