NO. I THE INSECT HEAD — SNODGRASS II 



where in the neck, and that the dorsal muscle fibers attached on the 

 head have become continuous through two consecutive segments. The 

 true condition here is hard to understand, and probably has not yet 

 been rightly explained, but it must be noted that muscles from both 

 the head and the prothorax may be attached on the lateral neck 

 sclerites. 



Theories of head segmentation, including the disputed question of 

 segments in the blastocephalon, will be discussed in a final section of 

 this paper (p. 38). 



II. GENERAL EXTERNAL STRUCTURE OF THE INSECT HEAD 



The typical insect head (fig. 4 A) is a craniumlike capsule mova- 

 bly supported on the thorax by a short membranous neck. The head 

 bears the eyes (E), the antennae (Ant), and the organs of feeding, 

 or mouth parts. The last include an upper lip, or lahrum (Lm), a 

 pair of mandibles (Md), a pair of maxillae (Mx), a lower lip, or 

 labium (B, Lb), and, enclosed between these parts (D), a median 

 tonguelike lobe known as the hypopharynx (Hphy). On the back 

 of the head (B) is a large opening (For) into the neck, analagous to 

 the foramen magnum of the vertebrate skull, but generally called the 

 occipital foramen. The only movable part of the head is the labrum, 

 which is either articulated on the clypeal area (A, Clp) above it, or 

 suspended from the latter by an ample membranous area, sometimes 

 called the anteclypeus. The labrum is really an appendicular struc- 

 ture provided with four basal muscles, two of which are anterior and 

 two posterior (C, 4), the latter attached on special sclerotizations 

 (Tor) known as the tormae. 



The cranial wall is continuously sclerotized, but it is usually marked 

 by grooves that appear to divide it into specific areas, which the 

 earlier entomologists regarded as sclerites united along "sutures." 

 This concept, however, is now seen to be entirely erroneous, as will be 

 shown in the next section, since the function of the grooves is to form 

 internal strengthening ridges. 



Enclosed by the mouth parts of insects such as the cockroach 

 and others that feed on solid foods is a space (fig. 4 D, PrC) that 

 serves for the intake of food and its mastication by the mandibles. 

 This space, therefore, has been known as the "mouth cavity" or 

 "buccal cavity" of the insect. However, the true mouth (Mth), or 

 opening into the alimentary canal, lies in the inner wall of this 

 cavity. The preoral food cavity (PrC), therefore, is merely a part of 

 the exterior enclosed between the labrum in front, the labium behind. 



