110 Annals Entomological Society oj America [Vol. XI, 



spiracles are ever present caudad of the eighth somite, and, in 

 most cases, where the somites caudad of the seventh are 

 retracted, spiracles are lacking in the eighth. Therefore it is 

 evident that the adult abdomen always presents, to the eye at 

 least, a far more simple composition than do the head and 

 thorax. In reality, in many larvae an abdominal somite is an 

 entire ring without differentiation into tergite or sternite, an 

 arrangement often seen in the lower thoracic somites, and a 

 more nearly primitive one than that above mentioned. 



The general shape of the abdomen, taken as a whole, also 

 varies considerably. According to the best authorities, one 

 ought to recognize the abdomen, systematically, from the 

 number of its somites and from its appendages. The fact sug- 

 gested above, that the caudal somites may be more or less, even 

 entirely, concealed by retraction, is responsible for the number- 

 ing of abdominal somites which is based only upon the number 

 of visible somites and has no justification in morphology. Dis- 

 agreement upon this point is found, however, not only among 

 the systematists, who formerly limited themselves to numbering 

 the abdominal somites which were undoubtedly visible, but also 

 among the morphologists. The latter differ in their results for 

 various .reasons, as follows: (1) Because of the loss of certain 

 sclerites. (2) Because of differences in the point of view: e. g., 

 some investigators begin to count at the cephalic, while others 

 begin at the caudal end of the abdomen, and still others do not 

 correlate closely dorsal and ventral sclerites. (3) Because, 

 undoubtedly, of failure to observe the sclerites present in the 

 caudal region, due in many cases to the lack of potash or other 

 suitable reagents and methods for exposing chitinized parts. 



2. Theories and Investigators. — There follows at this 

 point a list of prominent investigators together with the number 

 of abdominal somites which they have believed were present 

 in the adults of certain orders: 



Orthoptera. 



1872 Chadima 11 abdominal somites 



1876 Brunner von Wattenwyl .9 abdominal somites 



1882 Tarzioni-Tozzetti 11 abdominal somites 



CoLEOPTER.\. 



1894 Peytoureau (in Hydrophilus) 8 abdominal somites 



1894 Peytoureau (in Dytiscus) 9 abdominal somites 



1893 Kolbe (in lamellicornia) 9 abdominal somites 



1893 1894 1896 VerhoefT 10 abdominal somites 



1903 Berlese (in Coleoptera in general) 11 abdominal somites 



