ufansius:

Caddis Fly Larva – Wildscape

Caddis fly larva live in streams, building themselves little shells of whatever material they can find. As a child I had a little collection of these found in the stream in the woods near my house, little inch-long tubes of tiny pebbles or bits of stick; Wildscape created an artificial stream, seeding it with bits of various materials (opal, in this instance) for their captive caddis flies to use in building their shells. I posted another example here.