THE PROBLEMS OF TRANSFORMATION 255 



And the caterpillar type, preserving the prolegs jointed in 

 the antique Micropterygids and Panorpids can be derived 

 from the same hypothetical primitive larva by an elongation 

 of the body, a restriction of the thickened regions of the cuticle 

 to scattered plates, a shortening of the legs and the dis- 

 appearance of the cerci. The reduction or disappearance of 

 the legs has evidently occurred independently among the 

 beetles and the Hymenoptera, as well as among the primitive 

 Diptera, and in the last-named order we have already traced 

 the disappearance of the head-capsule, resulting in the evolu- 

 tion of the degraded maggot which exemplifies the extreme 

 type of divergence between an insect larva and its winged 

 parent. 



THE TWO TYPES OF WING-GROWTH 



In the preceding section we have compared the larvae of 

 mayflies with those of certain beetles, and with the archaic, 

 wingless bristle-tails. In this comparison there is a suggestion 

 of continuity through the whole class of the Insecta, since 

 mayflies belong to the Exopterygota, beetles to the Endoptery- 

 gota and bristle-tails to the Apterygota ; and of these three 

 great sub-classes there can be no doubt that the last named is 

 the most primitive, and that the Exopterygota are, both as 

 regards structure and life-history, less highly specialized than 

 the Endopterygota. The insects are the only winged arthro- 

 pods, and at some period in the history of their development 

 they must have passed from an originally wingless to a winged 

 condition. An analogous transition from creeping to flight 

 takes place in the life-history of the vast majority of insects 

 in the course of their individual lives after hatching. What 

 indications of relationship do we find in the facts of these 

 life-histories ? Did the group of insects that show the open 

 type of wing-growth and those that show the hidden type arise 

 independently from wingless ancestors ? Or were the more 

 specialized Endopterygota elaborated from an originally 

 Exopterygote stock ? 



After the demonstration that the same series of main 

 nervures can be traced in all orders of insects, and that these 

 are prefigured by a corresponding set of air-tubes in the 



