Is it possible that two rivers no more than thirty miles apart could have two distinctly different caddis that appear during the same time period?
Yes. I know I should probably leave it at that, but that's not the whole story.
PA has 6 recorded
Brachycentrus species, known variously as Apple Caddis, Shadfly, Grannom, Black Caddis, Penn's Creek Caddisfly, or even Mother's Day Caddis. Most of these hatch during the same general time period (mid-April to mid-May).
B. americanus is an exception. Its later, longer emergence often extends from late June through August in the Northeast.
The
Brachycentrus species that is known as the Apple Caddis on the Delaware is
B. appalachia. This species has very light wings (almost white) upon emergence, but goes through stages of darkening as the adults age, ending up with a brownish or grayish wing and a darker body. This species is also found on the Beaverkill, Willowemoc, and Little Beaverkill. Adult specimens from those waters are dated from April 30th to May 7th in the collecting record.
Another
Brachycentrus species that is found in those same waters (Delaware/Beaverkill system) is
B. solomoni, named for fly fishing writer Larry Solomon. This is a darker species, but it emerges at the same time. Adult specimens in the collecting record are dated from April 29th to May 7th. (These include the holotype collected by Solomon from the Beaverkill at Roscoe.)
Here is another example of
appalachia from that same river system (notice Jason's comments on the darkening):
http://www.troutnut.com/specimen/727
Here is another
appalachia from the same system that shows the dark wing veins:
http://www.troutnut.com/specimen/511
And here are two darker specimens collected from the same system at about the same time. (These are still listed as
Apatania, but Creno thought they were
Brachycentrus, and he should know. They might even be
solomoni.):
http://www.troutnut.com/specimen/728
http://www.troutnut.com/specimen/714
The name Apple Caddis is usually applied to
B. appalachia. This is a newer nickname reflecting more modern usage and a shorter fly-fishing tradition on the Delaware. The name Shadfly or Shad Fly (which is also sometimes applied to Green Drakes/
Ephemera guttulata and Hex/
Hexagenia) is much older and reflects older traditions on the Beaverkill. Its application to
Brachycentrus species on the Beaverkill can be traced back (at least) to Louis Rhead's attempt to describe Catskill hatches in
American Trout Stream Insects (1920).
My apologies in advance for droning on about this.