CLASSIFICATION 307 



never develop any. But in several lines of study we have 

 come across the interesting fact that some individuals of 

 the same kind may be winged and others wingless, even the 

 young of one mother, hatched from eggs laid, or bom at 

 the same time may grow into either winged or wingless 

 adults, like the aphids or the ants, or like the scale-insects 

 and those moths in which the males have wings and the 

 females none. As such wingless insects all belong to groups 

 typically winged, there can be no hesitation in regarding 

 the wingless condition as '' secondary." The wingless 

 aphid is obviously closely akin to her winged sisters, she 

 has, as it is commonly expressed, '' lost her wings," and a 

 classification that depends rigidly on the presence or absence 

 of wings is quite clearly unnatural, for it w^ould obscure 

 rather than express the relationship between the various 

 groups. Thus we incidentally reach the highly important 

 conclusion that a classification can be regarded as satisfactory 

 only in so far as it does indicate the mutual relationships of 

 the creatures classified. Again, we noticed in our discussion 

 of growth and development (p. 164) that certain entire 

 groups of insects, as lice and fleas, are wingless, though they 

 are shown by their form to be allied to winged groups. 

 They also, therefore, furnish examples of " secondary " 

 winglessness which may characterise not some members only 

 of a family or an order, but all. 



There are, however, certain families of entirely wingless 

 insects which may be distinguished from others by very 

 definite and important additional characters. The bristle- 

 tails and springtails have, in the adult state, paired limbs 

 on several at least of their abdominal segments, and they 

 have mandibles which resemble those of Crustacea much 

 more nearly than those of typical insects (Figs. 52, b. 

 79, c, e). These associated characters suggest that the 

 wingless condition of such insects is not secondary but 

 primitive, and that it is reasonable to separate these bristle- 

 tails and springtails (as Apterygota) from all the rest of the 

 class (Pterygota) in which the development of wings is a 

 typical character, though wings may be absent in many 



