Sunday, October 15, 2017

The "ventral apotome" and genus ID of caddisfly larvae

I've been working with a friend who wants to learn EPT genus level ID, and I've been struck by the number of cases in which the "ventral apotome" -- also known as the "gular sclerite" -- is one of the keys to the genus.  The ventral apotome is a sclerite on the venter of the head, as you can see in this illustration.

(The "gula" on the larva is "the ventral region between the base of the beak and the collar." [Merritt, Cummins, and Berg, ed., An Introduction to the Aquatic Insects of North America, 4th edition, p. 996.])

While I can't be sure that I have any readers who find this of interest, for those who might, here's how this is used when we identify a caddisfly larva to the level of genus.

1. Hydropsychidae, genus Arctopsyche (the larva in the photo at the top of the page.)  This is a common netspinner that I find in Montana, but it does occur in our region even though I've not come across it so far.

In this case, the sclerite runs the length of the gula, but the critical factor is that it narrows from top to bottom.

2. Hydropsychidae, genus Parapsyche.  Another netspinner that apparently occurs in our region, but again one that so far I've not seen.  This larva is one that I found in Oregon.  (Unfortunately, for this one I don't have a live photo.)

The ventral apotome again runs the length of the gula, but it narrows only slightly from top to bottom, more rectangular in shape.

With Arctopsyche and Parapsyche, the ventral apotome is the only thing you need to see to determine the genus ID.

3.  Hydropsychidae, genus Cheumatopsyche.  This is a netspinner we see a lot in our streams: our stream monitors will recognize this one for sure.

While the ventral apotome is not the only thing we need for our genus ID, it's where we begin.

The ventral apotome can consist of an anterior sclerite and a posterior sclerite or simply one or the other.  Here it is the anterior sclerite on which we must focus.  No tubercle -- a bump on the middle of the top of the sclerite.

4. One more netspinner -- Hydropsychidae, genus Hydropsyche -- the most common, common netspinner that we see in our streams. 

To be fair, it's not the ventral apotome that's important on this one.  Rather, we look for a pair of sclerites below the "prosternal plate," kind of the "neck" instead of the gula.

And that's all you need to see.

5. Lepidostomatidae, genus Lepidostoma and genus Theliopsyche. 

Lepidostoma is very common in forested streams -- cold water streams in the mountains -- throughout the winter and into the spring.  (We're not sure that those that we see in late spring/early summer are the same species that we see in the winter: probably not.)  Theliopsyche, however, is not at all common: I've only seen one (possibly two).

The two differ, as you can see, by the size of the ventral apotome, and the length of the median ecdysial line. 

6. Uenoidae, genus Neophylax

This is another taxon that we see a lot in the winter in the very same streams we find Lepidostomatidae.  It prefers those cold mountain streams, though some are found in streams that are not at the very top tier (e.g. Doyles River and Buck Mt. Creek).  Neophylax used to be a Limnephilidae genus; but now it's a family -- Uenoidae -- all on its own.   The ventral apotome is one of two things we use to establishing genus (Neophylax) ID.

The gular sclerite is unique being shaped like a "T".

7. One more, the "humpless" casemaker (Brachycentridae), genus Micrasema.

One of the keys to genus ID -- the ventral apotome is wider than it is long.


So there you have it.  I hope some of you find this instructive.  When you pick up a caddisfly larva -- one of the above anyway -- take a peek under the head.  Useful information down there.

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