Saturday, December 2, 2017

Our "Green Stoneflies" (Chloroperlidae), genus Sweltsa, are Sweltsa onkos

For sometime now, based on the appearance of the adults,

my friend and I have felt strongly that our Sweltsa "green stoneflies" were Sweltsa onkos in terms of the species.  With the help of our new key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- we can now confirm that ID.   (And yes, if the genus is Sweltsa, there is nothing "green" about either the nymphs or the adults of these Chloroperlids.)

Let me work through our key (pp. 206-208). 

72  Tibial fringe setae sparse and about half as long as median outer marginal seta......73
72' Tibial fringe setae more abundant and about as long as median outer marginal seta......77

The tibal setae is dense and long.  We move to couplet 77.

77  Longest dorsal setae on last five cercal segments about as long as 1.2- 1.5 cercal segments.......Sweltsa mediana
77' Longest dorsal setae on last five cercal segments about as long as 2 cercal segments........78

I'd go with 77'.  Certainly the longest setae on at least the last three segments are close to being the length of 2 cercal segments.

78  Abdominal terga dark brown, occiput about as dark as frons; meso- and metanota rather densely covered with long dark clothing hairs.......Sweltsa onkos
78' Abdominal terga pale brown, occiput almost entirely pale; meso- and metanota with sparse, brown clothing hairs.....Sweltsa naica

That's a no-brainer: Sweltsa onkos for sure. 

As we go through the winter, I'll keep checking Sweltsa nymphs just to be sure that onkos is the only species we have in our streams.  I can certainly say that every nymph that I have preserved keys out to onkos, and the nymphs in my photos match that description as well.

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