GENERAL EXTERNAL STRUCTURE OF INSECTS. 15 



of an insect, they are usually regarded as morphologically impor- 

 tant things in themselves, representing a tendency of the tergum, pleu- 

 rum, or sternum to separate into smaller plates for some reason. The 

 truth about them would appear to be just the opposite in most cases — 

 they are the unavoidable external marks of an internal thickening 

 and strengthening of the plates. In a few cases they may be the 

 confluent edges of separate centers of chitinization. Hence, most of 

 the sutural lines in insects apjDear to signify a bracing or solidifying 

 of the body wall rather than a division of it. 



Since the body wall of insects is continuous over all the surface it 

 contains no articulations of the sort that occur between the bones in 

 th'e skeleton of a vertebrate. Although insects and their allies be- 

 long to the class of animals known as the Articulata, j^et an articu- 

 late articulation is simply a flexibility — two chitinous parts of the 

 exoskeleton are movable upon each other simply bj^ the intervention 

 of a nonchitinized, flexible, membranous part. While there are often 

 special ball-and-socket joints developed, these are always produced 

 on the outside of the membranous hinge and simply control or limit 

 the movement of the articulation. 



The head of an adult insect is a thin- walled capsule containing the 

 brain, the ventral head ganglion of the nervous system, the pharynx 

 and anterior part of the cesophagus, the tracheal tubes, and the 

 muscles that move the antennae and the mouth parts. Its shape varies 

 a great deal in different insects, being oval, globular, elongate, or 

 triangular. In some it is flattened dorso-ventrally so that, the face is 

 directed upward and the mouth forward, but in most, including 

 the bee, it is flattened antero-jDosteriorly so that the face looks for- 

 ward and the mouth is directed ventrally. In a few it is turned so 

 that the face is ventral. The walls of the head are usually divided 

 by sutures into a number of sclerites, which in general are located 

 and named as follows: The movable transverse flap forming the 

 upper lip is the labrum. Above it is a sclerite called the clypeus, 

 which is a part of the solid wall of the head and carries the anterior 

 articulations of the mandibles. The clypeus is sometimes divided 

 transversely into an anteclypeus ("clypeus anterior," "epistoma") 

 and into a post-clypeus ("clypeus posterior"). Above the clypeus 

 is the front, a plate usually occupying the upper half of the face 

 between the compound eyes and carrying the antennae. The top of 

 the head is called the vertex, but does not constitute a separate scle- 

 rite. The sides of the head below the compound eyes are often sepa- 

 rated by sutures from the anterior and posterior surfaces and are 

 known as the gence. The back of the head is formed by the occiput, 

 which surrounds the large opening or foramen magnum that leads 

 from the cavity of the head into that of the neck. The parts pos- 

 terior to the gense, carrying the posterior mandibular articulations. 



