Monday, December 27, 2010

Common Stoneflies (family: Perlidae)

The most common stonefly we see is the "Common Stonefly" (family: Perlidae), and the most common Perlid genus we see is Acroneuria -- pictured in the photo above.  This is a beautiful stonefly: here is another photo; an Acroneuria stonefly with slightly different colors.  The tufts of gills behind each leg that distinguish Perlid stones from Perlodid stones are pretty clear in this photo.

The Common stonefly nymph, when mature, is a fairly large critter.  Both of the photos above are "live" photos taken at streamside with a macro lens: the actual size of each of these nymphs was slightly over an inch, but I've seen Perlids that are closer to two inches in length.   The Common stonefly is the only stonefly we can find in our streams anytime of the year.  Almost all insects are "univoltine" -- that is to say, they have a one year life cycle.  Perlid stoneflies, on the other hand, can take up to three years to mature.  Thus, at any given time of the year, there can be three generations of Perlids present in our streams, and we can find Perlids in the same sample that vary considerably in size.  Perlids hatch in mid-summer (the "Golden Stonefly hatch" to fly fishermen), so it is not unusual to see the largest Perlid nymphs in late winter and spring.

I have found five different genera so far in our streams, though not all genera seem to be found in all streams.  If there is only one genus present, it is likely to be Acroneuria.  Mature Acroneuria nymphs are often easy to identify by the yellow "M" on top of their heads (very clear in the first photo above).

Also common in some of our streams is the genus Eccoptura which also has a distinct pattern on the top of the head: the front part of the head is yellow and shaped like a drooping "T".  I have found a fair number of these in Powells Creek near Crozet.  Powells Creek in Crozet is also the only stream in which I have found the genus Neoperla (head pictured below).  This genus varies from all other Perlid genera by having only two ocelli (the black dots on the head) instead of three.  (Check it against the Eccoptura picture.)

Are certain stonefly genera "stream specific"?  Is there something about the size of the stream or the type of habitat which determines which genera one will find there?   Are there more genera found in our really good streams than in our so-so streams?  I don't know the answers to these questions; if someone out there does, please feel free to comment.

This is another genus that I have only found in one stream -- and it is not in our watershed.  This Common stonefly is genus Paragnetina, and I have only found this genus in the Rapidan River where it flows out of Shenandoah National Park (another live photo: note the bushy gills behind each of the legs).  This is a genus of stonefly that is very intolerant of stream impairment.  Is that why it is only found in this very clean stream?  (I should add that of all of the insects we study, stoneflies are the least tolerant of impairment: TV's (tolerance values) range from 0-2 on the scale of 10.  Paragnetina nymphs share a feature with Neoperla nymphs that is not found on Acroneuria and Eccoptura specimens.  They have a very distinct "occipital ridge" at the back of the head.  This is a line of close set dark hairs or spinules (clearly seen in the photo below).

Finally, there is one genus that I have only seen in two streams -- the Rivanna River at Darden Towe Park, and Buck Mt. Creek -- and I have only seen nymphs that are fully mature (in May and June).
This genus is very distinct in its colors, with the dark edges on both sets of wingpads.

The photo below shows the distinct Perlid gills -- behind each set of legs.  Rose Brown of StreamWatch calls these -- quite fittingly -- "hairy armpits"!

No comments:

Post a Comment