It's another dark, dreary day here in Virginia -- a good day for doing some microscope work! So I thought I'd do some diagnostics on some of the insects I found yesterday.
First -- I mentioned in an "addendum" to my post yesterday that I had found a couple of black flies, and that they were two different genera: Prosimulium and Simulium. This is the first time this has happened to me -- I've always found the same genus on the same day. Since my finding so far has been that black fly larvae here in the winter are Prosimulium in genus and that they change over to the more tolerant Simulium genus in spring, I think what I saw yesterday was a stream in transition. Let me be clear that this is an hypothesis -- not a proven fact.
How do we tell these genera apart? Let me read from Barbara Peckarsky (Freshwater Macroinvertebrates of Northeastern North America, pp. 204-207) on the two key features that we have to find. The first has to do with how far the "postocciput"(bottom of the head) extends around the back of the head. Prosimulium: "Postocciput nearly complete dorsally and enclosing cervical sclerites." Simulium: "Postocciput with a distinct and usually wide gap dorsally, not enclosing cervical sclerites."
I would conclude that the larva on the left in the photo above (I encourage you, as always, to click on the photo to enlarge it) is Prosimulium while the one on the right is Simulium. I have to admit that the postocciput on the Prosimulium larva does not seem to completely "enclose the cervical sclerites" -- but I think it's close enough. The gap between the postocciput edges is very small (vs. the one on the right).
The second critical feature -- the color of the antennae. Prosimulium: "basal 2 segments of antennae pale, strongly contrasting with darkly pigmented distal segments." Simulium: "basal 2 segments of antennae at least partially pigmented yellow to brown, not strongly contrasting with distal segments."
The photo below gives us a pretty good look at the antennae: Prosimulium is bottom left; Simulium, top right. (Please click to enlarge.)
The other diagnostic photos I'm posting today are to show how we know that the "Northern Case-maker" (Limnephilid) caddis that I found yesterday is genus Pycnopsyche. Here is a photo again of that larva.
And here is a look at another Pycnopsyche -- still in its three-sided leaf case -- that I found in this stream the last time I was here.
Again, let me read from Peckarsky on the key traits of the Limnephilid, Pycnopsyche. "1) Head pale with dark scars and blotches; a small sclerite at posterior of each lateral hump; case of vegetable matter or sand." And 2) "Metanotal SA1 sclerites not fused medially, although may be close together or joined by a suture." We already have the case of "vegetable matter": here are photos of the rest of the features that we need to see.
1) Head -- light colored with dark scars/blotches
2) Sclerite (yellow patch) to the rear of the lateral hump
and 3) Metanotal sclerites not fused (the metanotum is the third segment back from the head)
Clearly not fused: genus Pycnopsyche. What a nice way to spend a crappy spring day!
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