17] THE HEAD-CAPSVLE OF COLEOPTERA—STICKXEY 17 



sutures resulting from this migration. Yet the predominant condition 

 of the heads studied shows only a certain degree of removal from the 

 primitive type. Stronger chitinization has probably kept pace with the 

 cephalization of the dorsal surface, as well as the growing compactness 

 of such a structure as the submentum. But the very significant structure 

 of the ventral surface, the metatentorina, only shows a certain degree of 

 removal from the primitive condition in most of the heads. 



Developmental processes such as are discussed above can be best 

 worked out through a comparative study of a large series of forms. In 

 fact the determination of the homologies of some structures entering into 

 these developmental processes, as for example the various changes in 

 the epicranial suture, and the determination of the nature and line of 

 development of the area between the occipital foramen and the submentum, 

 would probably be most difficult without this comparative study. The 

 chief value of this study lies in all probability in the determination of 

 homologies, to the end of understanding the lines of development present. 

 The homology existing between the various structures of the head-capsule 

 of Coleoptera and other orders of insects, particularly the generalized 

 orders, seems to work out satisfactorily. From such an homology the 

 hypothetical type was constructed without much difficulty. In general 

 appearance the head is oblong and rather flattened dorso-ventrally. Such 

 a form is fairly characteristic of generalized insects and of the more 

 generalized Coleoptera. The mouth may be considered as directed 

 cephalad. Such a direction is representative of the vast majority of 

 the heads, and for purpose of convenience, at least, the following 

 discussion considers the head as extending cephalad. Some possible 

 exceptions in which the head appears to be directed ventrad are 

 found in Calopteron (Fig. 329), Macrosiagon (Fig. 342), Isorhipis (Fig. 

 355), Throscus (Fig. 356), Byrrhus (Fig. 368), and a few others. It 

 should not be forgotten, however, that in primitive insects the mouth is 

 directed ventrad, and the occipital foramen is on one side instead of at 

 the opposite end. 



The line of closure of the head in the embryo is represented by the 

 epicranial suture. The complete epicranial suture is typical of generalized 

 insects. Where it is present in Coleoptera, this denotes a generalized 

 condition. The primitive form of the epicranial suture is that of a deep 

 inverted Y, with the cephalic ends of the arms near the lateral border of 

 the labrum. The hypothetical head is represented as having a complete 

 epicranial suture. The epicranial stem extends to a transverse line drawn 

 through the middle of the compound eyes. Branching here the epicranial 

 arms continue to the margin of the head cephalad of the compound eyes. 

 A complete epicranial suture is not of general occurrence in the Coleoptera. 

 It is practically complete in Hydrous (Fig. 13) and Hydrophilus (Fig. 14), 



