STEPS IN SPECIALIZATION 121 



wing greatly thickened and those of the middle portion reduced 



to narrow lines Isoptera 



CC. Wings developed internally Neuroptera 



BB. Supernumerary veins of the intercalary type. 



C. Flight-function cephalized; the hind wings being greatly reduced 



in size Ephemerida 



CC. Flight-function not cephalized; the hind wings as large as or 



larger than the fore wings Odonata 



AA. Wings specialized by a reduction in the number of veins in the 

 preanal area. 

 B. Wings developed externally. 



C. The two pairs of wings similar in texture. 



D. With the tendency to develop accessory veins retained. 



Plecoptera 

 DD. With the tendency to develop accessory veins in the preanal 

 area lost. 

 E. With the courses of some of the longitudinal veins modified so 



that they function as cross-veins Corodentia 



EE. The transverse bracing of the wing attained in the usual 

 way. 



F. The veins of the wing bordered with dark bands. Embiidina 

 FF. The veins of the wing not bordered with dark bands. 

 G. Wings long and narrow, supplemented by a wide fringe 



of hairs Thysanoptera 



GG. Wings not greatly narrowed and not supplemented by 



a wide fringe of hairs Homoptera 



CC. The front wings more or less thickened. 



D. The front wings not greatly reduced in length as compared with 

 the hind wings. 



E. The front wings thickened throughout Homoptera 



EE. The front wings thickened at the base, the terminal portion 



membranous Heteroptera 



DD. The front wings greatly reduced in length. . . . Dermaptera 

 BB. Wings developed internally. 

 C. Fore wings' greatly thickened. 



D. Fore wings modified so as to serve as covers of the posterior 



wings COLEOPTERA 



DD. Fore wings reduced to slender, leathery, club-shaped appen- 

 dages Strepsiptera 



CC. The two pairs of wings similar in texture. 



D. With the tendency to develop accessory veins retained. 



Mecoptera 

 DD. With the tendency to develop accessory veins lost. 



