Sunday, October 5, 2014

The Rapidan River: a young Agnetina capitata and "hundreds" of caddis case-makers

It's a young one (11 mm) -- but it's already gorgeous: the common stonefly (Perlid), Agnetina capitata.  Actually, since I so commonly see Paragnetina immarginata at the Rapidan River, I thought that's what it was.  But the head pattern didn't look right -- and then I noticed the anal gills.  Had to be Agnetina.  (No anal gills on genus Paragnetina.)

Since I just wrote about Agnetinas, there's not much point in repeating the keys to identification (genus and species), but they all show up very well on this nymph.

This one has a long way to go before it's mature.  It was 5/26 that I found this A. capitata that was ready to hatch.


1. Odontocerids (strong-case makers)

But it was "case-maker" day at the Rapidan River.  I have never before seen so many case-makers at any one time in a relatively short stretch of water.  This is where I was looking.

Most prolific -- the strong-case makers.  There were hundreds of them lying on the tops of the rocks.  I don't know how many times I looked down and saw 10-12 at a time.  It was stunning.  What a hatch of "Dark blue sedges" there must be up here in the spring.    At this time of year, we often see them grazing on algae.  But they'll soon seal up those cases to pupate, hatching in April and May (see Thomas Ames, Caddisflies, pp. 210-214).

The species?  Quite sure they were all Psilotreta labida, just about the only species I see (though I have found Psilotreta frontalis, on which see the entries posted on 10/29 and 11/19 of last year).   One of the key things to look for on P. labida is the long, acute, anterolateral projection on the pronotum.  (Beaty, "The Trichoptera of North Carolina," p. 15)

I saw so many larvae this morning that I decided to only photograph those with the curved, tapered cases.  But many cases are straight.


2. Brachycentrids (humpless case-makers)

Also prolific.  I even found some in the leaf packs, though most were firmly attached to the rocks.  Species?  What we find is Brachycentrus appalachia, the one that makes these little 4-sided, tapered cases with thin ribbons of bark.  Again, I saw hundreds of them.  They too will soon seal up their cases, hatching as the "American grannom" in early spring (Ames, Caddisflies, pp. 175-181).

One very interesting photo.  The short, stubby front legs, in this photo, were fully extended.


3. Goerids (weighted-case makers)

And, yes, that's a Goera case on which the humpless case-maker is crawling.   I had not noticed Goera at all until fall of last year: today, I saw their little cases all over the place.  Species?  Goera calcarata.  Every larva I saw had the normal 2-stone case (see the entry of 9/21).  Unfortunately every larva I saw played shy.  This is the only time I saw one stick its head out of its case.


Since it's a time of year when I don't expect to see a whole lot at the streams, I was pretty pleased with my findings this morning.  And in future trips up to this river, I'll be watching out for the A. capitatas.

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