156 PROCEEDINGS ENTOMOLOGICAL SOCIETY 



arbitrary method but it has the advantage that, by stating where 

 the greatest width lies and then defining other dimensions in its 

 terms, one is able to describe the shape and proportions in brief 

 formula. 



THE FRONS. 



The shape and dimensions of the frons and its adfrontal margins 

 are characters of greater value and have been extensively used by 

 Dyar in his numerous descriptions, and by both Forbes (5) and 

 Fracker (6) in their diagnosis of families. The frons and adfrons 

 are usually treated as separate sclerites. In reality frons and ad- 

 frons are, as has been shown by Dampf (2), one piece, the only 

 suture being between the adfrons and epicranium, the line mark- 

 ing the division of frons and adfrons being merely an infolding. 

 The extent of this infolding varies considerably in different forms. 

 In the normal head of free-feeding larvae the external portion of 

 the adfrons is appreciable, and narrowly borders the frons, the 

 greater portion being folded in to form a strong chitinous, caudal- 

 ly pointed arch within the head. The points (Plate X, fig. 1) 

 where the frontal margins begin to converge sharply to make the 

 V-like line, indicate the attachment of the tentorial arms. In 

 sap-feeding larvae the frons is considerably widened and extends 

 back to the vertex of the head with little or no narrowing. In 

 these forms the infolding is greatly reduced and the adfrontal 

 margins absent. The points of attachment of the tentorial arms 

 are thrust correspondingly far back with a bridge between them, 

 connecting the dorsal hind margins of the epicranium. In 

 Mnemonica [Comp. Busck and Boring (1) ], on the other hand, 

 there is but a slight infolding to mark the lines of a rather normal 

 frons, and little or no reduction of the sclerite., the adfrontal mar- 

 gins being extended until they form a half circle covering a greater 

 part of the anterior dorsal surface of the head capsule. Such 

 skeletal modifications are necessitated by the environment and 

 biology of the larvae. Their degree would indicate therefor, the 

 extent to which any particular form had developed to meet a given 

 condition, but in spite of the fact that a similar biology will pro- 

 duce similar modifications, the different fundamental form of the 

 head structure remains distinguishable. 



A Cameraria and a Phyllocnistis rise to practically the same 

 level of development from different starting places. Their en- 

 vironment has caused similar modifications, but the type form 

 remains different. 



There is an internal cause for certain changes in the frontal 

 sclerite as well as the external one of alteration to accommodati 

 the larva to any particular mining habit, and that is the increase 

 or decrease of muscular tension at their points of attachment.' 



