Insects have two objects in their attack on trees, one is to obtain food, as when they are in the larval stage, and the other is to provide for offspring, as do certain beetles. The number of insect enemies of the forest is enormous. At the St. Louis Exposition, there were on exhibit nearly three hundred such insects. These belong to some twenty orders, of which the beetles (_Coleoptera_), which have horny wings and biting mouth parts, and the moths and butterflies (_Lepidoptera_), with membraneous wings and sucking mouth parts, are the most destructive. Insects attack every part of the tree, the seed, the shoot, the flower, the root, the leaf, the bark and the wood, both standing and cut. Of the fruit and seed pests, the most destructive are weevils, worms and gall insects. Of the twig and shoot pests, beetles, weevils and caterpillars are the worst. Among insects that attack roots, the periodical cicada (17 year old locust) may be noted. The leaf pests are far more serious. They include the true and false caterpillars, moths, gall insects and plant lice. Of the bark pests, the bark beetles are the most destructive. These are also called Engraver Beetles from the smoothly cut figures which are their burrows under the bark, Figs. 89, 90, 91. [Illustration: Fig. 89. Work of the Spruce Destroying Beetle: _a._ Primary gallery; _b._ Borings packed in side; _c._ Entrance and central burrow thru the packed borings; _d._ Larval mines. Note how the eggs are grouped on the sides. [_Agric. Year Book_, 1902, Fig. 24, p. 268.]] [Illustration: Fig. 90. Complete brood Galleries of the Hickory Bark Beetle in Surface of Wood. [_Agric. Year Book_, 1903, Fig. 28, p. 316.]] [Illustration: Fig. 91. Brood Galleries of the Oak Bark Beetle, showing Character of Primary Gallery at _b_; Larval or Brood Mines at _a._ [_Agric. Year Book_, 1903, Fig. 30, page 318.]] Many pairs of beetles make a simultaneous attack on the lower half of the main trunk of medium-sized to large trees. They bore thru the outer bark to the inner living portion, and thru the inner layers of the latter; they excavate long, irregular, longitudinal galleries, and along the sides of these at irregular intervals, numerous eggs are closely placed. The eggs soon hatch and the larvae at once commence to feed on the inner bark, and as they increase in size, extend and enlarge their food burrows in a general transverse but irregular course, away from the mother galleries (see illustration). When these young and larval forms are full grown, each excavates a cavity or cell at the end of its burrow and next to the outer corky bark. (Hopkins, _Agric. Yr. Bk._, 1902.) Some of the species attack living trees, causing their rapid death, and are among the most destructive enemies of American forests.