SEPTENARY SYSTEM. 243 



from the differences in the pairs of wings at all interfering with 

 each other, or with those in the prothorax, mesothorax and 

 inetathorax, and therefore requiring a different chain of rela- 

 tions, we find that all of them follow a similar system of varia- 

 tion, and each, used as a divisional character, would dissect the 

 figure in the same way. The following modications of struc- 

 ture are almost invariably found co-existent in the same group. 



Prothorax nearly obsolete, or appearing as a narrow ring imme- 

 diately behind the head : mesothorax fully developed and very 

 conspicuous : metathorax a narrow ring : fore wings ample, strong ; 

 the chief, sometimes the sole instruments, of flight : hind wings 

 nearly obsolete. 



These characters are, in a great measure, dependent on each 

 other. The circumstance of the fore wings being the chief or 

 sole organs of flight, insures an increase or maximum of volume 

 in the mesothorax, and a decrease in the volume of the pro- 

 thorax. The rudimental and inactive character of the hind 

 wings is accompanied by a decrease or minimum of volume in 

 the metathorax. These characters are those possessed by the 

 class Diptera : we will, therefore, draw a diagonal line through 

 the diagram from A O B, and we shall find tiiat all insects pos- 

 sessing ample fore wings, and a maximum of mesothoracic 

 development, are on the right side of the line. 



The characters so fully possessed by the class Diptera, are 

 also inagreat degree observable in HymenopteraandLepidoptera; 

 the power of the hind wings, however, has greatly increased, and 

 these become implements of flight, little inferior to the fore wings; 

 the mesothorax also yields a great portion of its volume to the me- 

 tathorax. Part of the class Neuroptera, the Phryganijia, theEp/ie- 

 meritia, and the Panorpina possess the whole of the characters 

 in question, but in a degree much modified, in accordance with 

 their supposed situation in the figure. We also find a portion 

 of the Hemiptera, (I allude to the true Cicadites,) possessing the 

 Dipterous character of mesothoracic development; and a corre- 

 sponding portion of the Hymenoptera, the Cepkites and Sirecites, 

 possessing a character belonging to the classes below the line, 

 that of prothoracic development. With these exceptions a com- 

 plete dichotomy is effected by the alary line marked A O B. The 

 exceptions, beautifully balanced as they are, serve to confirm 

 rather than invalidate the divisional character. 



