Wednesday, April 18, 2012
Going, Going -- Well, Not Quite Gone: "Yellow Sallies" (Isoperla namata)
I took this photo of a "Yellow Sally" last year on May 14.
"Yellow Sallies" are the adult, terrestrial form of the Perlodid stonefly Isoperla namata. At the moment, they're hatching throughout central Virginia. The hatch will continue into May, but I'm now seeing very few nymphs in the streams that I visit (I only saw one yesterday at Buck Mt. Creek).
I've been taking photographs of these nymphs -- this species -- every month, beginning in November last year. So, I have a good set we can use to trace the changes this insect goes through as it develops. Let's look at the following photo -- which I took one week ago on April 11 -- as a reference point to use as we go along.
The nymph in this photo is fairly mature -- the wing pads are starting to darken. Remember that I. namata Perlodid stoneflies have a dark transverse band on the head, from which two black lines drop down to the rear ocelli (lines in front of eyes). Often, there are two dark spots/lines at the rear of the head, though on some nymphs these rear spots/lines are missing. There are three stripes on the abdomen (this is true of all Isoperlas), two on the sides (lateral) and one in the middle -- the middle "line" is made by a sequence of "crosses" on each of the abdominal segments. Finally, when the nymph is mature, the rear wing pads angle away from the body more sharply than the front wing pads (note the angle of the arrows) -- but this is true of all Perlodid stoneflies.
Sample 1: November 8, 2011
Since the head pattern has not yet developed, I can't be positive of my identification. But, it was an Isoperla, and since I. namata is the first Isoperla we see at the start of any new season, I think I. namata is a reasonable guess. Features: 1) it is possible to make out the lateral stripes on the abdomen (click on the photo to enlarge it): 2) note that the rear edges of the wing pads at this point seem to be straight across, if not convex in shape.
Sample 2: November 18, 2011
We can see all three abdominal stripes. And on the head, the light spot at the front of the head is starting to show, and we can start to make out the "dark spots/lines" at the rear of the head. Note that the rear edges of the wing pads are starting to bow inward, more true of the rear wing pads than the front wing pads.
Sample 3: December 13, 2011
While the abdominal stripes are not very clear on this nymph, the head pattern is starting to round into form. We can see the transverse band, and we can see the black lines dropping down from it touching the rear ocelli, though they're still on the "thick" side at the moment.
Sample 4: December 26, 2011
The head pattern looks much the same as on the previous nymph. But note how you can distinctly make out the "crosses" forming the middle abdominal stripe.
Sample 5: January 4, 2012
No doubt about this nymph being Isoperla namata. The three abdominal stripes are prominent now, and the head pattern is well defined. However, note how the two "lines" that drop down from the transverse band on the head are still rather "thick" -- i.e. they've not really thinned into lines. Wing pads are still straight across on the back edges, though bowing in slightly.
Sample 6: December 21, 2011
One significant change: those lines that connect with the ocelli are thinning out as the pale space between them widens. Rear edges of the wing pads remain much the same.
Sample 7: February 2, 2012
The abdominal stripes are fully formed, and the head pattern is fully developed -- but, note that this is one of the I. namatas that does not have the dark spots/lines at the rear of the head. Big changes: 1) color -- the nymph is now a pale yellow/orange, shades that will soon be enriched; and 2) rear edges of the wing pads -- note how they are bowing/arcing in towards the middle.
Sample 8: February 23, 2012
Note the wing pads: they are much longer, and the rear wing pads have started to flare away from the body. The front and rear wing pads now move away from the body at different angles.
Sample 9: March 9, 2012
Richer colors. The lateral abdominal stripes are a little bit thicker, while the "crosses" in the middle stripe stand out in a prominent way. The wing pads are longer and more fully patterned than those in the previous picture.
Sample 10: March 28, 2012
This is a nymph that is fairly mature: the wing pads have started to darken.
Sample 11: April 11, 2011 (I had to go back to last year's collection for this one.)
Fully mature and ready to turn into the adult terrestrial that we see at the top of the page!