72 EPHEMEROPTERA. 



dragon-flies but with atrophied mouth parts. The 

 sole function of the adult is, therefore, the reproduc- 

 tion of the species, and some of this group (Ccenis, 

 etc.) have reached an extreme stage of specialization 

 by reduction, being affected not only in their mouth 

 parts, but also in the decrease of the number of wings 

 to one pair, as in the Diptera, the hinder pair having 

 become atrophied. Thus, while this group appears to 

 indicate in part of its developmental history a very 

 ancient and primitive origin, in another part, it shows 

 that specialization by reduction has been at work, 

 probably greatly altering the original ancestral form, 

 and not only producing an existing adult type, whose 

 field of work is solely the reproduction of the species, 

 but also in the limited group to which Ccenis belongs, 

 culminating with species that imitate Diptera. 



