Tuesday, September 10, 2013

The Main Story Was Stoneflies -- Young and Old -- at the Rapidan River

Obviously one of the "old" ones with those very black wing pads: Paragnetina immarginata.  This one was small, 15 mm.  This one was large, 25 mm.

What accounts for the differences in size and abdomen color?  I don't really know, but I wonder if it's a matter of gender.  In any event they're both the same species.

P. immarginata -- nymphs ?? mm; head M-pattern with medial pale line extended anteriorly, often connected to pale frontoclypeal margin; yellow femora distinctively patterned with dark brown longitudinal bar extending about 2/3 the length; abdominal terga banded, anterior half dark; anal gills absent.  (Steven Beaty, "The Plecoptera of North Carolina," p. 18)

We can see those features on both of the nymphs, but let me use the big nymph for purposes of illustration.

Both of these stones have "missed the boat" when it comes to the peak hatch of the Golden Stones: that took place in July and August, and stonefly "shucks" were all over the place on the rocks in the river.

Note how the head and thorax splits open to allow the adult to emerge.   Most of the mature nymphs have hatched, but the leaf packs were jammed with small and mid-sized Perlids, both P. immarginata and A. abnormis.  Some of those will be hatching next summer; some have two years to go.

Other insects around?  Some small minnow mayflies, mostly Acentrella nadineae, and still some E. vitreus flatheaded mayflies,  lots of common netspinners and fingernet netspinners (Dolophilodes), and caddis casemakers -- Saddle-case makers (Glossosomatids), Humpless case-makers (Brachycentrus appalachia), and some Strong case-makers (Odontocerids), the case-maker we commonly see here in the fall.

Odontocerid, Psilotreta labida

Brachycentrus appalachia (quite a few of their "log cabin" cases attached to the rocks)

And then there was this Giant stonefly.

Do you recognize it?  It's Pteronarcys proteus, the same species we see in the small streams in Sugar Hollow.   This is the first time I've seen it up here.  Actually, I've probably seen it before and mistakenly thought it was the species I'm used to finding in the Rapidan River, Pteronarcys biloba.  This one.

Note how the lateral abdominal hooks point sharply away from the body on P. biloba while they're "appressed"  -- i.e. tucked in close to the body -- on P. proteus.  Also, while there are sharp hooks on segments 1-8 on P. biloba, on P. proteus, the hook on segment 7 is small and virtually non-existent on segment 8.

So last week we discovered two Giant species in Buck Mt. Creek (P. dorsata and P. biloba), and today we find the same thing at the Rapidan River (P. proteus and P. biloba).

Always a treat to make this trip to Madison County.

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