Thursday, November 8, 2012

Looking For -- and Finding -- Small Perlodids in Sugar Hollow

It was my first trip of the fall to my favorite small mountain stream in Sugar Hollow, one that is filled with quality insects, and I was hoping to find some young Perlodid stoneflies.  And I did -- two different species, in fact.  The one in the photo above is one that I identified for the first time in January at the Rapidan River: Malirekus hastatus.  But I saw a lot of them in the winter in the very stream I went to this morning.  Here's a beauty that I found here in February this year.

The other Perlodid nymph that I came up with today was a very small -- 5 mm --  Isoperla similis, the first Isoperla I've seen so far this season.

This too will be a beautiful insect when it's mature.  This one is from April of 2011.

When you look at small nymphs of these species, it's not always easy to tell them apart.  With a microscope, you won't have a problem -- the mesosternal "Y" ridges are quite distinct as are the laciniae.

I. similis mesosternal "Y" ridge (and yes, they're very hairy):

M. hastatus mesosternal "Y" ridge:

I. similis lacinia:

 M. hastatus lacinia (thinner with a "knob" that is covered with setae):

Then too, there's the fact that Malirekus hastatus nymphs have conical-shaped submental gills.

But, can we tell them apart by simply "looking" at them?  Actually, we can.  Both have pale "M" marks in front of the anterior ocellus.  But M. hastatus nymphs have "two pale, ovate spots lateral to [the] ocellar triangle, and [a] pale interocellar spot." (Stewart and Stark, Nymphs of North American Stonefly Genera, p. 216).  These are easy to see on the nymph from this morning.

There is no pale interocellar spot on an I. similis nymph, and the pale spots that are lateral to the ocellar triangle are not "ovate" in shape.   These too are features we can detect even on the small nymph from this morning.

Just use your loupe.

Also found in this small stream this morning ---

1. Free-living caddisfly larva, Rhyacophila nigrita.  This is the only place, to date, that I've found this free-living species.  The dark front half of the pronotum is one of the keys to species identification.

2. Giant stonefly.  I used to think this was P. scotti; I now think it's P. proteus.  I must have seen at least a hundred of these this morning of various sizes.  They love the leaf packs.

3. And the "Common stonefly" that we commonly find in these small mountain streams -- Eccoptura xanthenses.


Malirekus hastatus

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