CLASSIFICATION OF INSECTS. 51 



zation in such cases as Locustidae, Perlidae, Termites, Neu- 

 roptera, is brought about by the addition of characteristics 

 to the supposed primitive winged ancestral types. The 

 existing animals being more complicated in structure than 

 their predecessors and representing progress in evolution, 

 the resulting structures should be considered examples of 

 specialization by addition. The huge third pair of legs in 

 Locustidae, the complicated mouth parts of Hymenoptera, 

 the ornamented wings of Lepidoptera, are good examples 

 of this kind of specialization. Such specializations may 

 indicate an enlargement of the field of work occupied orig- 

 inally by the type or a change in its habitat. Speciali- 

 zation may, however, take place by a different process ; 

 namely, through the reduction and suppression of organs 

 in certain parts of the body. Thus, the existing May-flies 

 have mouth parts in large part obliterated and unfit for tak- 

 ing food, while their hinder pair of wings is smaller than the 

 front pair. The Coccidae have only one pair of functional 

 wings, etc. ; the Diptera are similarly situated ; and almost 

 all the adults in Lepidoptera have lost the mandibles dur- 

 ing the transformation of the mouth parts into a sucking- 

 tube. This sort of specialization, in course of which organs 

 are lost by reduction, leads usually to a narrower field of 

 work. Thus, the adult May-fly exists only to reproduce 

 its species ; many of the dragon-flies and Lepidoptera have 

 the first pair of thoracic legs so much reduced in size that 

 they are of no use as supports, and these insects practically 

 rest upon four legs ; while the Lepidoptera and Diptera, 

 having the most complete sucking-tubes, can Hve only 

 upon fluids. Specialization by reduction gives greater 

 strength and efficiency to the parts which survive the 

 reduction, and this mode is the prevalent one among the 

 so-called "highest"' types. These are usually the descend- 

 ants of types which reached a certain acme of prog- 

 ress through specialization by addition, and consequently 

 possess highly complicated organs as compared with the 



