Wednesday, May 8, 2013

Update on our Isoperla Perlodid stonefly, species unknown

I've been discussing this Isoperla, Perlodid stonefly with Steven Beaty, and I thought I'd bring everyone up-to-speed on where things stand at the moment on determining the species ID.

I've now found three of these nymphs.  The one at the top of the page was found on 5/4 at Buck Mt. Creek.  The first one I found, I found at the upper Doyles River on 4/18, the immature nymph in the photos below.

Finally, I have this slightly damaged -- the leading edge of the pronotum is crushed -- specimen that I found at the Rapidan River on 4/24.

At the moment we'd have to say that these nymphs look a lot like Isoperla orata -- but they're not quite the same.   The safe designation right now is "Isoperla sp.," though Beaty plans to work on this further.

Let's see how they differ.  Beaty, "The Plecoptera of North Carolina," p. 24 --

I. orata -- nymphs 11.5-14.5; lacinia with apical tooth as long as sclerite bearing it with a tuft of setae below the subapical tooth; head with a dark transverse band through the median ocellus and ocellar triangle enclosed with a large pale spot; dorsum of abdomen with three dark, narrow longitudinal stripes, the central stripe often faint or discontinuous.

1. The nymphs I've collected so far are not nearly this big.  The smallest was 8 mm; the largest was 9 mm.

2. The laciniae on my nymphs do match the description.  Here is a microscope photo from the nymph at the top of the page.

However, there appear to be 4-5 setae in that tuft of setae below the subapical tooth; Beaty says that in all I. orata specimens he's seen so far, the number of setae is 3.

3.  In all of the nymphs that I have collected, there is a large pale spot in the ocellar triangle, but only one of those spots is clearly "enclosed" in front of the ecdysial line.  Here's what I mean.  This is the head of the immature nymph, and the spot is completely enclosed ahead of that line.

That is not the case with the other two nymphs where the pale spot virtually touches the ecdysial line.

3. Finally, the central abdominal stripes on the nymphs I have found are not at all "faint or discontinuous"; they look exactly like the lateral stripes.


The question, of course, is do these slight variations indicate a different species?  or, do they represent regional or individual variations?   I suspect that, in the end, there is only one way for us to know: collect some more nymphs and raise them at home until they hatch as adults.   Then, are the adults I. orata, or some other species, or a species that no one has seen which now needs a name?

That's the kind of thing I have to leave to the pros.

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