The "Tiny Mayflies" of Summer: the "Little Stout Crawlers" and the "Small Square-gills"

First of all, for my fellow fly fishermen -- "Little Stout Crawlers" are Tricos!  (family: Tricorythidae)
Wouldn't you think the entomologists would get with it?!  After all, "Trico" is a lot easier to say than "Little Stout Crawler," and it's easier to record on the data sheets monitors use.

Fly fishermen already know these are "tiny".  We all hate them; but we know that we have to have imitations to use come late summer and early fall when they hatch.  Tiny or not, when they're on the water, they're on the water in very large numbers, and the trout like to move through the water sipping them down as they go.  But tying on a size 20 or 22 hook is no easy task for those of us of the "senior" persuasion, and seeing them on the water is even more of a challenge!

Back to the entomology side of the coin.  Tricos have a very distinct feature that, if you can see it, makes identification a simple matter.  They have "triangular operculate gills".  Picture.

These are not so much "gills" as "gill covers."    They cover the "functioning gills" -- i.e. the gills that are absorbing the oxygen.   Since Tricos prefer the slow-moving water in rivers and streams, the "operculate" gills protect the real gills from a build up of silt.   In the StreamWatch data I've looked at, and in my own experience in working with StreamWatch, Trico nymphs are found almost exclusively in sites on the main stem of the Rivanna.

While the Trico ("Little Stout Crawler") is of great value to the fly fisherman, the "Small Square-gill" (family: Caenidae) is of little, if any, value at all.  The "Small Square-gill" is fittingly named since its "square gills" are used for identification.  Here is a close-up.

Note that these gills overlap;  the Trico's triangular gills do not.  Still, these gills serve the same purpose.
I.e. they are there to protect the functioning gills from a build up of silt: the "Small Square-gills" also prefer to live in the slow moving water of streams.   Some lists of "tolerance values" give the Small Square-gill a high value of "7": but StreamWatch has a tolerance value of 4 for both the Small Square-gill and the Trico.

Interstingly,  this mayfly is hardly ever found in main stem sites on the Rivanna.  It is found in a good many streams in our region but usually in small numbers.  Identification in the field isn't easy: this nymph is very small.   But the color pattern is such that when I see it, I see an insect wearing a white belt and shorts!  The Trico gills, on the other hand, look more like "chaps".