BY R. J. TILLYARD. 



569 



of M is very weak; and this vein is represented at its base, 

 in many forms, by little more than its trachea. In others it 

 is more strongly chitinised; but in no ease does the typical main- 



Text-Fig. 45. 

 Cubito-median Y-vein in the Order Planipennia. a, from forewing of 

 Meg apsy chops illidgei Frogg. (fam. Psychopsidae) ; b, from hind- 

 wing of same; c, from hindwing of Polystoechotes punctatus Say, 

 (fam. Polystoechotidae); </, from Osi>iv/its chrysops (Lmn.). (fam. 

 Osmylidae). Lettering as on p. 535. 



vein armature of macrotrichia begin on M until well past the 

 position of M s . Thus we cannot, unfortunately, apply the test 

 of the presence or absence of macrotrichia to M s in this Order. 

 The conclusion we are bound to come to is that, in the Plani- 

 pennia and Megaloptera, M s was originally present. The 

 specialisations it has undergone, especially in the forewing, are 

 due to the development of the many-branched Rs in these Orders 

 causing a correlated narrowing of the area supplied by M. The 

 general result has been that M has become squeezed into a nar- 

 row space between the two strong convex veins R and Cu, and 

 M 5 has therefore tended to become reduced, and finally 

 obliterated in the forewing, though remaining more normal in 

 the hind. 



