but someone will have to explain to me how this resembles, imitates or even suggests a beetle.
My personal theory (fortunately "unfalsifiable" in a Popperian sense

) is that trout (at least in current) don't really see insects as whole, intact images, but rather they view potential food sources "prismatically;" i.e.
something
in that whirl of motion of the fly tumbling downstream looks enough like
something they've eaten before that they're ready to give it a go. Absent a "negative trigger," that's enough to stimulate a reflexive behavior, even if the input is only (or especially?) that of simulated movement.
Or, to put it another way, trout look at the Bracken Clock they way, say, Picasso might: the bit of fluttery brown/maroon hackle might allude to the elytra of a Japanese Beetle, or an amber wing being crinkled by the current; the peacock herl might mimic the flash of light off chitin. In slower water, a smarter fish might see the dark tips of the hackle as the tibia/tarsus of a beetle's legs.
Or else, peacock herl is simply magic.
