642 THE PANORPOID COMPLEX, ii., 



of tlie wiiig-trichiation; and the two characters taken together 

 suggest that this family represents one of the most archaic types 

 of Dipteron still existing.* 



Text-fig. 26. 

 Portion of two main veins and the included membrane from the wing of 

 Rhyphus hreris Walk,, to show the trichiation; ( x l.SO). (See also 

 Plate Ixviii., fig. 12). 



From the type of trichiation still existing in the Rhyphidcf, all 

 other types to be found within the Order are derivable, by 

 elimination of the macrotrichia from the wing-membrane. There 

 are, however, two distinct lines of evolution to be traced, in one 

 of which the macrotrichia tend to become slender and elongated, 

 and finally develop into scales; while, in the other, they tend to 

 become short and stout spines, and also become gradually 

 eliminated from all except the anterior veins and costal border 

 of the wing. 



As conspicuous examples of the first line of evolution, we may 

 take the Moth Midges or Psychodkhf. (Text-fig. 27). Here we 

 see the macrotrichia as slender hairs abundantly present upon 



" In this connection, it should be noted that the short vein usually con- 

 sidered as a cross-vein connecting Cuj with M. in Rhyphn-s is reallj- the 

 basal piece of M^, since it carries macrotrichia. On the other hand, the 

 vein below it, usually considered to be the basal portion of Cuj , is a true 

 cross- vein and carries no macrotrichia. 



