Merrill decided

Maurice Burton, The sixth sense of animals, New York, Ballantine Books, 1973.

These larvae of caddis flies live in fresh waters. They have long bodies protected by a tube of silk threads stregthened by pieces of stick or leafg, tiny pebbles, even small shells of water snails. The tube reaches from the insect’s head to its tail, just long enough for it to cling by hooks on its rear end while pushing its front part out to frag the tubular case around and to feed. Merrill decided that the caddis worm uses sensory bristles on the hind end of the abdomen as measuring instruments, so he took one of the larvae and cut off these bristles. When put back into the water the larva started to build a new, and went on buiding until the case was three times as long as it should be- then gave up from sheer exhaustion. Without its bristles the larva had no means of telling that it had built a large enough case.