I] INTRODUCTION 7 



though lacking those high specializations of the wing-venation 

 which mark off the Odonata of to-day. Now we can also trace 

 back the Plectoptera to the Jurassic, where we find forms differing 

 little from those of the present day except in the possession of 

 less reduced hind-wings. The fossil record undoubtedly points to 

 a gradual reduction taking place in the hind-wings of the Plec- 

 toptera, from an ancestral form in which fore and hind- wings were 

 sub-equal. A study of Permian fossils also bears this out. Coming 

 at last to the Carboniferous period, we find evidence of the existence 

 of a group of moderately delicate insects with four almost equal 

 wings. These, under the name Protephemeroidea, are generally 

 admitted to have been the ancestors of our May-flies. It will thus 

 be seen that, in the Upper Carboniferous, the two Orders Plec- 

 toptera and Odonata were already differentiated, but far more 

 closely related than they are to-day. It seems therefore not un- 

 reasonable to recognize a very archaic connection between the 

 two, of a far more definite character than we can point to in the 

 case of the Perlaria. We need, however, only remember the 

 numerous and overwhelming points of difference between the two 

 Orders to realize how very slight the relationship really is. We 

 have only to recall the differences in the structure of the head and 

 thorax, the atrophy of the mouth-parts in the May-flies, the very 

 characteristic arrangement of interpolated sectors in their wing- 

 venation, together with numerous points of difference in the 

 internal anatomy, to feel the force of this. We have not mentioned 

 the sub-imago of the Plectoptera, since a somewhat analogous 

 process is evident in the case of the pronymph of the Odonata. 

 But for the fact that the Planipennia have a complete meta- 

 morphosis, we might be tempted to see a considerable resemblance 

 between the very ancient Protascalaphine genus Stilbopteryx and 

 the Odonata. That Stilbopteryx is remarkably like a Dragonfly 

 in its flight and appearance, nobody who has seen this insect alive 

 will deny. Moreover, the fact that it is admittedly the most 

 archaic of all the Ascalaphidae makes it impossible to accept this 

 resemblance as one purely due to convergence. The more highly 

 specialized Ant-lions depart widely from the type shewn by 

 Stilbopteryx, and do not resemble Dragonflies in the least. It 

 seems allowable to postulate an exceedingly remote ancestor 



