Thursday, April 25, 2013

Caddis delights at the Rapidan River: "humpless casemaker," Micrasema charonis

In the Rapidan River -- as they are in most of our streams at the moment -- the leaf packs and rocks are filled/covered with two insects -- spiny crawlers (E. invaria and E. dorothea) and Perlodid stoneflies (I. namata, or whatever we want to call them).   That's what I expected to see.

This insect wasn't expected.  This a "humpless casemaker" caddisfly larva (Brachycentridae), Micrasema charonis, and this is the first time I've seen one.  If you're a regular reader, you know that I often see Brachycentrids at the Rapidan River, but in the past, they've all been the "log cabin" casemakers -- Brachycentrus appalachia.  They look like this.  Four-sided case made out of pieces of bark.

So I was surprised and delighted today to see these little cylindrical cases made out of ribbons of vegetation (found two of them).

This is a much smaller insect than B. appalachia.   The B. appalachia case pictured above -- found on 1/4 of this year -- was 15 mm long: The M. charonis cases that I found today were 9 mm, the larvae were only 6.   For identification let's turn to Beaty ("The Trichoptera of North Carolina," p. 80):

M. charonis -- head with distinct dark muscle scars laterally, pale areas anterolateral of frontoclypeus.  Case vegetal and straight.  Mountains and eastern Piedmont.  

lateral muscle scars:

pale areas at front of head:

One other photo is also instructive.  In his genus ID of Micrasema, Beaty notes that the "ventral apotome [is] wider than long."  Our larva nicely flipped on its back for this photo!

So another addition to our EPT list: Micrasema charonis.  TV, 1.0.


But I spotted another casemaker in a leaf pack today, one that we've seen before, the Limnephilid (Northern case-maker) Pycnopsyche scabripennis.

Not the prettiest case that I'ved seen: these larvae just seem to throw these together with twigs and bark.  But note how the larva uses that large piece of bark to cover its head.  This case measured 36 mm (!); the larva inside was 29.  Big insect.

Now for the more common things that we see at the moment.

1. Perlodid stonefly, Isoperla namata (or I. nr. namata, or I. montana).

2. Spiny crawler, E. dorothea (9 mm).

3. Spiny crawlers, E. invaria (both 7 mm).

4. A brush-legged mayfly (Isonychia).  They're starting to mature.

5. And, surprise surprise, another one of those Isoperlas -- the same one that I found at the Doyles River last week -- species still unknown.  Still waiting to hear from Steven Beaty.

This one had lost its left eye.

But the real prize -- Micrasema charonis.

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