Thursday, March 17, 2011

Another "Secret Stream": Discovering "Mystery" Bugs

Today I explored a very small stream -- a tributary to the Moormans River.  This is on private land, so the stream goes unnamed.  It is a rocky, boulder-strewn gem that drops down a steep ravine in a place where you keep an eye out for bears!

I didn't find a whole lot -- but this stream was virtually dry until the rains poured down last week.   I did return with some "mystery" nymphs that required identification work in the lab.   What I did find, of course, was very good.  There were:

1) A lot of young Chloroperlids (Green Stoneflies), genus Sweltsa,

2) A handful of Roach-like stoneflies (Peltoperlids),

And 3) quite a few Ameletid mayflies.

And now for the problems.  I had two insects that I couldn't ID with certainty at the stream.  Number one:

When I first saw this in the tray -- because the tails were so short (they're broken off, as it turns out) -- I thought it was another Green Stonefly, one that was more colorful, more mature than the rest.  But the wingpads weren't right.  In this photo, you can barely make out the rear set of wingpads (but you might have success by enlarging the photo), but it's clear that the front pads are long and finger-like.  My guess became --  a "Small Winter".

Having looked at this now with the scope, I'm pretty sure it's a Leuctrid -- a Rolled-winged Stonefly.  If it is, it would be the first one that I've seen this year.  The rear wingpads are crucial in making  this identification.  If they're short and stubby -- it's a Small Winter; if they're "finger-like" like the front wingpads, then it's a Leuctrid.  Here's the best photo I could get in the lab.

The rear wingpads on a Small Winter stonefly are also "truncated" -- i.e. squared off on the bottom; these clearly are not.  Hence the identification.  The other determining factor -- and I failed to get a good photo of this -- is that the "ventrolateral folds" on the abdomen are only on segments 1-4 on this insect; on a Small Winter stonefly, they run the length of the abdomen, from segments 1-9 (see the entry on "Small Winter Stoneflies," 12/19/10, for more on these "folds").

Number two:

With these wingpads, this is either a Large Winter stonefly (Taeniopterygidae) or a Nemourid (Nemouridae).  But it's not the right color for a Taeniopteryx Large Winter Stone (plus it has no "coxal" gills), and it does not have the abdominal stripes we expect on a Strophopteryx Large Winter Stone.  On the other hand, if it's Nemourid, clearly it isn't genus Amphinemura since it does not have frilly gills sticking out from it's neck.  That left me with Nemourid, genus Nemoura -- but it doesn't look like any other Nemoura that I've ever seen!

Well, microscope work showed me that this is, indeed, a Large Winter stonefly, it's not a Nemourid:  tarsal segments 1 and 2 on the leg are equal in length (see the previous entry on the genus Nemoura, 3/10/11):

 and it also has the "triangular, ventroapical plate" characteristic of the Large Winter family.

But that still left me with the problem of genus identification.  It's not Taeniopteryx (for the reasons mentioned above), and I could not convince myself that it was Strophopteryx -- no abdominal stripes (take a look at the Strophopteryx pictures in the entry for 3/2/11) -- yet those are the two genera we see in our streams.  So, off I went to the "keys".  My conclusion is -- and it's a tentative one, I am open to other suggestions -- that this is a Large Winter stonefly, genus Taenionema.  I hope I'm right; it's exciting to find something new!  My evidence is drawn from two different sources.  In Peckarsky,, Freshwater Macroinvertebrates of Northeastern North America, p. 67, there are two keys that lead to the Taenionema label: 1) "[The] proximal cercal segments [are] without long dorsal hairs," (i.e. there are no long hairs on the cercal segments [that is, the "tails"] closest to the body -- see the photo above), and 2) "[The] body [is] dark brown, pattern indistinct: legs uniformly dark brown.)  I think the nymph in our picture above fits that description.  But I also looked for confirmation at the definitive work on stonefly genera by Stewart and Stark -- Nymphs of North American Stonefly Genera (Plecoptera).   On p. 240, the authors provide a drawing of the "male terminalia, ventral" -- it looks exactly like the "terminalia" in the photo directly above: Stewart and Stark also confirm that Taenionema nymphs (specifically, Taenionema atlanticum ) are present in Virginia (p. 241).

Counter proposals are invited.

No comments:

Post a Comment