Tuesday, December 21, 2010

Lickinghole Creek: Uenoid Caddisflies

I saw a good number of insects in Lickinghole Creek (near Crozet) yesterday -- but it was pretty much "the usual suspects": e.g. large winter stoneflies (both Taeniopteryx and Strophopteryx -- mainly the former) and  small winter stoneflies, a number of very large midges (Chironomids), a fair number of common netspinning caddisflies, and a few brushlegged mayflies.   But I also saw a large number of Uenoid case-making caddisflies, and they are increasing in size from my first sighting (last week at Buck Mt. Creek).

Cases of the caddis family Uenoidae often show up in our streams in early December, when their cases can be very small  (1/8"?): they look like compacted grains of sand that are stuck on a rock.  They can be prolific, with 10, 12, or more cases stuck to a single rock in a stream, but they are normally only found in small to mid-size streams (10'-30' wide); I have never seen them in the main stem of the Rivanna.  Their cases are made from grains of sand and small pebbles, but typically there are 3-4 larger pebbles attached to each side of the case (as in the photo above).   Cases can be colorful or drab, depending on the mineral composition of the stream in which they are found.  By February, they start to seal off both ends of their cases for pupation, often clustering together in large numbers -- 30-40 cases might be seen side-by-side on a rock.

(A young Uenoid that has slipped out of its case.)   Identification of the Uenoid cannot be determined for sure in the field.  However, since we know this is the most common case that we see in the winter in our streams, we tend to know what they are right away.   But Uenoid caddisflies can be confused with certain genera of Limnephilid caddisflies (Northern casemakers) who make a similar case out of pebbles and stones.  (Until recently, the family Uenoidae was considered to be the genus Neophylax in the Limnephilidae family.)  Exact determination requires microscope work in the lab.

There are two anatomical features that make a Uenoid a Uenoid.  They are 1) a "medial notch" in the anterior margin of the mesonotum.  In the photo above, that's the "W" shaped notch in the top edge of the brown, rectangular sclerite between the second set of legs.  (There is no such notch on the mesonotum of the Limnephilid caddis.

And 2) there is a "T-shaped" sclerite on the underside of the head on a Uenoid caddis: in the photo above, this is directly in the center.  For some beautiful photos of the terrestrial form of this insect, the reader is advised to consult the book by Thomas Ames Jr., Caddisflies: A Guide to Eastern Species for Anglers and Other Naturalists (Stackpole Books, 2009), pp. 257-262.

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