Hook Demmon E320 (big is better for this model), 10-14
Materials
Extended body Cinnamon, brown or olive (2mm micro chenille)
Hot spot / egg sac Chartreuse arctic fox (Troutline UV resin)
Wing Natural grey dryer lint (CDC feathers)
Hackle Rusty (Squirrel dubbing in a dubbing loop)
How to tie it
Cut a length of 2mm micro chenille in cinnamon, brown or olive, taper one end, and bind it along the top of the shank so it projects past the bend as an extended body. Tie big - 10 to 14. Because the extended body needs room and this fly works better large.
Bind the chenille down the full length of the shank to the 70% mark so it cannot rotate, and trim the excess.
Put a small blob of chartreuse UV resin at the very tip of the extended body and cure it. This is the egg mass, and that hard bright green ball is the fly's whole trigger for fish keyed on egg-laying females.
Tie in a bunch of natural grey CDC at the 70% mark as a low tent wing extending just past the egg blob, and trim the butts.
Load a dubbing loop with rusty squirrel dubbing, spin it, and take two or three turns in front of the wing as a hackle collar, sweeping the fibres back.
Whip finish a small head. The stiff squirrel guard fibres hold the fly up and let it be twitched on the surface, which is how egg-laying caddis behave. Dead drift catches fish, but a small skate or twitch catches more.