28 II.U.XOIS BIOLOGICAL MOXOGRAPflS [28 



The invaginations on the external surface of the head-capsule of the 

 metatentoria are the metatentorinae. In the Orthoptera the meta- 

 tentorinae are located along the cephalo-lateral or ventro-lateral border 

 of the occipital foramen as invaginations between the maxillariae and the 

 postgenae. They are not in any way associated with the submentum in 

 generalized insects or in the Coleoptera. The same relative position of 

 the metatentorinae is maintained in the Plecoptera. In a number of 

 Coleoptera, as in Helichus (Fig. 214), Stenelmis (Fig. 215), Heterocerus 

 (Fig. 216), and even in the platystomid, Eurymycter (Fig. 286), this same 

 generalized position of the metatentorinae is found. In a number of 

 Coleoptera the metatentorinae are situated considerably cephalad of 

 the occipital foramen. The question might be raised as to whether the 

 metatentorinae that are so located could possibly be more generalized in 

 position than those situated adjacent to the occipital foramen? In 

 every instance in which the metatentorinae are located cephalad of the 

 occipital foramen, a suture connects the metatentorinae with the occipital 

 foramen. In only a few cases does the suture extend much farther 

 cephalad than the metatentorinae. This suture in the vast majority of 

 heads studied does not extend cephalad. It is readily seen how the meta- 

 tentorinae might be drawn cephalad and as a result a suture be formed 

 marking their line of migration. In such a process one would naturally 

 not expect to find a suture located cephalad of the metatentorinae, and 

 in the cases in which the suture does extend so it is easy to understand 

 that the force of the cephalic pull might have been communicated to this 

 region, producing in consequence a suture or invagination. Due to the shape 

 of the head, it is most difficult to believe that the metatentorinae could be 

 drawn caudad, and if they were so drawn, it would seem that in this process 

 there would be formed a suture cephalad of the metatentorinae, marking 

 the line of migration. In this discussion the generalized position of the 

 metatentorinae will be considered as that of its generalized position in more 

 primitive insects, at or near the occipital foramen. 



In the development of the coleopterous head the metatentorinae have 

 shown a tendency to migrate cephalad. The cephalic migration of the 

 metatentorinae and the ventral migration of the pretentorinae and other 

 structures, were no doubt due to the same force, the result being a closer 

 approximation of parts, which naturally supplied increased firmness to the 

 head's mechanics of operation. The Dryopoidea show the most general- 

 ized position of the metatentorinae. Genera of this superfamily have been 

 mentioned above. The Elateroidea probably possess the next most gen- 

 eralized metatentorinae, such as in Sandalus (Fig. 208) and Alaus (Fig. 

 209). The Cucujoidea show the metatentorinae just a little removed 

 from the occipital foramen, as in Megalodacne (Fig. 234), Anchicera (Fig. 

 236), Philothermus (Fig. 240), and others. In the genera of other super- 



