AbstractIn four species of caddis‐worms,‐Ptilostomis semifasciata(Say),Platycentropus radiatus(Say),Limnephilus indivisusWalker, andPycnopsyche guttifer(Walker), —removal of the anal hooks and the posterior hair sensilla significantly prolonged case‐building activity with a resulting increase in case production. The extensive building was of the rough provisional type, which is normally completed within a few hours. There were differences among the species with regard to the amount of excessive building and the relative importance of the hairs.Transection of the ventral nerve cord appeared to block the extensive building that normally followed hook removal.It is proposed that the stimulus for building is an endogenous process which causes excitatory impulses to be emitted from the several ganglia. The impulses go forward to the integrative areas controlling the animal's building apparatus and stimulate building. The emission of impulses from the ganglia is inhibited by sensory input from contact with t