INSECTA 405 



Even as early as this, these orders had taken to a nymphal aquatic 

 existence. In the Permian rocks we find primitive dragonflies, stone- 

 flies and Hemiptera of which the Heteroptera with their characteristic 

 half-horny anterior wings appear to be the more recent development. 



Up to this stage none of the important endopterygote orders had 

 made their appearance. 



The mandibulate Mecoptera form an order which is most general- 

 ized in structure among the Endopterygota, and Permian Mecoptera 

 from Kansas and New South Wales have been discovered which have 

 wing features that link up five of the important higher orders, the 

 Diptera, Trichoptera, Lepidoptera, Neuroptera and Mecoptera. 



The highly specialized Hymenoptera make their first definite ap- 

 pearance in the sawfly form in the Jurassic, but remains from the 

 Permian have been described as Protohymenoptera. These had two 

 pairs of wings of equal size without coupling apparatus and a venation 

 of a generalized hymenopteran type. 



Hymenoptera of the specialized kinds — the bees, wasps, ants — are 

 found first in the Tertiary period. In the same way we find nemato- 

 ceran Diptera (craneflies, etc.), in the Upper Lias, but not till the 

 Tertiary age do we find forms more nearly resembling our highly 

 organized blowflies, etc. Little can be said here of the Lepidoptera 

 except that they occur in the Tertiary period. 



The Coleoptera are far older geologically than the Diptera, Lepi- 

 doptera and Hymenoptera. Already there were water beetles, weevils 

 and the leaf-eating chrysomelids in the Triassic, and recognizable 

 beetle remains, though scarce, have been extracted from the Upper 

 Permian. This is not without interest, since the Coleoptera as we 

 know them to-day possess, particularly in their mouth parts, a number 

 of features which place them in the generalized category. 



Now if we consider the order of events hinted at in the above brief 

 account, it will be seen that though the ancestors of the Hymenoptera, 

 Diptera and Lepidoptera may have existed in the Permian, the latter 

 age with the Carboniferous was essentially one of insects with in- 

 complete metamorphosis and with no feeding mechanism for dealing 

 with flowering plants. It has been suggested that the change from 

 the perpetual warmth and humidity of the Carboniferous to the 

 transitional epoch of the Permo-Carboniferous with its glacial con- 

 ditions may have accounted for the onset of metamorphosis, the pupal 

 stage being evolved for the purpose of surviving cold periods in a 

 quiescent state. 



The most interesting fact, however, is that the main evolution of 

 our specialized bees, flies and butterflies coincided in point of time 

 with the evolution of the flowering plants to which by their manner 

 of feeding they are now on the whole so inseparably bound. 



