592 ARTHROPODA. 



Parschlug, and Auvergne, and the Rhenish Brown Coals have been 

 scarcely less prolific " (Scudder). 



All the known Palaeozoic Insects are referred by Mr Scudder to 

 " a single homogeneous group of generalised Hexapods," which this 

 eminent authority has named Palczodictyoptera, and which " should 

 be separated from later types more by the lack of those special 

 characteristics which are the property of existing orders than by 

 any definite peculiarities of its own." With the exception of a few 

 forms from the Trias of North America, which are allied to the 

 Cockroaches, all the types included by Scudder under the name of 

 Palaodictyoptera are restricted to the Palaeozoic period. Of the 

 modern orders of Insects, the great divisions of the Orthoptera, 

 Neuroptera, and Coleoptera possess representatives in rocks of 

 Triassic age ; while the Hemiptera, Diptera, Lepidoptera, and 

 Hymenoptera existed under well-marked forms in the Jurassic 

 period. 



In the following brief summary of the orders of Insects, with more 

 especial reference to their geological history, Mr Scudder's treatise 

 on fossil Insects in Zittel's ' Handbuch der Palaeontologie ' has been 

 followed, with some variation as to the classification adopted : — 



Division A. Ametabolic Insects. 



Of the four existing orders of Ametabolic or Apterous Insects, — 

 viz., the Anoplura ("Lice"), Mallophaga ("Bird-lice"), Collem- 

 bola (" Springtails "), and Thysanura, — only the last two are known 

 to be represented by fossil forms, and these only in deposits of 

 Tertiary age. Thus, forms allied to the existing Podura and 

 Smynthurus have been recorded as occurring in amber (early 

 Tertiary), while a species of the recent genus Lepis?na has been 

 similarly preserved, along with a number of other allied but extinct 

 types. The Oligocene deposits of Florissant, Colorado, have also 

 yielded examples of insects belonging to the order Thysanura. 



Division B. Hemimetabolic Insects. 



Order I. Pal^eodictyoptera. — This order has been founded 

 by Mr Scudder for the reception of a number of Palaeozoic and a 

 few Triassic Insects, with the following characters : " Body generally 

 elongated, mouth-parts variously developed ; antennae filiform. Tho- 

 racic joints subequally developed ; legs moderately long. Meso- 

 thoracic and metathoracic wings closely similar, equally mem- 

 branous ; the six principal veins (fig. 442, a) always developed, the 

 marginal simple, and forming the costal border, the mediastinal 

 simple, or with superior branches only ; the other veins usually 



