Wednesday, January 26, 2011

The "Strong Case-maker" Caddis: (family: Odontoceridae)

The Odontocerid caddis larva (Strong Case-maker) has a tolerance value of 0.0.  That goes a long way in explaining why, in the StreamWatch data I'm using which goes through 2007, exactly 2 larvae are noted as having been found in this watershed's streams.   One larva was found at one of the reference sites, the other in the Doyles River.  On the other hand, I found them by the dozens in November of 2009 in the Rapidan River where it flows out of the National Park.  I guess that's pretty clean water.  They were lying loosely on tops of rocks.

The "Strong Case-maker" gets its name for the straight-forward reason that it makes a very strong case.
You won't break it by squeezing it with your fingers, and in the lab, we tried, unsuccessfully, to break one with metal tweezers.  The bits of rock that make up the case are held together by extra amounts of silk, the silk that caddisflies use to "glue" their cases together.    Thomas Ames speculates that this is done "as protection against the grinding motions of the substrate," in which they spend a good deal of their time (Caddisflies: A Guide to Eastern Species for Anglers and Other Naturalists, p. 209).

Dichotomous keys do note tiny things one can look for when working at identification.  I've never seen any reason to risk eye strain with identification of this particular larva.  Number one, the case is unique (and the "hard" case is one of the features used by the keys), and number two, also distinctive are the black and orange bands on the head.   One other thing that is striking is the clustered gills that cover the abdomen (see below) -- though such gills are not unique to this particular family.

While there is more than one genus in the Odontoceridae family, almost all larvae found are genus  Psilotreta.  This is also an easy identification to make:  with larvae in this particular genus, the front edges (anterolateral margins) of the pronotum come to a very sharp point.  The two pictures below make this point very well.

Odontocerids hatch in May and June in our region.  But they have a two-year life cycle, so they might be found at any time of the year in various sizes and various stages of development.

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