Wednesday, October 10, 2012

Hydropsyche rossi, Hydropsyche venularis: Do We Have Both Species in the Rivanna River?

I think we do, and I think this larva is H. rossi -- but I don't have a whole lot to go on.  I'll present the evidence that I find convincing, but I'm an amateur, and my conclusions should be regarded as tentative: Steven Beaty might tell me to leave this one alone!

I found the larva in the photo above on 9/29 in the Rivanna River at Crofton.  I've not posted an entry on that particular trip; this common netspinner is the only thing I found of interest (I collected three of them).  The genus is Hydropsyche -- no question on that -- the abdominal "scale hairs" are clear.

But what about species.  On 9/15 (see the entry posted that day), the common netspinners I found at this very location were Hydropsyche venularis, so let's recall the key features we use to ID that species.

Hydropsyche venularis:

I relied on Schuster and Etnier's "Manual," and Steven Beaty's "The Trichoptera of North Carolina" to make this ID.  Schuster and Etnier focus on the pattern on the head of two pairs of anterolateral spots, which are often fused to make two, large, anterolateral spots ("Manual," p. 96.).  The dots are fused on the specimen in the photo above (from 9/15).  Even better, here is a close-up.

Beaty, on the other hand, notes that, with H. venularis,  the "muscle scars on[the] side of [the] head...curve dorsad [i.e. up towards the top of the head] posteriorly" (Beaty, p. 76).  And clearly they do.

My initial impression of the larvae I found on 9/29 was that they were also H. venularis since, in some head photos, we can see two pairs of anterolateral dots (not fused).  For example:

Still, all things considered, the head pattern just did not look the same as that of H. venularis.

Hydropsyche rossi:

So I decided to check on feature two -- the rows of muscle scars on the side of the head, and here's what I found:

right side of the head:

left side of the head:

The muscle scars here do not turn up towards the top of the head, and this is the only distinction Beaty makes between H. venularis and H. rossi.  "H. rossi -- rows of muscle scars on side of head not curved dorsad posteriorly." (Beaty, p. 76)   In addition, he makes the point more than once that H. rossi and H. venularis are hard to distinguish, and identification should not be attempted if the larvae are young.  Rather, it's best to call them H. rossi/venularis.

The common netspinners I've found in the Rivanna are fairly mature, so I'm willing to go out on the limb and say that those found on 9/29 were H. rossi; those found on 9/15 were H. venularis.   That's my view at the moment.   If I find out that I'm wrong, I'll let you know.  Both species, by the way, are fairly tolerant:  H. venularis has a TV of 5.1, H. rossi, 4.8.  It would make very good sense that they both inhabit the same stretch of water.

The Rivanna River below the bridge at Crofton.

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