Monday, August 27, 2012

Arctopsyche grandis: An Important Common Netspinner in the Northwest

The common netspinner caddisfly larva in the picture above was one that I found -- you may recall -- in the Blackfoot River on 8/14; the one in the photo below is the one that I found in the Salmon River in Oregon on 8/19.

They turned out to be the same species, Arctopsyche grandis, and the yellow stripe that goes through the center of the head and the thorax is an important key to this identification.

Arctopsyche common netspinners can be found in the East, but according to Steven Beaty ("The Trichoptera of North Carolina," p. 72), they are only found in the mountains, and they're uncommon.

Let's work on the species ID by starting with the genus ID, using Beaty's description:

Ventral genae of head entirely separated by apotome which narrows posteriorly; thick sa2 and sa3 setae on abdomen not arranged in tufts...Usually collected on large bouders and in fast currents.

We might add that unlike the two genera we commonly see in Virginia -- Cheumatopsyche and Hydropsyche -- this is a genus in which the larvae have a single point on the fore trochantin; i.e. the fore trochantin is not "forked."  That feature is visible in one of the photos I took on 8/19.

In another photo I took of that same larva at the Salmon River, we have a clear view of the ventral genae and the apotome that "narrows posteriorly":

And here are microscope photos of the ventral apotome of both of the larvae.



For the thick setae at the sa2 and sa3 positions that are "not arranged in tufts," I can provide the following microscope shot:

The setae are clearly "thick," but there are no clusters of tufts.  (To see what those clusters would look like -- on Parapsyche larvae -- look at Figure 18.79 in Merritt, Cummins and Berg, p. 492.)

For the species ID of grandis, I had to turn to "Fly Fishing Entomology: Pacific Northwest Caddisflies" (, where we find the following on Arctopsyche grandis: "All males and most females have a yellow stripe that goes down the middle of the top of the head and thorax."

Arctopsyche grandis.

A. grandis is a common and important netspinner in the northwest, one for which fly fishermen had better have imitations.  Gary LaFontaine said the following on this species in his well-known study, Caddisflies (p.232):

"This is the most abundant and widely distributed western species in the genus.  It is a mountain caddisfly, associated mainly with pristine rivers and streams over 1,500 feet in elevation.  It is common in the Cascade range throughout Oregon, Washington, and British Columbia; in both the Siskiyou and Sierra Nevada ranges of California; in the Rocky Mountains, down the continental spine, through Montana, Colorado, and Wyoming; and in the Sangre de Cristo Park, and San Juan mountains of Colorado and New Mexico.  A. grandis is one of those special insects that is worth making the focus of a fishing trip because it gets the best trout in a river interested in feeding."

Below, the "pristine" waters of the Salmon River near Welches, Oregon.

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