The X-Caddis is Craig Mathews' refinement of the Elk Hair Caddis, built for the time when fish refuse the high-floating original. There's no hackle. The body is dubbing, the wing is a sparse fan of deer or elk hair lying low over the back, and a tuft of amber or brown antron trails off the bend as a trailing shuck. That shuck is the whole idea: it pins the fly in the film and reads as a bug that hasn't finished hatching.
Tie or buy it in tan and olive, sizes 14-18, matching the caddis already on the water. Tan covers most spotted-sedge and tan caddis; olive covers the green-bodied bugs.
Fish it during a caddis hatch when trout are bulging just under the surface or sipping rather than slashing - that subtle take means they're eating emergers, not adults. Dead-drift it through riffles and seams. Best in clear to lightly stained water, low to normal flows, with water in the low-to-mid 50s up into the mid-60s. Grease the wing, not the shuck, so the back end hangs in the film.