EVOLUTION OF INSECTS — CARPENTER 341 



less insects, which had no means of escape. It is not surprising, 

 therefore, that the ability to fly changed the direction of insect evolu- 

 tion and that in our present insect world only one-tenth of 1 percent 

 of the species are Apterygota. 



The process by which wings were acquired by insects * has been a 

 question of much speculation, for they are not modifications of pre- 

 viously existing appendages. However, significant evidence has been 

 provided by the study of fossils. All the more generalized Pterygota 

 of the Carboniferous period, and even some species of the Permian, 

 possessed a pair of membranous flaps, arising from the dorsum of the 

 first thoracic segment. These flaps contained veins and were covered 

 with minute hairs like those of the true functional wings borne on the 

 second and third thoracic segments. There is every indication that 

 the true wings began, like membranous prothoracic flaps, as lateral 

 tergal expansions. However, so far as we know, the prothoracic 

 flaps never developed into functional wings. In most insects the 

 flaps have completely disappeared and in others they have been 

 absorbed into a pronotal disc. 



The first winged insects, or Paleoptera, which we have been con- 

 sidering, had a simple wing articulation and were incapable of flexing 

 their wings back over the body at rest; hence, they were preserved as 

 fossils with their wings outstretched. Dragonflies and mayflies — 

 the sole living representatives of the Paleoptera — exhibit the same 

 limitations in wing structure. The third stage in insect evolution 

 began with the modification of certain plates of the wing articulation 

 so as to permit wing flexing ; these insects are known as the Neoptera.® 

 The survival value of wing flexing was great, for it enabled the insects, 

 between flights, to hide among foliage or under objects on the ground. 

 The fossil record shows that this stage was reached by early Upper 

 Carboniferous time, when many of the paleopterous insects were 

 predaceous and of great size, though no flying vertebrates had yet 

 appeared. Later, when the flying reptiles, or pterosaurs, and birds 

 appeared in the Mesozoic, the neopterous insects had all the advan- 

 tage. The paleopterous insects, which had been dominant during the 

 Carboniferous and Permian, began to wane and the Neoptera to flour- 

 ish. This trend in insect evolution has continued up to the present 

 time to such an extent that 90 percent of the existing orders, includ- 

 ing 97 percent of the species, are now neopterous. 



* That the Pterygota are of monophyletic origin seems almost certain. Lemche, however, 

 has advocated [4, 5] a polyphyletic origin, even claiming that such insects as the 

 Grylloblattidae and the females of Zoraptera and of certain lampyrid beetles are primitively 

 wingless (nonalate). The evidence for his conclusion seems insufficient (see, for example, 

 Carpenter, 194S [6]). 



•■' The phylogenetic groups which are here termed the "Paleoptera" and "Neoptera" were 

 recognized independently by Martynov [7, 8] and Crampton [9]. 



