Sunday, June 2, 2013

Rapidan River, Part II.: The Perlodid stonefly, Isoperla orata -- making the case

This is the Perlodid stonefly, Isoperla orata.

This, too, is Isoperla orata.  These are two of the four I. oratas that I found yesterday at the Rapidan River in Madison county.   This is the first time that I've seen this species, and it has been added to our EPT list (see the entries of 9/8/12 and 10/3/12).   This is a very intolerant insect, assigned a tolerance value of 0.0 by the North Carolina Division of Water Quality.

Since I have previously discussed the possible identity of three other nymphs I have found as Isoperla orata (see the entry of 5/8/13), I want to look at the identity of yesterday's nymphs in some detail.  I will return to the identity of those three previous nymphs at the end of this entry.

I will use two sources to establish the identity of Isoperla orata.  1) Steven Beaty's description in "The Plecoptera of North Carolina," p. 24, and 2) the much more detailed description of this species in T. H. Frison, "Studies of North American Plecoptera: With Special Reference to the Fauna of Illinois," pp. 323-325, in the Illinois Natural History Bulletin, 22 (2), 1942.  Frison was the first to describe I. orata as a new species.

I. Steven Beaty

I. orata -- nymphs 11.5-14.5 mm: 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 of discontinuous.  Nymphs primarily occur in the Mountains in medium to small mid-elevation streams from March through May.  Uncommon.

1. I'm not sure why NC I. oratas are as big as they are.  They nymphs that I found yesterday were ~ 8 mm -- which is exactly what Frison says they should be.  Frison, discussing the nymphs, "Approximately mature specimens with a body length of 8 mm."

2. Here is a microscope photo of the lacinia of one of the nymphs from yesterday.  The long apical tooth is very clear; so too is the "tuft of setae below the subapical tooth."

In private correspondence, Beaty has told me that with all of the I. oratas he's seen, there are exactly three setae in that tuft.  While it's difficult to count the setae using this photo, to me it does look like that there are three.

3.  Here are close-up views of the heads of the two nymphs in the photos above.


All of the features noted by Beaty show up as described.  True, the "pale spot" in the ocellar triangle of nymph #2 is not really "large," but I suspect that is because this nymph is very mature and dark markings tend to get darker as taxa mature.

3.  On the abdominal stripes -- here is a close-up of the abdomen of the nymph at the very top of the page.

The median stripe is not really entirely "faint," though that is true of the bottom of the stripes on terga 6 and 7, and the stripes on terga 9 and 10 are, in fact, discontinuous.

I'd call that a match -- Isoperla orata -- and this ID is even more certain when we look at Frison's description.

II. T. H. Frison (pp. 324-325)

NYMPH. -- General color yellow with dark markings on dorsum of head, thorax and abdomen.  Legs, antennae and anal cerci dominantly pale yellow.  Short, stout, spinelike setae, in addition to longer hairs, present on body and legs...  Pronotum broader than long with dark markings on disk.  Longitudinal dark stripes on abdominal tergites tend to be connected on hind margin of segments by narrow transverse line which gives tergites somewhat the appearance of having cell-like light spots each side of median, longitudinal stripe.

(Note: I have not cited Frison's entire discussion since there are things that don't seem relevant to our purpose [e.g. that the "ocelli are in the same relative position as in the adult"].  Also, I have italicized the discussion of the form of the stripes on the abdominal tergites since it seems to me very important.)

1. The colors and markings of the two nymphs in our photos are as they should be.

2. And, if you enlarge those photos, the "long hairs" on the body and legs show up clearly.  (Look at the photo of nymph #2.)

3. Yes, the pronotum is broader than long with dark markings.

4. On p. 324, Frison provides an illustration of the lacinia.  It looks exactly like the lacinia in the photo provided above, and, there are "three" setae in the tuft of setae below the subapical tooth.

5. On p. 325, Frison provides a full illustration of the nymph, and the head pattern is as Beaty describes -- the pattern we find on the heads of our nymphs.

6.  On the appearance of the abdominal segments -- let's look once again at the abdomen of nymph #1.

Perfect!  The "longitudinal dark stripes on abdominal tergites tend to be connected on hind margin of segments by narrow transverse line [my "dark stripe"] which gives tergites somewhat the appearance of having cell-like light spots each side of median, longitudinal stripe."  This photo looks exactly like Frison's illustration on p. 325.

The nymphs I found yesterday were Isoperla orata.

Now, where does that leave us with the identity of the three nymphs that we've looked at before?  These.

If you'll look back to the entry posted on 5/8, you'll see why Beaty has reservations about calling these Isoperla orata.  And now having seen the real Isoperla orata, I have reservations as well.  The abdominal markings just aren't the same, and there seem to be 4-5 setae in the tuft of setae on the lacinia (look at 5/8), and there are semi-circular gray spots behind the ecdysial line on the heads of these nymphs that do not show up on I. orata.   While Beaty has not yet expressed his final opinion, he has recommended we leave these at Isoperla sp. -- that is, "Isoperla, species unidentified."  They may be -- and this is me speaking -- close enough to I. orata to be called Isoperla nr. orata -- i.e. an Isoperla that is close in appearance (nr. = "nearly") to Isoperla orata.  That's, of course, unofficial, but it's the designation I'll use in our EPT list.
More photos of the two nymphs at the top of the page.

No comments:

Post a Comment