598 THE PANORPOID COMPLEX, 111., 



(from six to four), but also in those of Rs, which, in all known 

 C'horistid forms, from the Permian onwards, only number four. 

 Thus we find in Merope either five or six dichotomic branches of 

 M 1—4) and always five such branches of Rs. The only specimen 

 of this rare insect which I possess, and from which Text-fig. 61 

 was drawn, has five branches of M 1—4 : in all four wings; but 

 the specimen figured by Comstock (15, fig. 317) has six on the 

 right side, the extra one being due to a strong dichotomy of M 4b) 

 As it seems doubtful whether this extra fork may not be a recent 

 addition due to platygenesis, I have not counted it as an arche- 

 typic character. 



As regards Rs, the extra fork in Merope occurs on R 2 , and 

 is so deep that there can be no doubt of its being a primitive 

 dichotomy. The same fork persists throughout the genus 

 Panorpa. But, in the archaic genus Panorpodes there is to be 

 found a deep forking of R 4 which is also clearly archaic. Thus 

 the northern forms at present existing must have had an an- 

 cestor with at least a six-branched Rs, the extra forks being 

 present on R 2 and I< 4 respectively. 



Further valuable evidence on this point is to be gained by 

 the study of the Liassic genus Ortkophlebia (2, PL xlii.) and 

 allies. Many of these show Rs with no less than seven 

 branches, while the rest have six. In this group of fossils, the 

 extra branches all belong to R 2) and tend to be arranged pec- 

 tinately, — a very striking illustration of how very simple a 

 change might lead to the evolution of the Neuropteroid type of 

 venation, from the older Panorpoid type. Allowing fire 

 branches as the maximum for R 2+ 3> as in the Orthophlebiidae, 

 and three for R 4+5 , as in Panorpodes, we see that the Archetype 

 of the Order must have possessed at least eight branches to Rs, 

 as shown in Text-fig. 60. 



It will by now be clearly evident that the main line of evolu- 

 tion within the Mecoptera has been by narrowing of the wings, 

 with suppression of certain of the original archaic branches of 

 Rs and Mj_4. With the narrowing there has proceeded also a 

 lengthening process, which culminates in such forms as the 

 Bittacidae. Tims the name Mecoptera ("long-winged insects") 

 is fully justified for this ancient Order. 



(6) In all Mecoptera, and therefore in the Archetype of the 

 Order, Sc is shorter in the hindwing than in the fore, and Cu 2 

 is fused with 1A for part of its length, not far from the base, 



