34 CLASSIFICATION. 



and are wanting in several genera of Poduridce. 

 Lastly, the saltatorial appendage is absent in the 

 Lipuridce, and tliough present in all the Smynfhuridce, 

 Pajnriidas, and Poduridce, we must remember that the 

 saltatorial appendage of Podura and AcJiorutes is not, 

 in fact, homologous with that of the other genera, an 

 important point, the significance of which has been 

 overlooked by previous authors. 



Again, the mouth-parts of the Lejnsmidce and Po- 

 duridcB are constructed on a totally different plan. 



Dana^ reg^ards the Thysanura and Collembola as true 

 insects, but as forming one of three great groups into 

 which that class may be divided, namely — 



Ptero-prosthenics, or Ohenopters, including the Hy- 

 menoptera, Diptera, Aphaniptera, Lepidoptera, 

 Homoptera, Trichoptera, and Neuroptera ; 



Ptero-metasthenics, or Elytropters, comprising the 

 Coleoptera, Hemiptera, and Orthoptera ; and 

 thirdly. 



The Thysanura. 



It appears to me that the two former groups are 

 more nearly allied to one another than they are to the 

 third ; or, at least, to that portion of the third group 

 for which I have proposed the name " Collembola." 



As regards other Apterous groups of insects, Dana 

 himself observes^ that " the apterous Pediculi, as Nitzsch 

 long since observed, have no characteristics that would 

 separate them from Hemipters, and the Nirmids none 

 that would remove them from the Orthopters. They 

 are simply inferior wingless species of those types, as 

 much as the Coccids are of Homopters, and they have 

 nothing of the agility of the Lepismids. There are no 

 points of structure indicating an affinity to any two or 

 more of the higher subdivisions of insects, or to the 

 inferior Myriapods ; they are not urosthenic, being in 

 no way essentially different, as regards their legs, from 

 the types to which they are referred." 



' ' American Journal of Science,' Janiiary, 1864. 

 2 ' Loc. cit., p. 26. 



