NO. 1687. THE THORAX OF IXSECTS—SNODORASS. 517 



Comstock and Kochi (1902) attempt to assign the various head 

 sclerites of the adult to individual segments of the embryonic head. 

 Kile}' (1904:) in studying the cockroach arrives at different results, 

 hut he discredits the reliability of all attempts to map out the adult 

 head according to .segments. In discussing Comstock's view, he says: 

 '" My residts have convinced me that so intimate a relation between 

 primary .segmentation and the sclerites can not be shown." 



Of course, the ventral part of the preoral region becomes dorsal 

 so that the mouth, which is originally on the middle of the ventral 

 surface of the head, comes to be situated anteriorly. Thus the 

 labrum, cljqDCus, and front are developed from a primitive ventral 

 surface. So, in a general way, the other sclerites arise from definite 

 regions, but they are simj)ly secondary divisions of a continuous head 

 capsule, and the notion that they are modified terga, pleura, and 

 sterna of the head metameres appears to be entirely unsupported 

 by actual evidence. 



The last head segment is the one that chiefly concerns us in a study 

 of the thorax. All embryologists seem to agree that its body forms the 

 sclerites found in the neck of the adult and that only its f u.sed append- 

 ages, the labium, become asociated with the head, except when there 

 is a. gular plate prfisent, which sclerite is derived from its sternum. 

 This embryonic segment can, therefore, hardly be spoken of as a 

 head metamere. It is the segment of the neck and this, in the adult, 

 has received the name of " microthorax." 



Hence we would, accept six primitive head segments, providing the 

 a])parent superlingual segment is a real one, and one microthoracic 

 or neck segment. 



2. Segmentation of the Body. 



The foregoing discussion of the segmentation of the head has been 

 made more extensive, perhaps, than a mere introduction to the .study 

 of the thorax would require. But the writer wishes to illustrate to 

 anj'one not familiar with the subject the utter futility of attempting 

 a study of metamerism on an anatomicalbasis. The embryology of 

 the thorax has never brought out much more than that three segments 

 compose it, except in the Hymenoptera, where the first abdominal 

 segment is fused with the thorax. Hence there are no embryological 

 facts concerning the thorax by which anatomi.sts can be held in check, 

 but, with the unfortunate example of both the vertel^rate and the 

 insect head in mind, one must certainly regard with much doubt all 

 theories of thoracic metamerism based on a study of the plates form- 

 ing the very apparent three segments in the adult. Riley (1904) 

 makes the following appropriate .statement : 



It would seem that the definitive sclerites can afford little or no evidence as to 

 the primary segmentation of insects. Tliis is certainly true of the head sclerites 



