Tuesday, October 23, 2012

But That Doesn't Occur in Our Part of the Country: the Common Netspinner, Ceratopsyche bifida

I found this common netspinner larva in the Moormans River in Sugar Hollow on August 10th of last year (2011).  It keys out perfectly to Ceratopsyche morosa.  On C. morosa, Steven Beaty ("The Trichoptera of North Carolina," p. 73) notes the following features: "head with 5-6 large pale spots in checkerboard pattern on frontoclypeus and usually with three small white spots at posterior angle of frontoclypeus...pale muscle scars laterally on dark pronotum."  All of those features are clear in the following photo (6 pale spots on this one).  The p1, p2, and p3 arrows point to the three spots at the posterior of the frontoclypeus (central part of the head).

Schuster and Etnier ("EPA Manual," p. 41) say much the same: "Frontoclypeus with checkerboard color pattern; posterior corner of frontoclypeus always [note] with a cluster of three smaller yellow spots.  Posterior one-fourth of head yellow." They add on p. 42 "S. morosa [= C. morosa] can be consistently distinguished by the presence of the three small spots clustered in the posterior corner of the frontoclypeus.  No other checkerboard-patterned Symphitopsyche [= Ceratopsyche] species exhibits these spots.  In all of the other species, if any marking is present in the posterior corner of the frontoclypeus, it is a single, large, yellow spot."  Oddly, they say this right after noting that "..on rare occasions, there may be only two small spots rather than three." (???)

There is a reason for drawing attention to the one spot, two spots, three spots point, as you will see.

Now, let's look at a netspinner larva I found at the Rapidan River on 9/5 of this year.

Here is a better look at the top of the head since the pattern is not real clear in these photos.

Note that this has the very same six-spotted checkerboard frontoclypeal pattern, but there are only two pale spots at the back of the head (p1, p2).  Three of the netspinners that I found this past Sunday (10/21) at the very same spot in the Rapidan River turned out to have a head that looks much the same, but the anterior spot (number 3 in this photo) seems to be missing.

Here's a microscope view of the head.

Again, there are only two pale spots at the back of the head, spots that are each formed by a fusion of two muscle scars.

Given the checkerboard pattern of 5 or 6 spots on the head of our Rapidan larvae, I've assumed that they are a variant form of C. morosa -- but I've been bothered by the two posterior spots, versus the three that are supposed to be there.  (True, Schuster and Etnier do note two spots in rare cases.)  So,  I've decided to look into this further, and here's what I've found.

1.  At the end of Steven Beaty's description of C. morosa, he notes that "C. bifida is considered the west-central form of C. morosa with a head pattern similar to C. cheilonis and does not occur in NC."
So, C. morosa and C. bifida are much the same insect, but the former is found in the East; look for C. bifida in the central and western parts of the country.

2. In an article to which I was referred by a friend (Kurt L. Schmude and William L. Hilsenhoff, "Biology, Ecology, Larval Taxonomy, and Distribution of Hydropsychidae (Trichoptera) in Wisconsin," pp. 123-146 [The Great Lakes Entomologist, 19:3, 1986]), on the other hand, a distinction is made between two different forms of C. morosa: the morosa form and the bifida form.  Both forms are said to occur in the state of Wisconsin.

3. And finally, Schuster and Etnier describe Symphitopsyche (Ceratopsyche) bifida and  Symphitopsyche (Ceratopsyche) morosa as distinct species.

I think that the netspinner larvae I'm finding in the Rapidan River correspond pretty well to Ceratopsyche bifida and/or Ceratopsyche morosa, bifida form -- even though that species -- or that "type" of C. morosa -- does not occur, so we're told,  in this part of the country (!)  Keep the following photos in mind -- microscope photos of the head of one of the larvae I found on Sunday -- as we proceed.

1) top of the head, showing the pale spots on the frontoclypeus:

2) side view of the head:

3) ventral view of the head:

1) This last photo is important to keep in mind when we read what Schmude and Hilsenhoff have to say about C. morosa, bifida form:  "ventral areas of the head are variable.  The basic pattern is three dark areas halfway back, one each on the posterior half of the stridulatory surfaces [= 1 and 3 in the photo], and one mesally on the gular suture [pointed out with arrow 2].  These can fuse to form a transverse band or be reduced to three small spots.  More often the pigmentation is extensive, forming a wide "W" shape [emphasis added] as in C. alhedra; this is never the case with C. morosa (morosa form)  [emphasis added] or C. bronta." (p. 131)  I'd say our larva has a "wide "W" shape" which would make it C. morosa, bifida form.

2) Schuster and Etnier describe Symphitopsyche bifida (Ceratopsyche bifida) in the following way: "Seven spots in checkerboard pattern on frontoclypeus [but I think they have in mind a single anterior spot and a single posterior spot, not the seven spots pointed out on my photo].  Dark brown to black pigmentation on margins of genae forming epicranial arms; this pigment expanded behind eye and reaching ventrally to level of eye.  Expanded black area behind eye with number of yellow spots.  Posterior one-fifth of dorsum of head yellow; large area around eye and posterad of eye, ventrad of black mark, yellow.  Venter of head with large, dark, inverted, Y-shaped mark on stridulatory surfaces."  (p. 30.  All of those traits are visible in the second photo above: the side view of the head.)  On p. 31, they note some variation: "A certain degree of variation has been observed in the larvae of S. bifida, and this variation relates to the number of spots on the frontoclypeus.  The central five spots on the frontoclypeus are consistently present, but the medial anterior and posterior spots are variable.  Some individuals of the same population may possess all seven spots, while others have only six and lack either the anterior or posterior.  While still other individuals may possess only the central five and lack both the anterior and posterior spot."  The larva I found on 9/5 does have an anterior spot; those found on Sunday do not.  In neither case, however, is there a single posterior spot -- both have two light spots at the rear.

Well, that's where things stand at the moment on the C. morosa issue.  I'd be really inclined to say -- as Schmude and Hilsenhoff say about the state of Wisconsin -- that here in Virginia we have both types of C. morosa -- the "morosa form" and the "bifida form".  But "officially speaking," C. morosa: bifida  -- I take it -- doesn't occur in this part of the country.

Anatomical note: The "frontoclypeus" is that part of the top of the head that is enclosed by the epicranial suture.  The "genae" run from the outside of the epicranial suture on top of the head to the gular suture on the bottom of the head.

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