; _d_, sapwood; _e_, heartwood.]

[Illustration: Fig. 23. Work of Ambrosia Beetles in Oak. _a_,
_Monarthrum mali_ and work; _b_, _Platypus compositus_ and
work; _c_, bark; _d_, sapwood; _e_, heartwood; _f_, character
of work in wood from injured log.]

The characteristic work of this class of wood-boring beetles is shown
in Figs. 22 and 23. The injury consists of pinhole and stained-wood
defects in the sapwood and heartwood of recently felled or girdled
trees, sawlogs, pulpwood, stave and shingle bolts, green or
unseasoned lumber, and staves and heads of barrels containing
alcoholic liquids. The holes and galleries are made by the adult
parent beetles, to serve as entrances and temporary houses or
nurseries for the development of their broods of young, which feed on
a fungus growing on the walls of the galleries.

The growth of this ambrosia-like fungus is induced and controlled by
the parent beetles, and the young are dependent upon it for food. The
wood must be in exactly the proper condition for the growth of the
fungus in order to attract the beetles and induce them to excavate
their galleries; it must have a certain degree of moisture and other
favorable qualities, which usually prevail during the period involved
in the change from living, or normal, to dead or dry wood; such a
condition is found in recently felled trees, sawlogs, or like crude
products.

There are two general types or classes of these galleries: one in
which the broods develop together in the main burrows (see Fig. 22),
the other in which the individuals develop in short, separate side
chambers, extending at right angles from the primary galleries (see
Fig. 23). The galleries of the latter type are usually accompanied by
a distinct staining of the wood, while those of the former are not.

The beetles responsible for this work are cylindrical in form,
apparently with a head (the prothorax) half as long as the remainder
of the body (see Figs. 22, _a_, and 23, _a_).

North American species vary in size from less than one-tenth to
slightly more than two-tenths of an inch, while some of the
subtropical and tropical species attain a much larger size. The
diameter of the holes made by each species corresponds closely to that
of the body, and varies from about one-twentieth to one-sixteenth of
an inch for the tropical species.

Round-headed Borers

[Illustration: Fig. 24. Work of Round-headed and F