Sunday, January 15, 2012

Finding Glossossomatids (Saddle-case Makers)

It's actually pretty easy.  Saddle-case makers (family Glossossomatidae; genus Glossossoma) attach their cases to the tops of rocks, so they're easy to see when you're walking around in a stream.  Look for a little oval made out of sand grains and pebbles.  Something like this:

But you'll never see the larva until you turn the "dome" over: the larva does all of its eating without exposing any part of its body.  With the case turned on its back, using a loupe, you'll see the head at the top of the case and feet sticking out of the bottom: the body stretches over a "saddle" of sand grains that functions much like the plastron in the shell of a tortoise.

Glossossomatids, you may recall from earlier entries, are not true "case-makers": they differ anatomically from things like Lepidostomatids and Uenoids -- the case-makers we see so often at this time of year.  They also differ in terms of the way they make use of their "cases".   Anatomically, they are like freeliving caddisflies and Micro caddisflies in having sharp points on the terminal segments of the maxillary palps as adults.  Together this group of caddisflies is called Spicipalpia (i.e. "spiky" palps).    (See Glenn Wiggins, Caddisflies: The Underwater Architects, p. 107, and pp. 20-26.)  And in terms of their cases, unlike the true case-makers (known as Integripalpia), when they outgrow their cases, they abandon them and start building all over again.  It's when they are "caseless," by the way, that they are vulnerable to trout, and fly fishermen do well when they imitate these freely drifting larvae.  (See Thomas Ames, Caddisflies, pp. 89-91.)

Glossossomatids are easy to identify when they're in their cases.  But, they are usually out of their cases when you're dealing with preserved larvae you need to ID in the lab.  In that case, here's what you do.
Number one, they often assume a "hands up" position (!) when they're preserved.

That's a "dead" giveaway (hmm... guess that has double meaning!).  If you don't see that, you have to look at their bottoms where you will see 1) a "sclerite" on the top side of the last abdominal segment:

and 2) you will notice that only the tips of the anal prolegs stick out free from the body, the rest is fused with the final abdominal segment.

This is the first time this season that I've picked up some Glossossomatids: I was at Buck Mt. Creek.  I also saw, for the first time this year, this little stonefly --

That's a Nemourid stonefly, genus Nemoura.  I hope to go to the Rapidan River next week, where I expect to see a lot of these nymphs, and I'll spend more time on them when we have some better samples to view.  (Or, you can look back to the entries of 2/20/11, 2/21/11, and 3/2/11).

My other findings today were large winter stoneflies -- Strophopteryx fasciata -- and, to my surprise, some small winter stoneflies.  I photographed two of the small winter stoneflies, neither of which looked like the small winter species I found at Buck Mt. Creek.  This one, I found on November 25.

Fairly mature -- note the dark wing pads.  But look at the two that I found today.

The one on the right is distinct with that very orange abdomen, while the one of the left, which is fairly big, has wing pads that look like they've "lightened," not darkened.  Hmm...?  I suspect this is a matter of species and that the species we're finding now (are the two in the photo above the same species?) differ from the one that we found in November.  Beaty notes that "there are at least eight species of Allocapnia in North Carolina many of which are undescribed as nymphs"  ("The Plecoptera of North Carolina," p. 1).  So we should not be surprised if all of our small winter nymphs don't look the same.

"Light" wing pads with maturity might also be true of the large winter stonefly Strophopteryx fasciata.  I seemed to see that last year, and it was also the case with the largest S. fasciata nymph that I found today.

Always more things to learn.

Note:  After another look, I'm not really sure the top, right, small winter stonefly in the picture above is genus Allocapnia.  But I didn't keep it, so I can't key it out.  The hind wing pads don't look right.

1 comment:

  1. I've been following your blog for several months now. I like how you feature aquatic insects. Say, could you help me with the ID of an old post of eggs and cases?