392 ENTOMOLOGY 



far from primitive, and their ancestors have been obliterated. 



The general plan of wing- structure, as S'cudder finds, has 

 remained unaltered from the earliest times, though the De- 

 vonian specimens exhibit 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 tho- 

 racic 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 dissimilar tho- 

 racic segments. The Carboniferous precursors of our cock- 

 roaches, phasmids and May flies have been mentioned. Palae- 

 ozoic insects are grouped by Scudder into a single order, 

 Palaeodictyoptera, 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 continuously traceable. In this era the large fam- 

 ilies became differentiated out. 



Most of the Tertiary species are referable to recent genera, 

 peculiar families being highly exceptional, while all the Quater- 

 nary species belong to recent genera. 



Hemiptera appear in the Silurian; Neuroptera (in the old 

 sense) in the Devonian; Thysanura and Orthoptera, Carbonif- 

 erous ; Coleoptera and Hymenoptera, Triassic ; Diptera, Juras- 

 sic ; and Lepidoptera not until the Tertiary. 



