They are no doubt

Evelyn Cheesman, Everyday Doings of Insects, New York, MacBride, 1925, p. 155.

The cases in which some grubs live during the whole of their larval existence are more interesting than cocoons, for they are caravans, movable houses which they carry with them wherever their needs urge them. They are no doubt adopted as a protecting against ennemies, those grubs which use them being much sought after as food by other animals.