12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. lO/ 



In the primitive condition of the nervous system, accompanied by 

 what may be supposed to have been the primitive musculature of the 

 anterior part of the head (fig. 2C), the frontal ganglion (FrGng) 

 must have lain between the compressor muscles of the clypeus (cprdp) 

 and the postoral group of frontal muscles. The latter included dilators 

 of the pharynx (dlphy), and possibly a pair of retractors of the mouth 

 angles (rao). The clypeal compressors become in the modern insect 

 (D) the dilators of the cibarium (dlcb), and, with the development 

 of the hypopharynx, the mouth-angle retractors become the muscles 

 of the hypopharyngeal suspensoria (hphmcl). In generalized ptery- 

 gote insects the frontal muscles of the hypopharynx are functionally 

 productors of this organ, but they are retained in Neuroptera, 

 Coleoptera, and Hymenoptera, in which the hypopharynx is inti- 

 mately united with the labium (fig. 13 A, F) and has no independent 

 movement. 



The development of the insect hypopharynx (fig. 2 D, Hphy) from 

 a metastomial lobe of the head creates a food passage (/w^) leading 

 to the mouth between the inner wall of the labrum and clypeus and 

 the anterior wall of the hypopharynx. At its inner end the food 

 passage is usually enlarged by a depression of the hypopharyngeal 

 surface to form a food pocket, the cibarium {Cb), beneath the clypeus. 

 The pocket, therefore, may be expanded by the contraction of the 

 compressor muscles of the clypeus, which thus become dilators of the 

 cibarium (dlcb) ; it is compressible either by transverse muscles on 

 the clypeal wall, or by elasticity of the latter. The cibarium is there- 

 fore capable of a sucking action, and in the liquid-feeding insects it 

 is elaborated into a highly efficient ingestion pump. 



With the greater development of the cibarial pump, the clypeal 

 dilator muscles become greatly increased in size, and, to accommodate 

 these muscles, the outer wall of the clypeus is correspondingly en- 

 larged by an upward extension on the face. There is thus always a 

 close correlation between the size of the clypeus and the size of the 

 pump muscles, and in most of the insects that feed exclusively on 

 liquid food the clypeus, or the clypeal area of the head wall, occupies 

 a relatively large part of the facial surface of the cranium. If the 

 clypeus is not set off from the frons by an epistomal sulcus, its 

 proper area is always to be identified by the origins of the dilator 

 muscles of the sucking pump, and in no such case is it necessary to 

 suppose that the clypeus in its upward or posterior extension has 

 taken over the muscles of the invaded frons. 



