20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I07 



clypeus are usually separated by an epistomal sulcus and ridge 

 (fig. 5 A, G) ; but in the Heteroptera, as noted by Spooner (1938), 

 the f rons and the postclypeus are always confluent. In the latter sub- 

 order, therefore, the respective areas of the frons and the clypeus 

 can be determined only by an examination of the muscle attachments 

 on their inner surfaces. The muscles of the sucking pump pertain 

 always to the clypeus. Since the usual labral and hypopharyngeal 

 muscles are absent in Hemiptera, only the pharyngeal muscles of the 

 frons are present, and they are to be identified as such by their sepa- 

 ration from the pump, or cibarial, muscles by the frontal ganglion and 

 its brain connectives. 



The variations in the point of forking of the cleavage line on the 

 head of Hemiptera are clearly correlated with the posterior extent 

 of the frontoclypeal muscles on the dorsal wall of the cranium, since 

 the arms of the line always diverge from the coronal stem above or 

 behind these muscles (fig. i C-G), In the lygaeid Oncopeltus, for 

 example, the frontoclypeal muscles spread over a large part of the 

 upper wall of the head (fig. 6F, dlch, dlphy), and consequently the 

 frontal arms of the cleavage line branch far back on the head from a 

 very short coronal line (A), leaving only a small area to the vertex. 

 In the Pentatomidae, however, the cibarial muscles (G, dlcb) of the 

 sucking pump are so large that they crowd the pharyngeal muscles 

 (dlphy) to the extreme posterior edge of the cranium. In this family, 

 therefore, the frontal arms of the cleavage line branch directly from 

 the dorsal cleft of the thorax (D, E), leaving the median area of 

 the cranium intact. The dorsal part of the vertex in such species is 

 eliminated, and the frons is in contact with the neck membrane. On 

 the other hand, in the Fulgoroidea the clypeal nmscles and the clypeus 

 itself are relatively small (fig. 5 G, Pclp), while the frons {Fr) is a 

 large area on the front of the head, and the vertex forms the upper 

 surface limited anteriorly by the arms of the cleavage line (D, E). 



The Aphididae appear to be exceptions among the Hemiptera in 

 their manner of splitting the head cuticle at ecdysis. According to Hen- 

 riksen (1932) there is no forking of the cleavage line on the head in 

 either the Aphididae or the Psyllidae. Exuviae of Hyadaphis xylostei 

 and those of another species examined by the writer show only a 

 median split of the head cuticle from the occipital margin to the post- 

 clypeus (fig. 5 H, I). Weber (1928) in his study of Aphis fabae 

 Scop, concludes that the frons and the vertex are fused since there is 

 no line of separation between them, but he says that in the wingless 

 form there is a suggestion of a median line on the head. However, 

 considering the small size of the frontal area between the arms of 



