Why was this?

N. S. Easton, « Original Observations », The Swiss Cross: a monthly magazine of the Popular Sciences. Vol 3-5, New York, Juin, Hodges, 1888, p. 191.

Caddis-Flies Construct New Cases

The water in a jar containing the larvae of caddis-flies became coated one night with ice an eighth of an inch thickl. The larvae withdrew themselves entirely from their cases, and when the ice thawed next morning they constructed new cases. Why was this ?