Many of the members of the sub-family Hydrophilides construct egg-cocoons. In the case of _Hydrophilus piceus_, the {218}boat-like structure is provided with a little mast, which is supposed by some to be for the purpose of securing air for the eggs. _Helochares_ and _Spercheus_ (Fig. 100) carry the cocoon of eggs attached to their own bodies. _Philydrus_ constructs, one after the other, a number of these egg-bags, each containing about fifteen eggs, and fixes each bag to the leaf of some aquatic plant; the larvae as a rule hatch speedily, so that the advantage of the bag is somewhat problematic. [Illustration: Fig. 100.—_Spercheus emarginatus_ ♀. Britain. A, Upper surface of beetle; B, under surface of abdomen, with the egg-sac ruptured and some of the eggs escaping.] The larvae of the aquatic division of the family have been to a certain extent studied by Schiödte and others; those of the Sphaeridiides—the terrestrial group of the family—are but little known. All the larvae seem to be predaceous and carnivorous, even when the imago is of vegetable-feeding habits; and Duméril states that in _Hydrous caraboides_ the alimentary canal undergoes a great change at the period of metamorphosis, becoming very elongate in the adult, though in the larva it was short. The legs are never so well developed as they are in the Adephaga, the tarsi being merely claw-like or altogether wanting; the mandibles are never suctorial. The respiratory arrangements show much diversity. In most of the Hydrophilides the process is carried on by a pair of terminal spiracles on the eighth abdominal segment, as in Dytiscidae, and these are either exposed or placed in a respiratory chamber. In _Berosus_ the terminal stigmata are obsolete, and the sides of the body bear long branchial filaments. Cussac says that in _Spercheus_ (Fig. 101) there are seven pairs of abdominal spiracles, and that the larva breathes by presenting these to the air;[98] but Schiödte states that in this form there are neither thoracic nor abdominal spiracles, except a pair placed in a respiratory chamber on the eighth segment of the abdomen, after the manner described by Miall as existing in _Hydrobius_. No doubt Cussac was wrong in supposing the peculiar lateral abdominal processes to be {219}stigmatiferous. In _Berosus_ there are patches of aëriferous, minute pubescence on the body. The pupae of Hydrophilides repose on the dorsal surface, which is protected by spinous processes on the pronotum, and on the sides of the abdomen.