Wednesday, January 5, 2011

The "Common Netspinner" Caddisfly (family: Hydropsychidae)


The "Common Netspinner" caddis larva, some might say, is the bane of our existence.  It is present -- I think -- in all of our streams, and in large numbers in many cases.    We normally find two different genera -- Cheumatopsyche (above left) and Hydropsyche (above right).  Both genera are tolerant of stream impairment (a tolerance value of 6 in the scale of 0 - 10), thus large numbers of them in a sample are bound to drag down the score for a stream.   I participated in two samples -- one last summer, the other the summer before -- in which in a one second net, we found more than 400 netspinner larvae.
Our goal in a sample is to have a TOTAL bug count close to 200!  (Clearly, not all of our streams are this bad.)

The netspinner is easy for monitors to get to know (perhaps because we see so many of them!), because of a number of features.  To begin with, almost all netspinners are green (though we do find some that are cream colored or tan).  Secondly, their stomachs are covered with branching gills (or as Rose Brown in StreamWatch likes to say, they have "fuzzy bellies".)  Thirdly, the three plates behind their heads (the pronotum, mesonotum, and metanotum) are all "sclerotized" -- i.e. they're hardened and darkened, not fleshy.

                                            (Close-up of a netspinner on a tablecloth.)

Netspinnners do in fact "spin nets"; it's how they gather their food.  They form little seine nets, much as a spider spins out its web, by spinning silk that they produce from their bodies.  This little net they stretch between rocks where it "filters" out food (they are "filter collectors" in terms of Functional Feeding Groups), then they move out of temporary sand and rock shelters to gather these morsels to eat.


They do something else of great interest.  They have incised grooves on their chins (visible in the photo above) which they rub with a knob on the edge of their femurs (also visible in the photo above), making a noise to scare off opponents.

While netspinners have a bad reputation, not all netspinners are bad.  That is to say, they are not all equally tolerant of stream impairment.   In a few of our streams we have found the genus Diplectrona which has a TV of 2.2 (vs. 6).    This is a genus that thrives in small, mountainous, fast flowing streams where the levels of dissolved oxygen in the water are high.  As you can see in the photo below, the Diplectrona larva has fewer and sparser gills than the more common Cheumatopsyche and Hydropsyche.   The dense gills on the more common genera allow them to absorb a lot of oxygen in water that is oxygen poor.   Diplectrona larvae can take in more with less.


I have seen this genus in very few streams.  The netspinners in the upper Doyles river are genus Diplectrona, as are the netspinners in the stream used by StreamWatch as its reference site for Albemarle County.  I have also found Diplectrona larvae in the Whippoorwill Branch of the Mechums River and in Long Island Creek.  These netspinners are visibly different from those that we normally see.  They are not green; their bodies are a purplish gray (the one in the photo above has lost its natural color because of preservation.)


There is one other netspinner genera that we occasionally see in some of our streams that has a tolerance value of 3.6.  This is the genus Macrostemum (in the photo above).  I saw large numbers of these this summer in the North Fork of the Rivanna, and I saw some again in the fall in Buck Mt. Creek.  It's quite possible that other streams have them as well -- streams that I did not explore.

This is a very strange looking larva.  It's much larger (fatter and longer) than the other netspinners we see, and it's a unique shade of green (somewhat visible in the photo above, though the color has faded through preservation).  But the strangest feature is clearly the head: it's flattened down -- one might call it concave.  This is often visible without magnification.



This is a netspinner larva that few monitors get to see since very little sampling is done in the summer when it's around, and since it's only present in a select number of streams.

(Below: another photo of a common netspinner, genus "Cheumatopsyche").

2 comments:

  1. Hi Bob, I've been looking for a macrostemum for my personal collection. If I paid for shipping and whatnot and you have extra, would you be able to send me one?

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    1. No problem. Shipping isn't expensive. Just give me your address.

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