10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO7 



mandibular muscles and the facial group of frontoclypeal muscles. 

 The shift in the lines from one position to another affects the manner 

 of ecdysis, but the cranial structure is thereby no more altered than 

 is an orange changed in structure according to the way it is peeled. 



An understanding of the structure of the insect cranium must take 

 into consideration certain facts in the basic organization of the head. 

 Though the frons and the clypeus present externally a continuous 

 surface, which may be indented by an epistomal sulcus for purposes 

 of reinforcement, a real anatomical difference between the two areas 

 arises from the fact that, while the frons is a part of the postoral 

 anterior wall of the cranium, the clypeus, in its generalized state, is 

 preoral, since it is the basal part of a hollow lobe projecting in front 

 of the mouth (fig. 2 D) divided into clypeus {dp) and labrum (Lm). 

 Both the labrum and the clypeus, therefore, have an outer surface and 

 an inner "epipharyngeal" surface. Each has a set of internal com- 

 pressor muscles (cprlm dlch), and the movable labrum has muscles 

 (Ibrmcls) arising on the frons. On the frons are attached also, as 

 above noted, the precerebral dilators of the pharynx (dlphy), and 

 a pair of muscles (hphmcl) inserted on the oral arms of the suspensory 

 apparatus (HS) of the hypopharynx (Hphy). Since the clypeal 

 muscles and the frontal muscles are invariably separated by the frontal 

 ganglion (FrGng) and its brain connectives, this fact would seem 

 to have some important significance bearing on the fundamental 

 structure of the head. 



The frontal ganglion in its embryonic origin is derived from the 

 dorsal wall of the ectodermal stomodaeum just within the mouth ; its 

 nerve connections are with the tritocerebral lobes of the brain (fig. 

 2 A, T). The tritocerebral brain lobes, however, as shown by phy- 

 logeny and embryogeny, have been added secondarily to the primitive 

 suprastomodaeal brain, which included only the ocular and the 

 antennal centers (A, P, D) that become the protocerebrum and deu- 

 tocerebrum of the definitive brain. Primarily the tritocerebral lobes 

 (A, T) were the ventral ganglia of the premandibular somite 

 (B, GngI). If, therefore, the head ganglia are visualized as they 

 would be if restored to their primitive places (B), it is clear that the 

 frontal ganglion {FrGng) must have been an unpaired, or possibly 

 paired, preoral center of the ventral nerve cord with its connectives 

 from the premandibular ganglia {GngI) embracing the stomodaeum 

 just within the mouth. With the later transposition of the pre- 

 mandibular ganglia to the brain (A), of which they constitute the 

 tritocerebral lobes (T), they carry the frontal-ganglion connectives 

 with them, but the ganglion itself maintains its primitive position. 



