Thursday, December 27, 2012

Cheumatopsyche: the common netspinner genus that we "commonly" see in the winter

When I was at the upper Doyles river on 12/23, I found this odd-colored common netspinner larva.  I preserved it, to work on species ID when I returned home, thinking it might be a Ceratopsyche species I had not seen before.  But when I looked through the microscope, I quickly saw that it was a genus Cheumatopsyche which, with a TV of 6.6 is nothing special -- so I ignored it in the blog that I posted that day.  But I think that this might deserve comment.

I generally think of summer as "common netspinner season."  In a lot of our streams, this taxon is the most common insect we see from July through September -- even into October.  The tangled vegetation that grows on the rocks in the Rivanna is so loaded with them at that time of year that it sometimes gives me the creeps!  But, we find a lot of them in all of our streams at that time of year.  And if you look back at the blog entries written during that stretch of time, you'll see that that is when I was working intently on netspinner ID, finding mostly Ceratopsyche and Hydropsyche species.

After October, they just disappear -- well, the big numbers disappear anyway.  At the moment, I don't see many netspinners at all.  True, I found a Ceratopsyche bronta at Lickinghole Creek on 12/2, and I found a Ceratopsyche alhedra at the Rapidan river on 12/6.  But most of the common netspinners I see at the moment are genus Cheumatopsyche.  I can usually tell by the color: most that I see are bright green with a dark brown head and nota.  (Not true, as you can see, of the netspinner I found at the Doyles.)

Is there a reason why this is our "common" common netspinner during the winter?  I think that there is.  As it turns out, Cheumatopsyche larvae commonly start to hatch -- as the "Little Sister Caddis" -- in April, "as early as mid-April in more southerly locations..." (Thomas Ames, Caddisflies: A Guide to Eastern Species for Anglers and Other Naturalists, 2009, p. 131).  But we will continue to see some during the summer.  Again, let me quote from Ames: "Peak flight periods in the East are from April through July, with continued emergence into October." (p. 130)

While most of the larvae of this netspinner genus I see are bright green, they do come in a number of colors, so we can't rely on color to decide on ID.  Certain ID often requires microscope work -- but you often can see the critical features using a loupe in the field.

Look for two things.  1) There is a notch in the leading edge of the frontoclypeus (essentially the top of the head).  This:

2) If you can see the larva while it's on its back, you'll notice that -- in contrast to the Hydropsyche larva -- it lacks sclerites below the prosternal plate.  Let me illustrate:



I'll continue to look for common netspinners throughout the winter: that Cheumatopsyche is the most common genus we see at this time of year is a thesis that needs to be tested.   Of course, there is another netspinner we commonly see in the winter and spring -- but only in small mountain streams: Diplectrona modesta.  This one:


Cheumatopsyche photos:

3/7/12 (Whippoorwill Branch of the Mechums)

8/30/12 (lower Doyles River)

12/23/12 (Upper Doyles River)

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