Sunday, December 19, 2010

Small Winter Stoneflies (Capniidae)

Small Winter stoneflies are "small."  In the picture above the relatively mature nymph is almost exactly 1/4" long (not counting antennae and tails); the immature nymph, more like 3/16".  We find these nymphs in our samples from October to February: the last nymph I found last winter was on February 19, the first one I found this fall was on October 24.  In other words, their presence in our streams corresponds closely to that of the Large Winter stoneflies, genus Taeniopteryx.  Small Winters hatch from November through February, though I've seem some terrestrials as early as October.

Field identification of Small Winter nymphs, especially with immature nymphs where the wingpads are not fully developed, is difficult and probably should not be attempted.  They are easily confused with Green stonefly nymphs (Chloroperlidae) and with Rolled-winged stoneflies (Leuctridae).  All three are tiny, yellow in color, and they have long, thin abdomens.  Identification can even be difficult with a microscope in the lab.  What must be detected are what are called "ventrolateral pleural folds" (see the photo below).  With Small Winter stones, these can be seen on abdominal edges of segments 1-9; on Rolled-winged stoneflies, they're only found on segments 1-4.

Field identification is easier to do once the nymphs are mature and the wingpads can clearly be seen.  The primary wingpads are long and thin; the secondary winpads are short and stubby.  And, the secondary wingpads sometimes have a notch in the bottom edge of the pad (as in the photo below: this is genus allocapnia).

While immature nymphs are very plain looking with few distinguishing features, the mature nymphs can be very colorful and highly patterned.  The one pictured below is probably not far from hatching.  The terrestrial form of this stonefly is uniformly dark brown or black and unattractive -- something that does not seem to bother the Trout!

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