Aquatic Insects of Central Virginia: The "Other" Freeliving Caddisfly

Saturday, February 19, 2011

The "Other" Freeliving Caddisfly

This is a freeliving caddisfly larva (family: Rhyacophilidae) that I found yesterday in Elk Run, a tributary to Buck Mt. Creek.

One of the things that I noticed in working with StreamWatch volunteers was that they invariably expected freeliving caddisfly larvae to be green. Some are (see my photos in the entry for 1/7/11); most that I've seen in our watershed streams are not! They look like the larva in the picture above, or, in some cases they're grayish purple in color. I suspect that a lot of volunteers sampling streams would count the larva above as a "fingernet" caddis, based on the burnt orange colored head. (The mistake of thinking that all freeliving caddis larvae are green is made, I suspect, by relying too much on the illustration that's found on p.164 [Plate 77] of J. Reese Voshell, Jr.'s A Guide to Common Freshwater Invertebrates of North America.)

What are the features we look for in a freeliving larva? First and foremost -- whether they're green, or tan, or gray -- their abdominal segments are deeply incised. We can see that in the photo above -- even better in the photo below of a green one. Rose Brown nailed this perfectly: she always said, "It's the Michelin man!"

Secondly -- and there are exceptions to this -- in most cases, the posterior edge of the pronotum, as in the picture above, looks like a "black collar" in the shape of an inverted "U". This shows up even better in this close-up of the larva I found yesterday.

Be careful -- fingernet caddis larvae also have a black line on the posterior edge of the pronotum -- but it goes straight across. See below.

There are three other features that help in freeliving caddis identification. 1) There are sparse hairs on the abdominal segments.

2) There is a sclerite (brown patch in the photo below) on the top of abdominal segment 9.

And 3) the anal claws are long, curved, and sharp. The first photo below is a close-up of the freeliving anal claw. The second compares the anal claws of the freeliving and fingernet caddis larvae.

(Let me careful to note that of the freeliving features I've noted, "keys" do not mention the "sparse abdominal hairs" and the "black collar" as defining characteristics.)

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One final note. When I worked in the lab with Rose Brown and John Murphy, on occasion we'd receive data sheets from volunteers listing large numbers of "freeliving caddis" in streams where it would be very unlikely to find them (remember that the freeliving tolerance value is "0"). While we couldn't be sure, we felt strongly that the larvae in question were, in reality, dark green "common netspinners."
Look at the picture below.

That's a "common netspinner," genus Hydropsyche. It could, indeed, be confused with a freeliving caddis. But, note the lack of "deep incisions" in the abdominal segments (i.e. it's not a "Michelin man"). And, remember, if you look at this with a loupe you'll see three features you will not see on a freeliving caddis larva: sclerotized plates on the pronotum, mesonotum, and metanotum; "fuzzy bellys" (frilly abdominal gills), and hairy tails. E.g. ...