Friday, November 4, 2016

Agnetina annulipes (the Southern Stone) at Buck Mt. Creek

Not really the stonefly I was expecting to see this morning.   While this species is common in the Rivanna at this time of year (see the entry from 9/24), this is only the second (third?) time that I've seen it at Buck Mt. Creek.  Agnetina annulipes, the Southern Stone.

The key features are easy to see in this photo.  Agnetina nymphs have a complete setal row on the occiput, and anal gills are present.  And on annulipes nymphs -- vs.  what we find on flavescens and capitata -- tergite 10 is completely dark.

It's one of the smaller stonefly nymphs that I see.  This one measured just a little over 5 mm.  Not sure how much it will grow as it matures.


Another surprise this morning -- and like A. annulipes, a nymph that's fairly common in the Rivanna but uncommon here -- the giant stonefly, Pteronarcys dorsata.  This one measured 21 mm.

Distinguishing features?  The lateral corners of the pronotum are "produced," those in front almost hook-like.  Also distinctive are the "dots" on the abdominal terga.  While only 4 rows are visible in this photo, there were actually 5.

P. dorsata, you may recall, is the most tolerant of the "giant" stoneflies with a TV of 2.4.  (For more on P. dorsata, see Steve Beaty's "The Plecoptera of North Carolina," vers. 4.0, p. 77).

And while we're on "tolerant" insects, the most common insect I found today was the flatheaded mayfly, Maccaffertium modestum.  (TV, 5.7).  It may be the only Maccaffertium species that I see in this stream -- but I'll have to check that to be sure.    Here are photos of three of the nymphs that I found.

There are two different patterns  -- both dorsal and ventral -- on nymphs of this species.  On this type, the armature (dark bands) on the femora is very pronounced while ventrally, we find "anteromedial bars" on segments 8 and 9, that on 9 in the form of an inverted "U".  (See Beaty, "The Ephemeroptera of North Carolina," vers. 4.0, p. 35, also Bednarik and McCafferty's "Biosystematic Revision of the Genus Stenonema," Canadian Bulletin of Fisheries and Aquatic Sciences, 201, 1979), p. 68 figure 67)  The venter looks like this.


I was expecting to see lots of small winter stoneflies in the leafpacks today -- and I did find a few.  Unfortunately, when I went to take photos, they had disappeared from my bowl.  Alas!

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