Wednesday, March 27, 2013

Mature! Heterocloeon amplum and Helopicus subvarians -- Doyles River

The water in the Doyles River has finally dropped and cleared enough to allow me to resume looking for insects.  This morning, I was hoping to find two different things: the small minnow mayfly, Heterocloeon amplum, and the Perlodid stonefly, Helopicus subvarians.  Both, by this time of the year (end of March/early April) should be mature.  And they are.

In the photo at the top of the page, a beautiful, male, H. amplum small minnow mayfly.  The color is odd -- totally black -- but I've seen it before at the Doyles and up at Lynch River.  It was also large, about 8.5 mm.  The mature H. amplum that I normally see -- in terms of the color -- is this one.

Again a male, this one was about 7.5 mm.  Here are additional photos of both.

They're beautiful insects when they're mature, and the "Blue-winged Olive" hatch should begin anytime now.

And here's the Perlodid stonefly, Helopicus subvarians.

You'll notice that this nymph was chewing away on a black fly larva: they must be pretty tasty (though I have no intention of checking that out!).

H. subvarians is the largest Perlodid stonefly I see in the course of the year (this one measured 20 mm), and for that matter, H. amplum is the largest small minnow mayfly I see in the course of the year.

I thought I might review how we ID this particular stonefly using Steven Beaty's "The Plecoptera of North Carolina," p. 21.

Genus diagnosis: Lacinia triangular, bidentate sometimes with a small tuft of setae originating on a small knob and with a row of marginal hairs approaching base.

Definitely.  He continues, "right mandible with five teeth; prominent submental gills, about three times longer than wide."  I didn't examine the mandibles, but the submental gills are easy to see.

"Frons with a complete dark transverse band through ocellar triangle; cerci with a dorsal fringe of setae."  I'm afraid that the dorsal fringe on the cerci does not show up very well in my photos: the dark transverse band on the head/face, on the other hand, is obvious.

On the species ID (H. subvarians), Beaty says the following:

H. subvarians -- nymphs 17-20 mm; anterior margin of dark, transverse ocellar band a straight line or mostly so; occipital spinules grouped into broad patch of 2-3 irregular rows.  Nymphs occur September through April in the Mountains and Slate Belt.  Relatively uncommon.

The transverse band is indeed "mostly" straight, and there are indeed 2-3 irregular rows of occipital spinules.


I hope to get up to the Rapidan River on Saturday where all sorts of things should be maturing.

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