Thursday, June 30, 2011

Aw "shucks"! Stonefly Remains at the Rapidan River

Yet another sign of late spring/early summer -- stonefly shucks on stream side rocks (and leaves!).  We also find them on rocks in midstream, rocks whose tops are exposed.  When stoneflies mature and "emerge" as terrestrial insects, they crawl up on rocks, split open their shells, dry off their wings, and fly off to mate, lay eggs, and die.  The cases I saw today at the Rapidan River were -- I'm fairly certain -- those of Perlodid stoneflies: they weren't very big, maybe 1/2" or so, and almost all Perlodids by now have hatched.  Common stoneflies -- Perlids -- on the other hand, are in the process of hatching right now.  But Perlid shucks are typically 1" or so long.  Here are some Perlid shucks that I brought back from Montana two years ago.

If you look closely at the shuck on the left (click to enlarge) you can still see how the case split open when the insect emerged -- the exoskeleton is split open from the base of the head back through both sets of wing pads.

Now, there are still stoneflies in our streams, some Perlids will hatch throughout the summer.  And, remember that Perlid stoneflies (common stoneflies) can take 2-3 years to mature.  So, in addition to the mature nymphs yet to hatch, there are presumably 2-year old nymphs around and 1-year old nymphs as well.  In fact I've been seeing a lot of what I assume are 1-year old nymphs -- but they're too small to photograph at the moment.  Here's a nice common stonefly that I found today -- genus Paragnetina -- that may still hatch this summer, but it could also be a 2-year old with 1 year to go (the wing pads are not well developed).

I also found a Perlodid stonefly, one that will surely be hatching before very long (notice the dark wing pads).  This is an Isoperla holochlora -- a species that likes these cold, mountain streams, and one that tends to be a "late bloomer".

But all in all, it was a slow day of looking for bugs since so much has recently hatched.  I did find, in addition to the stoneflies pictured above (and those pictured below), some flatheaded mayflies -- Maccaffertium (pretty big), Leucrocuta, and Epeorus vitreus; a few Brushlegged mayflies; lots of common netspinners (genus Hydropsyche); some "Humpless case-maker" caddisflies (Brachycentridae); scads of baby Peltoperlid (Roach-like) stoneflies; and a large number of small Giant stoneflies.  But for the really interesting insects of summer -- small minnow mayflies, damselflies, dragonflies, genus Macrostemum common netspinners, Tricos, etc. -- I think we have to ignore the "pure" waters of mountain streams and look further along down in the valleys.  Time for me to think about spending more time at the Rivanna.

Three other good photos from today's trek: 1) a Glossosomatid (Saddle-case maker) caddisfly that actually crawled out from under its case;  2) the Perlodid stonefly riding a Perlid stonefly while the Glossosomatid looks at its reflection in the petri dish; and 3) another shot of a Perlodid shuck.

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