Tinea pellionella is a type of clothes moth that makes a case out of fibers of its food (often wool) tied together with silk. The caterpillar drags its case wherever it goes and will retreat into its case when disturbed. As the larva grows in size, it slits the case and adds new patches to expand the case.
Tinea pellionella feeds on woolens, often in environments with low humidity. It can survive at 0% relative humidity. Chauvin and colleagues* studied the effects of the case on growth, water loss and water retention by the caterpillar. The case helps prevent water loss by the caterpillar and buffer against changes in relative humidity. Between 85% and 50% relative humidity, the case will lose accumulated water as the RH decreases with no loss of water from the caterpillar. Once the case has lost all water, the caterpillar loses water only slowly and far more slowly than a caterpillar that is not in a case. At humidity of 100% RH (saturated) the case accumulates maximum water and becomes heavy. Too high humidity can induce the caterpillar to leave its case and construct a new one.
The case can be a food reserve and a shelter for the pupa. Caterpillars that are denied food will start to consume the case after several days of starvation. When it is time for the larva to pupate, the larva will expand the case and pupate inside.
*G. Chauvin, G. Vannier & A. Gueguen. Larval case and water balance in Tinea pellionella. Journal of Insect Physiology, Volume 25, Issue 7, 1979, Pages 615-619.