Tuesday, April 19, 2011
Stream Report: Long Island Creek -- Important Revisions!!!
Having looked through my microscope at the insects I brought home from Long Island Creek, I find that I have some revisions and additions to make to the entry I posted this morning.
First, the picture above does not represent a revision so much as an addendum: this is a microscope photo of the Nemourid stonefly for which live photos were previously posted. In this, the image is sharper, so the Nemourid features noted this morning can be more readily seen. (But his one tail is still missing!)
Second, I mentioned that I had found some black fly larvae. When I looked at them under the scope I realized that they were genus Simulium, not genus Prosimulium, the genus I've been finding all winter.
My feeling last year was that there was a changeover from Prosimulium to Simulium larvae sometime in April, and this provides more evidence for making that point. Remember that Prosimulium black flies have a tolerance value of 2-3: Simulium, in most lists, are given a value of 6. But, it is apparently the Prosimulium black flies that amass the huge "colonies" we see in the winter: Simulium black flies seem to stick to rocks in smaller clusters. (John Murphy suggested this to me, and I intend to pay careful attention to this through the spring.)
Two features distinguish Simulium larvae from Prosimulium: 1) Simulium antennae are clear; Prosimulium antennae have a black tip (this was mentioned in a previous entry): and 2) on Simulium larvae the "postocciput" (the back edge of the head) is incomplete, leaving a big gap at the base of the head. We can document those features in the larvae that I found this morning.
2) postocciput and gap
The third thing I discovered when I looked at the bugs through my scope? -- what I had called a "trumpetnet" caddisfly (Polycentropodidae) was not: it was a fingernet caddisfly, and I think it's genus Wormaldia. That would make it a first for me. This is one of three fingernet genera, the other two being Chimarra (our most common), and Dolophilodes. Of the three, Wormaldia is the least tolerant of stream impairment with a TV, in some lists at least, of .4 (Chimarra is 2.8; Dolophilodes is 1). So, "if" I'm right -- and that's a big "if" -- this would be very exciting. Here's a picture of the larva in question, then we'll look for the features we need to find for confirmation.
Wiggins -- Larvae of the North American Caddisfly Genera (Trichoptera), p. 316 -- notes three features that distinguish the Wormaldia genus. 1) "The anterior margin of the frontoclypeal apotome is symmetrical." That is, the front edge of the head is symmetrical. Frankly, I can't really be sure of that from what I can see with my scope, but I think that it is: the piece sticking out from the head in the picture below is part of the mandible.
2) "The fore trochantin projects freely as a small, knob-like process," and 3) "On the ventral surface of the head, seta (hair) no. 18 is ... stouter than any of the dorsal head setae." Those features I think can be seen in the following photo. (Please click on the photo to enlarge the image.)
I invite other opinions. If there are experts out there who feel I am mistakenly reading the data, I am happy to be corrected. Until then, I think that this is a Wormaldia fingernet caddis.