The downwardly directed head is
covered by the pronotum, and the three terminal antennal segments form
a distinct club. To this group belong the _Bostrychidae_ and
_Ptinidae_, well known (especially the latter family) for their
ravages in old timber. The larvae are stout and soft-skinned, with
short legs in correlation with their burrowing habit. The noises made
by some _Ptinidae_ (_Anobium_) tapping on the walls of their burrows
with their mandibles give rise to the "death tick" that has for long
alarmed the superstitious.

[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
numero