Bob,
I know this will probably bring a chorus of disagreement (it has in the past), but here goes anyway. The early fly fishers (talking very early) did not recognize the importance of the caddis to the trout and the fly fisherman. Some of the early authors discuss the caddis fly and then state that it is not an important food item to imitate because "they are on the water late in the evening". That is NOT to say that nobody knew of them and their behavior. They are mentioned in old books and their entomological classifications to be sure, but their importance to the fly fisherman was not widely accepted.
Since the late 50's (with Al Troth's EHC), perhaps, the caddis fly has been more generally recognized as a very important food item for trout and for the fly fisherman to imitate. Of course the work done by Gary LaFontaine (in the late 60's-early 70's), and others as well, did much to popularize patterns and the science of their life cycle and behavior brought the caddis into the mainstream of fly fishing....in my opinion.
Anyway, it is interesting that you point out the ubiquitous nature of the Caddis on the Broadhead where Lessening and Hidy Tied and fished their Flymphs........