46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO7 



cibarial muscles lying anterior to the ganglion are therefore primarily 

 preoral, while the hypopharyngeal and pharyngeal muscles attached 

 on the head wall behind it are postoral. 



10. The facial area of the head on which the cibarial muscles take 

 their origin is, in generalized insects, the outer wall of a preoral lobe 

 of the head known as the clypeus, which supports the labrum, and 

 carries on its basal angles the anterior articulations of the mandibles. 

 The cibarial muscles are primarily compressors of the clypeus. 



11. The facial area of the head above or behind the clypeus, on 

 which the hypopharyngeal, pharyngeal, and labral muscles take their 

 origin, is commonly termed the frons. In some insects the frontal 

 area is separated from the clypeal area of the head wall by a trans- 

 verse groove, the epistomal sulcus, but in others the frontal and 

 clypeal areas are continuous. When they are not thus separated the 

 respective areas are to be identified by the muscles attached on them, 

 and the identity of the two sets of muscles is to be determined by the 

 position of the frontal ganglion and its brain connectives. 



12. The term "frons" cannot be given a precise anatomical mean- 

 ing. If "the frons" is defined as the facial area above the clypeus 

 embraced by the arms of the cleavage line, it will vary in extent 

 according to the position of the arms, in some insects it would carry 

 the antennae, in others it would not. The term "frons," therefore, 

 is serviceable only when applied to the general facial area of the head 

 above the clypeus or the clypeal area. 



13. The clypeus, being the area of attachment of the cibarial 

 muscles, varies in extent according to the size of these muscles ; the 

 size of the muscles depends on the development of the cibarium. The 

 clypeus, therefore, is usually relatively large in liquid-feeding insects, 

 in which the cibarium forms the principal sucking pump, and in these 

 insects the clypeus is extended upward, or posteriorly, at the expense 

 of the frons. When, in such cases, an epistomal sulcus is absent, the 

 muscle attachments relative to the frontal ganglion still serve to 

 identify the respective areas of the clypeus and the frons. 



14. Just as the frons is usually cut into three sections by the arms 

 of the cleavage line, so also the clypeus may be divided when the arms 

 extend through the clypeal area. The facial apotome, or triangle of 

 the head cuticle cut out at ecdysis, therefore, may be either the median 

 part of the frons alone, or the median parts of the frons and clypeus, 

 depending on the length of the ecdysial clefts. 



