Thursday, September 21, 2017

Temporary housing or permanent shelter? Some unusual Lepidostomatidae cases

Yesterday, I was looking through some of the photos I've taken this year and was kind of struck when I saw this one.  That's a Lepidostomatid -- photo taken on 2/13 -- that is not a Lepidostomatid case.  You may recall that I ran into the very same thing three years ago (12/19/14) when I found this larva in one of our small streams in Sugar Hollow.  (See the entries of 12/19 and 12/21/14.)

At the time I pointed out that this type of case -- three-sided case made out of sections of bark or leaves -- is normally made by the Limnephilid casemaker Pycnopsyche gentilis.

Lepidostomatidae cases are normally made out of sand grains, or "they're four-sided and constructed of quadrate pieces of bark or leaf," (Glenn B. Wiggins, Larvae of the North American Caddisfly Genera, p. 156 in the 1977 edition), or a mix of the two.   For example...

Wiggins goes on to note that "Final instars in some other species assigned to Lepidostoma have cases of plant materials placed spirally or transversely."  In 2014 I thought that might explain what I had found, but there is reason to re-consider.  For one thing, the larva inside that case in 2014 was genus Theliopsyche, not Lepidostoma, and for another, the case that I found was not constructed of materials laid out spirally or transversely.   So what to make of these cases?  (I should also point out that known cases of Theliopsyche are made out of grains of sand.)

I contacted Steve Beaty.  While he didn't know the answer for certain, he suggested a number of things.  E.g., some caddisfly larvae, if they abandon their cases willingly or unwillingly, make a temporary case for protection while they make the type of case in which they're normally found.  (Interesting.  Did not know that.)  Or, it could be that the larva moved out of its own case and "borrowed" a Pycnopsyche case for awhile.  Or, this might be a case that's "typical" for one of the as yet unidentified Lepidostomatid species.   There are, by the way, 29 species of Lepidostoma in the southeast.  The larvae remain unassociated and therefore as yet undescribed.   (See Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species, p. 354.)

I decided to re-examine the larva I found in 2014 and quickly discounted the second solution.  While this cases resembles a Pycnopsyche case, it's much smaller.  It measured something like 10mm; Pycnopsyche cases are normally closer to 20.  That leaves us with the other two possibilities: either it's "temporary housing," or it's an unusual, but permanent, case of a Lepidostomatid larva that at this time is unknown.   That's about as far as we can go.

I should also be clear that while I can vouch for the identity of the 2014 larva  -- genus Theliopsyche -- I didn't keep the larva that I found this year.  Could have been Theliopsyche, could also have been Lepidostoma -- which raises even more questions.

Theliopsyche and Lepidostoma differ in the size of the ventral apotomes and the length of the median ecdysial lines.  They look like this.

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