994 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
biting type, without molar structure, and the ventral mouth parts are 
joined in a trapezoidal unit by fusion of stipes and labium. The 
mesothoracic spiracle is apparently pushed forward into the prothorax. 
The true bark-inhabiting forms are much thicker through the thoracic 
segments than elsewhere and are strongly curved. In many species 
of ambrosia beetles, the larvae are more elongate and more uniformly 
cylindrical. Although considerable work on scolytid larvae has been 
attempted, no entirely satisfactory larval characters separating the 
family from the Curculionidae or capable of being used in a key to 
the genera and species have been found. 
THE HABITS OF BARK BEETLES 
The Scolytidae differ from other beetles even more sharply in habits 
than in structure, and a species often may be recognized by its burrow 
as readily as by its body structure. Nearly all scolytids are borers, 
both as larvae and as adults, in the bark or wood of living or dead 
trees or shrubs, and all but a few hours of their existence is passed 
within their burrows. The most outstanding biological characteristic 
of the family is the secluded life of all stages; even the adults of most 
species are in the open only long enough to leave the old burrows ana 
find new hosts in the proper condition for attack. 
The family may be separated into the three following groups as re- 
gards the location of their burrows and the character of the food of the 
larvae and adults: (1) The true bark beetles, which construct their 
burrows either entirely in the inner bark or at the juncture of the bark 
and sapwood; (2) the xylophagous, or wood-eating scolytids, which 
excavate burrows through the sapwood and heartwood and feed on 
the ligneous tissues at all stages of their active life; and (3) the am- 
brosia beetles, which construct their burrows in the sapwood and feed 
both in the larval and adult stages on fungi, known as ambrosia, which 
they cultivate within their tunnels. 
Forest trees serve as the breeding place of most species of bark 
beetles, but fruit trees and even herbaceous plants are subject to the at- 
tacks of a few species. Ornamental trees, which are usually forest 
trees growing under unnatural conditions, are often affected, and 
this is especially so in the case of conifers. In tropical countries, and 
to a much less extent in temperate regions, a number of species attack 
seeds and nuts, and in some cases casks containing water, wine, or 
other liquids may be damaged by the burrows. But for the most part, 
both adults and larvae live in the bark or wood of trees. 
Bark beetles usually show a rather close discrimination in their 
choice of a breeding place. Usually a certain species has adopted as 
its host either a single species of tree or any one of several closely allied 
species. Many species confine their attacks to pies, whereas others 
may breed in either pine, spruce, or larch. A few species, however, 
show little discrimination between several genera of trees not closely 
related botanically, as in the case of Dryocoetes betulae Hopk., which 
breeds not only in several species of birch, but also in beech, wild 
cherry, and red gum. 
The part of the tree chosen for attack by bark beetles also is often 
indicative of rigid discrimination by certain species. Some are found 
only in the lower trunk, others only in the upper trunk, some in the 
larger limbs, others in the smaller limbs, still others in the small twigs, 
