26 EN TOMOLOGY 
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. 
