244 ENTOMOLOGICAL NEWS. [June, 'l6 



The Lines of Descent of the Lower Pterygotan Insects, 

 with Notes on the Relationships of the other Forms. 



By G. C. CRAMPTON.* 



It is a rather interesting' fact that those structures which 

 are apparently of no vital importance to the organism, are 

 frequently of the greatest value as indications of relationships 

 between the larger groups (orders, etc.) of insects. This may 

 perhaps be explained by the fact that structures which are of 

 vital importance to the organism would be the ones most 

 directly concerned in the struggle for existence, and would 

 therefore be the ones most acted upon and most profoundly 

 modified by natural selection or, as a Lamarkian would ex- 

 press it, these structures would be the ones most frequently 

 employed, and would therefore be the ones the most pro- 

 foundly changed by use. 



The nature and arrangement of the thoracic sclerites furnish 

 examples of such structures, which are of little or no vital 

 importance to the organism (since they are practically want- 

 ing in some forms), yet are among the most important charac- 

 ters for determining the relationships of the different groups 

 of insects, since they remain as "true to type," or as little 

 modified, as any structures of the body ; and in most instances 

 have furnished the clues which were followed out and verified 

 by the study of other structures. 



Other structures which have proven to be of considerable 

 value in determining the relationships of the lower insects 

 are : the character of the antennal segments, the nature and 

 location of the compound eyes, the outline of the head, the 

 character of the mouthparts (particularly of the labium), the 

 nature of the terminal abdominal segments and their append- 

 ages (the female external genitalia being especially "true to 

 type," or but little modified) and the appendages of the thorax. 

 Conclusions based largely upon an examination of the wings 

 alone, which is apparently necessary in the study of fossil 

 insects, may lead to entirely erroneous results, unless verified 

 by the study of other sets of structures, and this may in some 



* Contribution from the Entomological Laboratory of the Massa- 

 chusetts Agricultural College, Amherst, Mass. 



