Nymphal growth, instars, and frequency of molting

Yesterday, I went to the lower Doyles River -- shortly before it enters the Moormans.  While I didn't see much of interest, I did find -- as I often do at this time of year -- a Perlid nymph that had recently molted.  At this stage, early in a new instar, it has not yet sclerotized (darkened with patterns); it has just shed its previous exoskeleton, and the cuticle is in the process of hardening.  When that happens it will look something like this (a second young nymph that I found yesterday).

While I see freshly molted nymphs at various times of the year, if memory serves, I see them more frequently at this time of year -- September through December.  (E.g. here's one that I found on September 26, 2012.)

Why?  Is it possible that growth is more rapid during late summer and fall?  Rapid growth would mean a shorter time between instars, and that would increase the odds of seeing this kind of nymph.

Last night I dug around to see what I could find in the internet sources, and I suspect this conclusion is true.  The key thing -- stonefly nymphs grow rapidly throughout the fall then slow down in the winter.  Data supporting this fact was compiled by George L. Vaught in his M.A. thesis, "The History and Ecology of the Stonefly Neoperla Clymene." (1974, North Texas State University)

In his thesis Vaught notes early on (introductory summary of conclusions), "Nymphs required one year of development from the egg stage to adult, water temperature was the main factor influencing seasonal growth."  (emphasis added by me)  Obviously, the water temperatures are warm in the fall, cool to cold in the winter, them warm up again in the spring.   In line with these temperature changes, "Most early nymphs begin appearing late in the summer from the June-July adult peak emergence and oviposition.  A short spurt of fall growth is experienced before a period of slow size-increase sets in through the winter."  "Growth is resumed in March and continues until emergence." (p. 19)

Vaught tracks growth by looking at two different things: 1) wing-pad length (Fig. 5, p. 23), and 2) changes in head-capsule width (Fig. 6, p. 25)  If I'm reading his data correctly, the mean changes in wing-pad length (Fig.5) go from .5 mm to .62 mm from September to October and they jump again from November through December, but they drop to .61 to .62 from December to January.  The same is true for changes in the head capsule width (Fig. 6).  The mean width change from September to October is from .9 mm to 1.16 mm but from December to January it's only 1.15 mm to 1.16 mm.
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Interesting.  Sort of fun to know these things.  Oh, and one other thing, just in case you've been wondering how many instars are involved in these changes, Vaught notes for the Neoperla nymphs that he studied,  "Nymphs underwent up to 23 instars (males from 18 to 20, and females from 20 to 23 instars)."  That's a lot of changes, and it's no wonder that we run into these pale looking creatures every so often -- especially during the fall.
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Peltoperla nymph from 11/11/11.

By the way, though species ID is difficult with nymphs that have only recently molted, I suspect that our nymph from yesterday was Acroneuria abnormis.

No anal gills and no occipital setal row (= Acroneuria), and the posterior tergal margins appear to be light (= abnormis).