Friday, December 17, 2010

Large Winter Stoneflies (genus Strophopteryx)

The other genus of Large Winter stonefly I have seen in our streams -- at least to date, I have only seen two -- is Strophopteryx (on the genus Taeniopteryx, see the entry for December 16).   Strophopteryx nymphs appear later than Taeniopteryx nymphs: my first sighting last year was on December 28, this year I found them as early as December the 8th.  In December the nymphs are still very small; fully mature nymphs can be found from late January into March (by February, most Taeniopteryx nymphs have hatched).

They differ markedly in appearance from the Taeniopteryx nymphs.  They are yellow, with brown or green markings. The horizontal stripes on the abdominal segments can be seen with the naked eye when the nymphs are fully mature, as can the "mottled" patterns on the wingpads and pronotum (see photo above: this photo was taken in March of this year).

Strophopteryx nymphs do not have "coxal gills."  Rather, the distinguishing feature on this genus of large winter stonefly -- in addition to the colors and patterns --  is called the "ventral, triangular, apical plate".  This feature can clearly be seen on this young Strophopteryx nymph.  It is the plate that sticks out from the end of the abdomen on the ventral side -- right above the base of the tails.

Here is a closeup of the feature in question.  While Taeniopteryx nymphs can often be identified in the field using a loupe to see the gills, Strophopteryx nymphs are often difficult to distinguish until they are fully mature.
Oh, one more thing.  When Strophopteryx nymphs are preserved in alcohol, they always -- well, almost always at least -- curl up in the shape of a "C" as in the second photo above.

No comments:

Post a Comment