Saturday, August 4, 2012

Hydropsyche venularis: The Common Netspinner that Flourishes in the Rivanna

I was on a mission today: collect, photograph, and identify the common netspinner (Hydropsychidae) that
flourishes in the Rivanna during the summer.  If you pick up a rock that's covered with that gnarly vegetation and wait a few seconds, you'll see them starting to move all over the place.  Of course, I can't really be sure that there's only one common netspinner species in the Rivanna, and I plan to see what I find at other sites very soon (I was at Darden Towe Park this morning), but I suspect that this is the main one we see.  It is, as you might expect, fairly tolerant (TV = 5.1); it's also "very common and widespread" (Beaty, "The Trichoptera of North Carolina," p. 76).

Let's look at more photos before we get down to the work of determining species ID.

Close-up of the head:

Ventral view:


Beaty (p. 76) is not very helpful on identifying this species since he does not go into a lot of detail.  He does note that H. venularis can be confused with H. rossi, so let me begin with his description of H. rossi (which is also very brief):   "... rows of muscle scars on side of head not curved dorsad posteriorly; yellow band behind eye separates dorsal and ventral pigmentation."  That yellow band is very clear in the close up of the head that we just saw.  Look again.

Where H. venularis differs from H. rossi, is that on H. venularis larvae, the "muscle scars on [the] sides of [the] head in rows ... curve dorsad posteriorly."  I.e. the rows turn up at the back.  Have a look.

So we can probably determine the ID as H. venularis using Beaty, but for greater detail, let's turn to Schuster and Etnier's "A Manual for the Identification of the Larvae of the Caddisfly Genera Hydropsyche Pictet...," pp. 96-97.

Their description begins "Frontoclypeus with two pairs of yellow spots anterolaterally; anterior spots larger than posterior spots.  Spots often fused to form two, large, anterolateral spots."  This feature is not necessarily clear in our live shots, but it is clear in a microscope photo, where the spots are almost fused into one.

Also visible here are the "Two, large, posterior, sub-rectangular spots on top of genae."  Their description of the rows of muscle scars on the side of the head is phrased in this way: "Laterally behind eye, bronze with many small, yellow spots [i.e. the muscle scars] on four or five rows."

I'll note one final feature from their description: "Posterior half of frontoclypeus with minute spine-like setae located in yellow pockets."  Sure enough.  (Click to enlarge to see those "spine-like setae".)

Mission accomplished.  But I do want to look at more Rivanna netspinners over the summer to see if this is the only species around.  It was the only one that I saw this morning.

Other photos taken this morning --

1) Some nice shots of a male Heterocloeon curiosum small minnow mayfly.

And 2) our summer spiny crawler -- Serratella serratoides (missing one of his tails).


But this is the one I was after: Hydropsyche venularis.

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