654 THE PANORPOID COMPLEX, ii., 



The formation of scales in the Lepidoptera and Trichoptera 

 agrees with this; since, in archaic famiHes of the former Order, 

 and in all scale-bearing genera of the second, we can still find 

 some at least of the macrotrichia on the main veins remaining 

 in the form of hairs. Judging also from the tendency, in the 

 Lepidoptera, for the broadest (most highly evolved) scales to 

 appear furthest from the veins, while narrow lanceolate scales 

 are found nearest to the veins, we are led to the same conclusion. 

 Actually, the covering of scales is not needed so much upon 

 the veins as upon the membrane, and the original condition of 

 the macrotrichia is such that the need of the insect is to be met 

 along the easiest line of specialisation; i.f\, those macrotrichia 

 whose alteration into scales would most benefit the insect also 

 happen to be those in the best condition to undergo the change. 



In the other Orders in which scales appear (Diptera, Plaiii- 

 pennia) it should be noticed that they ai'e a comparatively late 

 effort, which takes place after the macrotrichia have been com- 

 pletely eliminated from the membrane of the wing. The scales, 

 in these cases, are formed from the macrotrichia left on the 

 veins. If these are still somewhat bristly, as in the Planipennia, 

 a hard, seed-like scale is produced (Text-fig. 24); if, however, 

 they have become slender and delicate, as in the Psychodidoi and 

 CnlicidcB, then the resulting scales will resemble very closely 

 those of the Lepidoptera and Trichoptera. 



The changes in position of the maci'otrichia, in connection 

 with the disappearance of the archedictyon, have been already 

 dealt with above. 



Taking all the above evidence into account, we may reason- 

 ably come to the following conclusions, as far as the limitations 

 of this Part of our study permit: — 



(1) The fossil Order Protomecoptera is undoubtedly the most 

 archaic type known within the Panorpoid Complex, as regards 

 the characters of its Wing-trichiation. 



(2) A single line of descent from the Protomecoptera to the 

 Mecoptera, and from the very base of this latter Order to the 

 Diptera, is strongly indicated by the uniform type of trichiation 



