88 THE PALEONTOLOGICAL DATA 



from any part of the spiracles of the mesothorax or nietathorax ; and con- 

 cludes "that the wings have been derived as lateral outgrowths or folds of 

 the hypodermis of the pleurum or tergum, or both." 



Some authors, as Handlirsch and also Lameere, regard the wings as 

 homologues of the pleurae of Trilobites; the Trilobites being regarded as 

 the stem form from which the insect series as well as the arachnid series 

 have been evolved. Handlirsch gives a series of diagrams illustrating this 

 supposed homology. It seems to me, however, hardly necessary to believe 

 that this homology means more than that in the two cases corresponding 

 parts of the body are expanded. I do not believe that the broadly expanded 

 plura of Triolbites were preserved by all of the forms intervening between 

 the Trilobites and the winged insects. In fact Handlirsch describes the 

 primitive insect, the Protentomon of Paul Mayer, as having the body 

 moderately slender, almost cylindrical. 



We can imagine that from a form like the supposed Protentomon forms 

 were evolved in which the body became shortened and flattened; and that 

 the subsequent course of development was as follows : 



The flattened form of the body was well-fitted for running over the 

 trunks of the carboniferous plants and for creeping between fronds of the 

 giant ferns of that time. Lateral expansions of the pleura of most of the 

 segments were developed which facilitated leaping from trunk to titink in a 

 manner analogous to that now used by our flying squirrels. 



At first nearly all of the body-segments, all except those at the caudal 

 end of the body, were furnished with the lateral expansions. Later there 

 was an increased specialization of the lateral expansions of the mesothorax 

 and metathorax and a reduction of those of the other segments, resulting in 

 a form with two pairs of fixed wings. 



The next step was the development of a hinge at the base of each of these 

 four wings; but at first these wings could be moved only in a vertical 

 direction, and were held outspread when at rest. Later the ability to fold 

 the wings over the abdomen was attained. 



The paleontological data on the origin of wings. — The account of the 

 origin of wings just given is largely hypothetical. Let us now see what 

 data bearing on the subject we actually have. 



We naturally turn to paleontology for this data. Forttuiately the 

 paleontological data has been made very available by the magificcnt work 

 of Handlirsch, in which every known paleozoic insect is figured, except 

 some recently described. 



Although nearly 900 paleozoic insects are known (Handlirsch lists 884), 

 unfortunately we have no specimens of the insects that preceded those with 

 well-developed wings. The oldest fossil insects known to us had wings as 

 large as those of modern insects. But although some of the connecting 



