Nos. 455-456.] 



HEAD OF BLATTA 



799 



of this structure have already been discussed. Regarding the 

 sclerites he states that the labrum and the clypeus arise from 

 the procephalon, while the remainder of the head capsule is 

 formed mainly by the bending over dorsally and the backward 

 extension of the cephalic lobes. 



It is Heymons who has devoted the most attention to a study 

 of the embryological development of the head sclerites. Study- 

 ing mainly the development of various species of the Orthoptera, 

 he very definitely stated his conclusions in his paper on the seg- 

 mentation of the insect body ('95a). The labrum and the clyp- 

 eus he derives from the procephalon, while the front, he con- 

 cludes, is developed from the fused cephalic lobes. The vertex, 

 occiput and genae he believes are developed from the fused 

 tergites of the mouth part segments, while the hypopharynx 

 arises from the fusion of the sternites of these same three seg- 



Janet, '99, finds in the sclerites but little indication of the 

 primitive segments. Thus he finds that the front " appartient, 

 par sa partie anterieure, au somite clypeopharyngien ; par sa 

 partie posterieure, au somite du labre ; par ses parties laterales, 

 au somite antennaire." Based upon years of study of the mor- 

 phology of certain Hymenoptera, Janet's paper presents a fund 

 of valuable anatomical data. From the embryological view point 

 it is less satisfactory. Moreover, though recognizing the con- 

 fusion in the use of terms to designate the head sclerites, Janet 

 has made no attempt to determine homologies. Until this has 

 been carefully done, results obtained from a study of such a 

 group as the Formicoidea can be of but little general application. 



On the other hand, Comstock and Kochi, : 02, attach very 

 great importance to the relation of the primary segmentation to 

 the sclerites of the adult insect. Their conclusions were based 

 upon comparative anatomical studies of a large series of the 

 more generalized insects, as well as upon embryological data. 

 According to their view, the typical segment is composed of two 

 subsegments. Ventrally the Hne of union of these subsegments 

 is determined by the position of the furcae, while laterally it is 

 similarly marked by the position of the lateral apodemes. Be- 

 tween the segments lie the tracheal invaginations, which these 



