Vol. 1.] Woodivortli . — Wi)ig Veins of Insects. 9 



wing should be traced from the first fundaments of an out- 

 growth of the thoracic integument, through the modifications 

 of form and structure essential to the performance of its func- 

 tion in each stage of its evolution, into a fully developed organ 

 of flight. 



The earliest insects of which we have any certain remains 

 had well-developed wings, and wings have in all proliability 

 arisen but once among insects. All existing insects are, there- 

 fore, to be considered descendants of flying Paleozoic ancestors 

 and to have inherited their wings through all these ages. The 

 meagerness of all ancient geological records and the necessary 

 absence of all direct evidence in later time make the problem 

 one of great difficulty. The process would be almost unin- 

 telligible were it not that organs similar to those from which 

 the wings have in all probability arisen appear to be easily 

 acquired by insects; for this reason we have preserved to us 

 abundant data for our investigation. 



The first step in the development of wings, in accordance 

 w4th this theory, is brought about as a result of a change of 

 habitat — an insect, formerly terrestrial, becoming aquatic. If 

 this occurred in a small insect, which was probably the case, 

 the only adjustment necessary would be a reduction of the 

 thickness and firmness of the cuticle. The insect was doubtless 

 one of those living in damp situations, such as serve at the pres- 

 ent time as the habitat of Thysanura. The situations in which 

 these insects may be found vary from subaquatic to almost 

 terrestrial, and the direct effect of the environment would be 

 sufficient in the primitive insects to accomplish the differentia- 

 tion of the older insects into two groups of individuals, not 

 sharply distinct, but looking toward the specialization that 

 later occurred: On the one hand, those that remained in the 

 more moist localities and retained a character of skin that made 

 possible the free absorption of oxygen from the water; and, on 

 the other hand, those whose surroundings resulted in the older 

 individuals quite losing this power, through the drying and 

 hardening of their cuticle, thus confining the adult individuals 

 to such situations as permitted the continuous breathing of 

 free air. 



The danger from disease would in time render such a neutral 

 condition as we have imagined for the subaquatic forms quite 

 untenable and required a modification that would harden 



