256 INSECT TRANSFORMATION 



nymphal or pupal wings of members of various orders, 1 it is 

 hard to avoid the conclusion that the whole series of winged 

 insects come from one original stock, that they are a mono- 

 phyletic group. If this be so, and if the conclusion set forth 

 earlier in this chapter, that in the course of the evolution of 

 insects there has been a divergence between the adult and 

 larval stages, be justified, we are led inevitably to the further 

 conclusion that those orders which practise the hidden method 

 of wing-growth must have arisen from ancestors in which the 

 more primitive open method was the rule. The Endopterygota 

 must have been derived from the Exopterygota. 



Here we are confronted with one of the most interesting 

 and difficult problems of insect transformation : how is the 

 transition from the older to the newer mode of wing-growth 

 to be explained ? The divergence between them goes deep and 

 begins at an early stage in the life-history, according as the 

 incipient wing-rudiments grow outwards so as to be covered by 

 the cuticle and to appear after the next moult, or arise as 

 buds to be enclosed by the inpushed pouches so as to remain 

 invisible outwardly until the pupal stage. So great difficulty 

 has been felt in imagining a process of change from one method 

 of growth to the other, that the suggestion has been made of 

 the interpolation of a temporary wingless condition between 

 the two. In previous chapters (notably in Chapter V) many 

 examples have been given of wingless insects that, being 

 clearly related to winged groups, must be presumed to 

 have lost their wings, and it has been seen how among the 

 aphids (p. 73) winged and wingless generations may appear 

 successively in the yearly life-cycle of the same species. The 

 orders of secondarily wingless insects, such as Mallophaga, 

 Anoplura and Aphaniptera, have been grouped together- 

 somewhat unnaturally in a section called the Anapterygota, 

 and it has been suggested that the ancestors of the endoptery- 

 gote orders were Anapterygota, that these races having in 

 the course of evolution lost the wings once developed visibly, 

 reacquired them, but with a new method of concealed growth. * 



The sudden and revolutionary changes necessary in such 

 a transition as this render the explanation incredible, and 



1 J. H. Comstock : " The Wings of Insects ". Ithaca, New York, 1919. 



2 D. Sharp : " Insects " in Encycl. Brit., loth ed., XXIX. 1902. 



