Saturday, January 15, 2011
Where are the Glossosomatids (Saddle-case Makers)?
I'm not finding them. My "stream notes" from winter 2009 indicate that by now I should be finding them in pretty good numbers. I've been to Lickinghole Creek; to the Lynch River; I've been to the Doyles River; and I've been to Buck Mt. Creek, all places where they've been in the past. So far, I think I've found two or three. I guess this might be too early: maybe February will bring me more sightings.
The "Saddle-case maker" (family: Glossosomatidae) is a "case making caddisfly" only in a qualified way. In the three caddisfly groups now distinguished by the expert in all of these things, Glenn B. Wiggins (see his Caddisflies: The Underwater Architects, pp. 11-28) -- 1) Case-making Caddisflies, 2) Retreat-making Caddisflies (netspinners), and 3) Cocoon-making Caddisflies -- "Saddle-case makers" are, strictly speaking, "Cocoon-making Caddisflies." The main reason for this is that in contrast to the true "case-maker," the Glossosomatid does not make a permanent case until its final instar, i.e. when it's preparing for pupation. True "case-makers" live in the same case throughout their various instars; as they grow, they simply add material to their existing cases. As it grows, the Saddle-case maker abandons its "too small" case and builds a new one from scratch. In waiting to build a permanent case until its final instar, the Saddle-case maker belongs with the Free-living caddis and the Micro caddis (family: Hydroptilidae). Together, this group is known as the Spicipalpia caddisflies. (For another look at the arguments forming this case, see Thomas Ames Jr., Caddisflies: A Guide to Eastern Species for Anglers and Other Naturalists, pp. 88-89.)
The Saddle-case "case" is shaped like a dome and is roughly cobbled together from whatever pebbles and grains it finds in the substrate. The larva "wears" this dome, covered much as a turtle is covered by its shell, and if you lift this case off of a rock (they're usually visible right on top of the rocks) and turn it over, you can often see the larva as it straddles its "saddle," head sticking out at one end, the abdomen and anal prolegs at the other -- as in the photo below. (Head to the left; anal prolegs sticking out at the right.)
But people who sample with nets only rarely see the larva and case together. When disturbed, the Glossosomatid quickly abandons its case. Thus it is the larva alone that shows up on the net. A Glosso that is not in its case is hard to identify at the stream. So for this, it's back to the lab and microscope work.
The feature that gives this one away -- in addition to the "Hand's up; you're under arrest" position of its front legs! -- is the fact that the "anal prolegs" at the very end of the body are almost fully attached to the end of the abdomen; only the very tip of the proleg projects beyond the body (in the photo, the "anal proleg" is covered with a brown, rectangular sclerite). Also note that only the pronotum is sclerotized with this larva: the mesonotum and metanotum are both "fleshy" like the abdomen segments.
Glossosomatids hatch in late spring and early summer, so we often see them on rocks in the winter and early spring. In our watershed, they are normally present in pretty good numbers in Buck Mt. Creek, the Doyles River, Lickinghole Creek, the Moormans River, the Lynch River and Raccoon Creek. We often find them in the same streams and at the same time as we find the Uenoid caddis -- which is a true "case-maker".
(It's worth noting that not all Glossosomatid cases are as neatly put together as the one pictured above.
In some cases, not much thought goes into construction -- as in the picture below!)