Wednesday, July 20, 2011

Our Summer Spiny Crawler: Genus Serratella

My main goal this summer is to find and photograph those nymphs and larvae that are only around this time of year -- e.g., damselflies, dragonflies, Long-horned case-maker caddisflies, and the Macrostemum common netspinners that I found on Sunday.   This morning, for the first time this summer, I found a few spiny crawler mayflies that were genus Serratella, and I found them in the Rivanna River in Darden Towe Park, one of the few places I saw them last summer.

The spiny crawlers we saw throughout the spring in HUGE numbers were genus Ephemerella.  They are plentiful, and they tend to hatch in May and June, and they account for some major hatches that are much anticipated by fly fisherman: E. invaria and E. rotunda become "Sulphur Duns," and E. dorothea hatches as the "Pale Evening Dun."  Serratella nymphs are not plentiful, we see them sporadically throughout the summer, and they are of little or no interest to fly fishermen.  They are also small, probably 1/2 the size of the spring Ephemerella.  And, they are drab in appearance -- dark brown or black.  Ephemerella nymphs -- olive or brown -- are richly patterned.  For example, this one that I found in late May.

Anatomically, however, the two genera are similar in constitution, in that the gill structure is exactly the same.  On both, there are gills on top of abdominal segments 3-7, though the gill on segment 7 is usually partially obscured by the gill on segment 6.  Here's a good picture of that from one of the Serratellas that I found this morning.  The second photo shows the actual "gills" under the gills.

However, the tails -- or "cerci" -- differ.  Ephemerella tails have "intrasegmental setae" -- i.e. they're very hairy; Serratella tails have whorls of spines at segmental joints but little or no intrasegmental setae.  Compare.

Ephemerella cerci:

Serratella cerci (also click on the photo at the top of the page to enlarge it):

So nice to see this genus around again for the summer.

The other insects of interest that I found this morning -- I can't get real excited about "tons" of netspinners, some Maccaffertium flatheads, and a few small hellgrammites! -- were small minnow mayflies, quite a few of them actually.  They were all Heterocloeon in terms of genus -- in fact they were, again, Heterocloeon curiosum.   Some nice pix:


We identify Heterocloeon nymphs by 1) the two tails (vs. three), and 2) by the gray pigment in the center of the gills (that's actually pretty clear in all of the photos above -- click on them to enlarge them.)
Here's a close-up of the gills on one of these nymphs.

And we know they are species curiosum because they have "procoxal gills," tiny finger-like gills that stick out from the base of the front legs.

One final photo.  This is another Heterocloeon curiosum small minnow mayfly, but a younger one with somewhat muted colors.

It's beginning to look like the small minnows that flourish in the Rivanna River throughout the summer are, for the most part, Heterocloeon curiosum.  Remember, this is what I found last week in the Rivanna at Crofton as well.   But, this is just an hypothesis to be tested.

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