My biology background gives me a preference for using scientific names. Thus I know my aquatic insects by their scientific familes far better then by the common names used by fly fishers. Heptageniidae includes the March Browns and Cahill mayflies. My last trip on the Guadalupe for trout brought about this design after reeling in a leaf with a few of these mayflies on it. So here is my attempt to match the hatch.
Hook: Gamakatsu C13U
Thread: UTC Olive 140
Head: Brown Glass bead
Thorax: Olive Rabbit dubbing (topped with UV resin)
Body: Wrapped extra small blue wire ultrawire
Tail: Hack fibers