[Illustration: FIG. 20.--_Hydrophilus piceus_ (Black Water Beetle). Europe.]
CLAVICORNIA.--This is a somewhat heterogeneous group, most of whose members are characterized by clubbed feelers and simple, unbroadened tarsal segments--usually five on each foot--but in some families and genera the males have less than the normal number on the feet of one pair. There are either four or six malpighian tubes. A large number of families, distinguished from each other by more or less trivial characters, are included here, and there is considerable diversity in the form of the larvae. The best-known family is the _Hydrophilidae_, in which the feelers are short with less than eleven segments and the maxillary palpi very long. Some members of this family--the large black _Hydrophilus piceus_ (fig. 20), for example--are specialized for an aquatic life, the body being convex and smooth as in the _Dyticidae_, and the intermediate and hind-legs fringed for swimming. When _Hydrophilus_ dives it carries a supply of air between the elytra and the dorsal surface of the abdomen, while air is also entangled in the pubescence which extends beneath the abdomen on either side, being scooped in bubbles by the terminal segments of the feelers when the insect rises to the surface. Many of the _Hydrophilidae_ construct, for the protection of their eggs, a cocoon formed of a silky material derived from glands opening at the tip of the abdomen. That of _Hydrophilus_ is attached to a floating leaf, and is provided with a hollow, tapering process, which projects above the surface and presumably conveys air to the enclosed eggs. Other _Hydrophilidae_ carry their egg-cocoons about with them beneath the abdomen. Many _Hydrophilidae_, unmodified for aquatic life, inhabit marshes. The larvae in this family are well-armoured, active and predaceous. Of the numerous other families of the Clavicornia may be mentioned the _Cucujidae_ and _Cryptophagidae_, small beetles, examples of which may be found feeding on stored seeds or vegetable refuse, and the _Mycetophagidae_, which devour fungi. The _Nitidulidae_ are a large family with 1600 species, among which members of the genus _Meligethes_ are often found in numbers feeding on blossoms, while others live under the bark of trees and prey on the grubs of boring beetles.