8 INTRODUCTION [CH. I 



common to these two groups an ancestor with an incomplete 

 metamorphosis, and only such structure of wing-venation as 

 underlies both types. From that ancestor, the Odonata broke 

 away by the adoption of the aquatic larval habit. The consequent 

 change in the source and direction of the oxygen-supply to the 

 developing wings may have been the determining factor for those 

 wonderful specializations in wing-venation in which the Odonata 

 are unique. On the other hand, a more direct line of descent 

 would lead to Stilbopteryx by the intercalation of a pupal stage, 

 and high specialization in the habits and form of the larva. 

 Palaeontology cannot help us here, since any connection of the 

 kind supposed must have existed before the Protodonata became 

 differentiated out. 



To conclude this discussion, we find in the Odonata a singularly 

 isolated group, marked by very high specializations of structure, 

 superimposed upon an exceedingly archaic foundation. No near 

 relatives of these extraordinary insects exist to-day, nor have any 

 such existed since Palaeozoic times. We may, however, most 

 certainly point to the Plectoptera as being the group which has 

 the closest affinity with them, slight though it be. In a serial 

 arrangement of the Orders of the Insecta, the Odonata are best 

 placed next to, and just after, the Plectoptera. 



