324 ENTOMOLOGY 



synthetic indeed as compared with their modern allies, are at the same 

 time highly organized, or far from primitive, and their ancestors have 

 been obliterated. 



The general plan of wing structure, as Scudder finds, has remained 

 unaltered from the earliest times, though the Devonian specimens ex- 

 hibit many peculiarities of venation, in which respect some of them are 

 more specialized than their nearest living allies, while none of them have 

 much special relation to Carboniferous forms. 



Carboniferous insects are more nearly related to recent forms than are 

 the Devonian species, but present a number of significant generalized 

 features. Generally speaking, the thoracic segments were similar and 

 unconsolidated, and the two pairs of diaphanous wings were alike in 

 every respect in groups that have since developed tegmina and dis- 

 similiar thoracic segments. The Carboniferous precursors of our cock- 

 roaches, phasmids and May flies have been mentioned. Palaeozoic in- 

 sects are grouped by Scudder into a single order, Palagodictyoptera, on 

 account of their synthetic organization, though other authors have tried 

 to distribute them among the modern orders. This disagreement will 

 continue until, with increasing knowledge, our classification becomes less 

 arbitrary and more natural. 



Mesozoic insects are interesting chiefly as evolutionary links, notably 

 so in the case of cockroaches the only insects whose ancestry is con- 

 tinuously traceable. In this era the large families became differentiated 

 out. 



Most of the Tertiary species are referable to recent genera, peculiar 

 families being highly exceptional, while all the Quaternary species belong 

 to recent genera. 



Hemiptera appear in the Silurian; Neuroptera (in the old sense) in 

 the Devonian; Thysanura and Orthoptera, Carboniferous; Coleoptera 

 and Hymenoptera, Triassic; Diptera, Jurassic; and Lepidoptera not 

 until the Tertiary. 



