710 THE PANORPOID COMPLEX, iii., 



The four Orders Mecoptera, Parameeoptera, Protomeeoptera 

 and Paratriehoptera represent a group of closely related forms, 

 which will probably be included in a single enlarged Order 

 Mecoptera when further discoveries in the Permian and Triassic 

 Beds have been made. Every new type discovered from now on, 

 belonging to this group of Orders, will manifestly tend to help 

 to close up the gaps that at present exist between them. Con- 

 sequently I have no hesitation in stating that Mecopteruid forms 

 were the first Holometabolous Insects to appear, and that they 

 were well represented, probably by all these four Orders, both 

 in the Upper Permian and the Trias. 



Prom the Table, it would appear that the Protomeeoptera 

 are, on the whole, the nearest to the Archetype of the Complex. 

 This Order must clearly have existed along-side the Mecoptera 

 and Parameeoptera in the Upper Permian, and serves in some 

 measure to connect the two. Neither the Protomeeoptera nor 

 the Parameeoptera can be derived from the other ; and we are 

 compelled to assume that both arose from a common ancestral 

 stem in the Permian. This common stem was evidently also the 

 original main stem of the Panorpoid Complex. From this main 

 stem, apparently as a separate line of specialisation, in which 

 Cui became an unbranched vein, there arose the common stem 

 of the true Mecoptera and the Paratriehoptera, — two very closely 

 allied Orders, neither of which is derivable from the other, see- 

 ing that the Paratriehoptera possessed originally the basal branch 

 of Sc, which the otherwise more archaic Mecoptera lacked. 

 Thes^ relationships between the four Mecopteroid Orders are 

 indicated by broken lines in the Phvlogenetic Diagram (Text-fig. 

 112). 



In the Upper Trias of Ipswich, the Paratriehoptera were well 

 represented by some fine forms. These serve to connect the 

 Mecoptera with the Diptera, and indicate very clearly the line 

 of descent of this latter Order. While it is not claimed that 

 any of the known types of Paratriehoptera from the Ipswich 

 Beds are in themselves the actual ancestors of the Diptera, I 

 think that there can be no doubt that some types within this 

 fossil Order, by reduction of their hindwings and narrowing of 

 the bases of their forewings, gave rise to the dominant Order 

 Diptera of to-day. True Diptera first appear in the Lias; and 

 these are, as might be expected, chiefly Nemocerous types, 

 though a few show the condition of Rs typical of the Brachy- 

 cera. The Cyclorrhapha probably arose in the Cretaceous, as 



