This "specimen" is actually two caddis larvae fighting each other over a case. The case is a hollow tube; one larva would go in the back end, presumably bite the other, and chase it out. The invader crawled forward into the case while the other one fled, and then it went around to the back and bit the first one. They did this several times, and I recorded it on video.

This nymph has double front tarsal claws and double gills on the first three abdominal segments.

This nymph has double front tarsal claws and double gills on the first three abdominal segments.

This nymph has double front tarsal claws and double gills on the first three abdominal segments. This one has a distinct dorsal stripe.

This nymph has double front tarsal claws and double gills on the first three abdominal segments.

This nymph has double front tarsal claws and double gills on the first three abdominal segments.

This nymph has double front tarsal claws and double gills on the first three abdominal segments.

I took a few group picture of a bunch of Baetisca laurentina nymphs to show the degree of individual variation in size, color, and shape that can occur within the same species in the same pool of the same river. This variation is one important reason why trout are forgiving of some small degree of variation in our imitations--the naturals themselves vary, too.

Here's an unusually small early instar nymph.