In the ants, the size and shape of the mandibular flexor muscles largely determine the form of the head and face, and functional peculiarities of the mandibles are closely correlated with the character of the flexor muscles. This holds for species and for castes within species. Types of mandibles may be placed in 9 groups: cutting, biting, gnawing, crushing, grappling, clipping, piercing, vestigial, and aberrant. The 1st 4 types are large and powerful, with heads correspondingly developed. The last 5 types are relatively weak, with less powerful muscles and small heads. As extremes in the same species, soldiers and large-headed workers contrast with small-headed [male][male] with mandibles feebly developed. A similar condition exists among species of beetles, except that here the [male][male] have the more powerful mandibles. In certain species, conspicuous, frontal convexities are due to the great development of flexor and extensor antennal muscles. Some adaptive structures in the head configuration, such as scrobes, are due to antennae. In [male] ants having large eyes, the shape of the head is correspondingly modified. This is more conspicuous in certain haustellate insects (Diptera, Lepidoptera, Mecop-tera, Heteroptera, etc.). In some of the latter the eyes may form most of the [male] head. Development of hairs on the gula, mandibles, and clypeus of certain species of ants indigenous to dry regions gives them a "bearded" appearance. Adaptations to non-living environment are found in the elongated head and body of ants living in narrow cavities, and in peculiar modifications ("phrag-mosis") for closing the burrows. Shape of thorax is largely dependent on development of wing musculature, although development of leg muscles is also important. The abdomen physiognomy is determined more by volume of viscera than by development of segmental musculature.