338 THE BIOLOGY OF INSECTS 



were precursors of the two distinctive modern orders of 

 the Ephemeroptera (mayflies) and Odonata (dragon-flies). 

 In both of these there is, as has been mentioned, a curious 

 mingling of primitive with specialised characters, and though 

 they practise the open type of wing-growth they pass through 

 a remarkable transformation in their development from 

 aquatic larvae. The Protephemerida differ from living 

 mayflies in the comparatively slight differentiation of the 

 wings of the two pairs ; the neuration shows an approach 

 to the mayfly type, but there is no such extreme reduction 

 in the hindwings as is distinctive of recent Epheme- 

 roptera. What kind of life-history the Protephemerida 

 passed through we do not know, but it is likely that there 

 was at least the beginning of larval adaptation to aquatic 

 life, as in Russian rocks of the succeeding (Permian) period 

 has been preserved a typical mayfly larva with nine pairs 

 of abdominal gill-appendages. The lower Permian beds 

 of Kansas, North America, preserve numerous remains 

 of primitive mayflies. The Ephemeroptera, therefore, 

 essentially as we know them to-day, were already well 

 established early in the great Secondary Era of the history 

 of life on our earth. It is interesting, however, to notice 

 that the hindwings of mayflies were in the Jurassic Age 

 less relatively reduced in size than those of living forms. 



Turning to the evolution of dragon-flies, we find that the 

 Protodonata of Carboniferous times were insects with 

 the general aspect of modern dragon-flies but without the 

 characteristic specialisation of body-structure and wing- 

 neuration. Some of them were of gigantic size : the huge 

 Meganeura, preserved in the Commentry beds of Northern 

 France displayed a wingspread of some two feet. In late 

 Palaeozoic (Permian) times dragon-flies were living which 

 can be referred to the same order (Odonata) as those around 

 us to-day, though they were more primitive in form than 

 members of the two modern sub-orders Zygoptera and 

 Anisoptera ; their remains are known from the Lower 

 Permian beds of Kansas. A special sub-order, Archi- 

 zygoptera, includes the early Mesozoic genus Protomyrme- 



