ILLINOIS BIOLOGICAL MONOGRAPHS [16 



HEAD-CAPSULE 



There exists a distinct homogeneity in the general character of the 

 structure of the head-capsule of Coleoptera. Its uniform strong chitiniza- 

 tion is typical. So is the spacious area occupied by the mouth-parts, 

 producing a relatively broad cephalic end. Especially characteristic 

 is the wide space between the occipital foramen and the submentum. 

 Then, there is that indescribable similarity of structure, even between 

 groups widely separated, that can best be appreciated from a thorough 

 knowledge of the morphology. As an illustration, there is little super- 

 ficial resemblance between Harpalus (Figs. 5 and 153) and Phalacrus 

 (Figs. 96 and 244), either in external or in internal morphology. Their 

 distinct differences are merely due to two divergent lines of development. 

 The structures of the dorsal surface of Phalacrus have become highly 

 specialized, whereas those of Harpalus are relatively generalized. On 

 the other hand, on the ventral surface the metatentorina has remained 

 in a relatively primitive condition in Phalacrus, whereas in Harpalus its 

 position is highly specialized. The internal structures of Phalacrus are 

 rudimentary or lacking, while in Harpalus they are in a well developed 

 primitive state. Considering the degree of generalization of each species, 

 there can hardly be any question that Harpalus is the more generalized. 

 The above comparative description simph' illustrates roughly the problems 

 of complexity of development that are encountered. The two distinctive 

 kinds of development as shown above for Harpalus and Phalacrus, that of 

 specialization of the dorsal surface and that of separate specialization of 

 the ventral surface, do not in the least necessarily parallel one another in 

 the Coleoptera. Indeed, these two lines of development are predominantly 

 divergent. In the majority of species, the development is trending towards 

 the obliteration of sutures and consequent consolidation of sclerites, and 

 towards the development of a compactness of form of the sclerites that do 

 not consolidate. The general trend towards a cephalization of migratory 

 structures is a part of this process, too, as well as the development of a 

 stronger chitinization of the head-capsule as a whole. The entire phenome- 

 non appears to be for purposes of strengthening the head. Besides 

 Phalacrus typical examples are Tenebrioides (Fig. 78), GHschrochilus 

 (Fig. 80), Megalodacne (Fig. 86), and all the Scarabaeoidea. On the 

 other hand these processes have lagged behind on the ventral surface in 

 Phalacrus and others. It is true that the hypothetical type (Fig. 149) 

 shows a marked cephalization of the submentum, with obliteration of 



