BY K. J. TILLYARD. 



579 



Text-Fig.52. 

 Evolution of the eubito-anal Y-vein in the order Planipennia. a, un- 

 fused condition in fore wing of Osmylits chrysops (Linn.) (fam. 

 Osmylidae) ; b, the same condition in forewing of Osmylcps placi- 

 dus Gerst. (fam. Myiodaclylidae) ; c, condition of first contact in 

 forewing of Nymphes myr))ieleonides Leach (fain. Nymphidae) ■, d, 

 condition of fusion in forewing of Glenoleon pulchellus (Ramb.) 

 (fam. Myrmeleonti&ae) ; b', c', d', the more advanced conditions of 

 fusion in the hindwings of the same insects as in b, c, d, respect- 

 ively. Lettering as on p. 535. 



Jugo-frenata. But, in the other genera of this group, more or 

 less reduction of this formation is generally evident, owing to 

 a progressive weakening of 3A, which results in that vein being 

 unable to maintain its loop intact with 2xV. The various stages 

 have been figured already by me in my paper on the Microptery- 

 gidae (26) and are shown in Text-fig. 53. 



It is important to note that, in Sabatinca and a number of 

 the Trichoptera. the cross-vein ia^ persists within the loop 



