All of the above indirectly affect both the quantity and quality of the wood supply. They can be studied more in detail in the publications of the U.S. Bureau of Entomology. Of the insects directly attacking wood, the most important are the ambrosia or timber beetles, the borers, the ants, and the carpenter bees. The most remarkable feature of the beetle is the manner of its boring into the harder parts of the wood. Its jaws are particularly constructed for this work, being heavy and strong. The boring is done something after the manner of countersinking, and the jaws are believed to be self-sharpening, by reason of the peculiar right to left and left to right motion. _Ambrosia_ or _timber beetles_, Fig. 92. This class of insects attacks living, dead, and felled trees, sawlogs, green lumber, and stave-bolts, often causing serious injury and loss from the pin-hole and stained-wood defects caused by their brood galleries. The galleries are excavated by the parent beetles in the sound sap-wood sometimes extending into the heart-wood, and the young stages feed on a fungus growth which grows on the walls of galleries. (Hopkins, Entom. Bulletin No. 48, p. 10.) The growth of this ambrosia-like fungus is induced or controlled by the parent beetles and the young are dependent on it for food. (Hopkins, _Agric. Yr. Bk._, 1904.) [Illustration: Fig. 92. Work of Ambrosia Beetle, _Xyloborus celsus_, in Hickory Wood: _a_, Larva; _b_, Pupa; _c_, Adult beetle; _d_, Character of work in lumber cut from injured log; _e_, Bark; _f_, Sap wood; _g_, Heartwood. [_Agric. Year Book_, 1904, Fig. 44, p. 384.]] [Illustration: Fig. 93. Work of Ambrosia Beetles in Oak: _a_, _Monarthum mali_, and work; _b_, _Platypus compositus_, and work; _c_, Bark; _d_, Sap-wood; _e_, Heart-wood; _f_, Character of work in lumber from injured log. [_Agric. Year Book_, 1904, Fig. 45, p. 384.]] There are two general types or classes of these galleries, one in which the broods develop together in the main burrows, the other, in which the individuals develop in short separate side chambers extending at right angles from the primary gallery, Fig. 93. The galleries of the latter type are usually accompanied by a distinct staining of the wood, while those of the former are not. (Hopkins, _Agric. Yr. Bk._, 1904, p. 383.)