630 THE PANORPOID COMPLEX, ii., 



and the higher faniihes of tlie Planipennia, in which the macro- 

 trichia appear upon the cross-veins as well as on the veinlets. 



Having now clearly explained the difference between veinlets 

 and cross-veins, let us now go back to the most primitive fossil 

 insects, and study the venational scheme that they present to us. 

 In the oldest known Orders, the Palteodictyoptera and their 

 allies of the Upper Carboniferous, we find the same scheme of 

 main veins as in recent insects. The spaces between these main 

 veins and their branches are, however, filled up with an irregular 

 meshwork of cellules, quite unlike anything to be seen in the 

 Holometabola of to-day, though very probably homologous with 

 the still-existing dense meshwork of cellules to be found in certain 

 Orders of Hemimetabola, such as the Odonata. To this oriyinal 

 meshwork of cdhdes, as developed in the Palseodictyoptera and 

 their allies, I propose to give the name archedictyon. A portion 

 of a typical archedictyon is shown in Text-fig. 19. 



Text-fig. 10. 

 Portion of wing of the fossil Hypermegethes shuckerti Handl., (Order Palaso- 

 dictyoptera), to show the archedictj^on; (nat. size). 



It is here necessary to disabuse our minds at once of the idea, 

 if it exists, that the dense venation found in certain Planipennia 

 is an archedictyon. Practically all known fossils of this Order 

 (Triassic, Liassic, and Jurassic) have either no or few cross- 

 veins, and the spaces between their numerous main veins and 

 branches are devoid of any meshwork. Further, the oldest types 

 still extant, such as the Fst/chopsidte, If/io)ddce, etc., have no 

 meshwork present; and it is demonstrable that the apparent 



