A small mothlike insect of the order Trichoptera that lays its eggs in, water (6)
I believe the answer is:
caddis
(Other definitions for caddis that I've seen before include "Aquatic larva of fly" , "Fly of ponds and streams that camouflages itself" , "worm" , "Immature swimmer" , "Mothlike insect's larva" .)