Tuesday, January 21, 2014

Large winter stoneflies: how to distinguish Taenionema nymphs from Strophopteryx nymphs

At the moment we're seeing all three of our large winter stoneflies: Taeniopteryx burksi/maura (probably burksi), Strophopteryx fasciata, and Taenionema atlanticum.   These are all photos that I've recently posted.

T. burksi/maura:

S. fasciata:

T. atlanticum

T. burski/maura nymphs are already hatching: S. fasciata and T. atlanticum will probably start hatching sometime next month.  It is common to see T. burksi/maura and S. fasciata in the same streams, and I sometimes see T. atlanticum and T. burksi/maura together (like Sunday at the Rapidan River);  I don't know that I've ever seen T. atlanticum and S. fasciata in the same riffles. T. atlanticum nymphs prefer mountain streams, especially headwater/1st order streams.

Now, to my point.  When I first found Taenionema nymphs I was told by entomologists to be sure they were not Strophopteryx nymphs.  When you look at the photos above you'll wonder how on earth the two species could be confused.  Strophopteryx fasciata is very distinct: the head, pronotum, and thorax (wing pads) are mottled -- a mix of yellow and brown -- and the abdomen is clearly banded;  Taenionema atlanticum is totally brown.  Here's the catch -- while there is only one Taenionema species found here in the East (atlanticum), there are three Strophopteryx species here in Virginia  -- S. appalachia, S. fasciata, and S. limata -- and, S. limata, like T. atlanticum, is totally brown and is also found in the mountains (Beaty, "The Plecoptera of North Carolina," p. 7; Stewart and Stark, Nymphs of North American Stonefly Genera, p. 238).

So if you're out sampling this winter and start seeing brown, large winter stoneflies, chances are good that you've run into T. atlanticum nymphs, but there's the odd chance that you've found S. limata.  How to be sure which you have?

All Strophopteryx nymphs have a "ventroapical plate" which you can see by placing the nymph on its side.  It looks like this on S. fasciata.

But, Taenionema nymphs have this feature as well.  So you need a direct, head-on, as it were, view of the ventroapical plate.  This is how you tell the two species apart.

Strophopteryx ventroapical plate.  The plate is narrow and bends in at the sides.

Tanenionema atlanticum ventroapical plate.  The plate is broad, and the sides move down in a uniform way.

(For good illustrations of both ventroapical plates -- and how to distinguish the male from the female -- see Stewart and Stark, pp. 237 (Strophopteryx) and 240 (Taenionema).  There are probably very few readers who will need to make this distinction -- but that's how it's done.

With predictions of snow and freezing cold weather about to move in, I made a quick trip yesterday to the Doyles River in search of our "winter" small minnow mayfly, Heterocloeon amplum.  I did find a few, but they're still very small.

It will be quite awhile before we see the beautiful mature males and females.

Always something to which we can look forward.

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