It is not soluble in water

Mary Roberts, The annals of my village: being a calendar of nature, for every month in the year, Londres, J. Hatchard, 1831.

Now, also, insects of the Trichoptera order prepare to cast inside their envelopes, and to leave their watery abodes. They made be seen on the gravelly beds of clear and shallow pools, where they resemble little moving piece of straw, or leaves, wood, or stones ; some enwrap themselves in four or five pieces of wood, which they glue together into a nest oblong case; others use for this purpose the leaves of aquatic plants ; others, again, make choice of the minute shells of young fresh-water muscles and snails, to form a moveable grotto; while the beautiful caddis-fly weaves together light, bending, fragile stems into an oval case, and in the interior of this contentedly resides. Yet, however differing in form, a silken grate or porteullis is uniformly affixed at either end. This is of woven silk- it is not soluble in water, and while it admits the aqueous fluid, it keeps out all intruders.