5(30 THE PANORPOID COMPLEX, iii., 



Having now analysed the complete Y-vein formation in the 

 Order Pararuecoptera, let us trace its evolution through the other 

 Orders of the Complex. 



(1) The Mecoptera. 

 These may be taken first, as they are the only Order known 

 to have existed alongside the Paramecoptera in Palaeozoic times. 

 Three stages in the evolution of the Y-vein are to be found 

 within this Order: — 



(a) In the pupal wing of Chorista (23), the Y-vein is com- 

 plete, though the upper branch, M 5> is shortened. It is very 

 important to notice that, in spite of the fact that this Order 

 has merotraeheate wings, yet trachea? R and M still exist, and 

 from the latter trachea a small branch penetrates down the 

 upper branch of the Y-vein. This seems to me to be very sig- 

 nificant of the true nature of this vein. At the time that I 

 described this pupal wing first (23), I had not studied the 

 arculus-formation outside of the Odonata, and knew nothing of 

 the Paramecoptera; hence I regarded this vein as a cross-vein, in 

 spite of its trachea. The new evidence has compelled me to 

 alter my opinion. 



In the fossil genus Stereochorista (Upper Triassic, 28), the 

 Y-vein may be seen completely formed.* 



(b) In the forewing of the genus Panorpa, the upper branch 

 of the Y becomes transverse, and takes on the appearance of a 

 cross-vein. A similar condition is to be seen in the forewing 

 of Merope (Text-fig. 62) ; but in this genus the lower branch of 

 the Y is much shorter than the upper. It is a pity that the 

 pupal wing of Panorpa has so far not been examined as to the 

 condition of M 5 , especially during the first day or two of pupal 

 life. 



(c) In the hindwings of the Meropidae and Panorpidae, and 

 in both fore and hindwings of the Choristidae, Nannochoristidae 

 and Bittacidae, the upper branch of the Y becomes eliminated, 

 and there is a greater or less degree of fusion between M 1—4 

 and Cuj. It will be seen that this high degree of specialisation 

 is not confined to the Mecoptera, but also characterises the ma- 

 jority of forms in the Trichoptera and Diptera, as well as cer- 

 tain groups in the Megaloptera . 



•Since this was written, Mr. J. Mitchell has forwarded to me from the 

 Upper Permian Beds of Belmont, a new type of Mecopterous wing with 

 the Y-vein present. 



