644 



THE PAVORPOID COMPLEX, 11., 



liecome arranged two, three, or even four rows deep, so that the 

 whole vein takes on a spiny appearance. Concurrently with this 

 chanue in the macrotrichia, we find two lines of evolution of the 



inicrotrichia. They either undergo 

 gradual reduction also, until they 

 become partly or entirely eliminated, 

 as in the family Syrphichf: or they may 

 remain well developed, with a differ- 

 entiation setting in in the size of the 

 two series on the upper and lower sur- 

 faces of the wing. In this latter case, 

 the microtrichia on the upjjcr surface 

 increase considerably in size, while 

 those of the lower surface remain fairly 

 constant. This de\'elopment takes 

 place in the Myiodaria, and is especi- 

 alty Nvell shown in the Muscidce. in 

 the Blowfly, for instance, the micro- 

 trichia of the upper surface of the %ving 

 are quite 30/x in length, and are set in 

 large, swollen bases, while those of the lower surface are only 

 about half as long, and have the usual small bases of insertion. 

 (Plate Ixix., fig.14). 



Text-fig. 28. 



Text-fig. 29. 



Small portion of a main vein from the win^i; of Tabanu.^ sp., (Hornsbj', 



N.S.W.), to show the trichiation: ( x 200). 



While, therefoi-e, the main mass of the Diptera shows consider- 

 able specialisation in its wing-trichiation, we have to postulate 



* Small portion of a main vein from the wing of a Mosquito, Mncidni< 

 alternans Westw., 9 , to show scales; { x 200). 



