26 ENTOMOLOGY 



hypothetical thysanuroid source of all insects, including Thys- 

 anura themselves. Though Thysanura and Collembola show 

 no traces of wings, even in the embryo, it should be borne in 

 mind that all the other insects probably had winged ancestors 

 and that it is more reasonable to assume a single winged group 

 as a starting point than to suppose that wings originated inde- 

 pendently in several different groups of insects. 



