Friday, January 7, 2011
"Free-living Caddisflies" (family: Rhyacophilidae)
Free-living caddisflies are "free living" because they do not depend on a case for protection (as do their case making cousins), nor do they build temporary retreats from which they emerge to clean food from their nets (as do the "netspinning" caddisflies: common netspinners, fingernet caddis, and trumpetnet caddis). The family Rhyacophilidae is the oldest caddisfly family for which there is fossil evidence; it goes back to the Jurassic period (see Glenn B. Wiggins, Caddisflies: The Underwater Architects, p. 76).
Thus, the free-living caddisfly stands at the head of caddisfly evolution. Current thinking is that netspinning caddisflies evolved from the free living caddis, then the "temporary" retreats of the netspinners became permanent cases for the case makers.
The free-living caddis is for the most part predaceous as an eater, and it requires a habitat of cold, well oxygenated water, tending to live in rocky, small, head-waters streams. That means we don't see them a heck of a lot in the watershed that I explore! The larva in the photo above was in a sample we did last summer in the Doyles River where it flows out of the National Park. I've also found them in both of the reference sites used by StreamWatch, and I've seen them in Raccoon Creek and the North Fork of the Moormans.
Not all free-living larvae are green. In fact, most larvae I've seen are a purplish gray color with a solid orange head (see the photo at the bottom of the page). But when they are green, they can be confused with common netspinners by inexperienced monitors (since common netspinners are normally green). Fortunately, the free living caddis has a number of features that make it distinctive.
What stands out most clearly is that the abdominal segments are deeply constricted -- more so than you would ever see with a common netspinner. Also note that the three segments that follow the head are not sclerotized plates. Finally the anal prolegs (the "tails" at the end of the abdomen) have a long anal claw, not the bushy filaments of the common netspinner. These are especially clear in the photo below (also note the "sparse" hairs on the abdomen -- another common trait).
If there is any doubt about the identity of this caddis larva, the monitor should preserve the specimen and let the lab experts decide.