Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
the more important feature to me is the the core of most chenille is cotton thread and the core of vernille is some sort of plastic thread
That becomes important when tying San Juan Worms or Poopah Caddis patterns. Both patterns call for melting the material so the exposed ends don't unravel. The chenille cotton core burns rather than melting..