BY R. J. TILLYARD. 637 



an indication of that disappearance. The macrotrichia have also 

 disappeared from the niejiibrane, but are still present on the 

 veins. As a high s})ecialisation, only to be })aralleled in the 

 highest families of the Planipennia, we note also thai the- macro- 

 trichia appear for the first time upon the true cross-veins, thus 

 rendering them indistinguishable from veinlets except by the fact 

 that they are not preceded by tracheae in the pupal wing. 



We may sum up the tendency of evolution of the trichiation 

 in this Order by saying that both njacrotrichia and microtrichia 

 appear to have been, from the \ ery first, of small size; and that, 

 although the most archaic genera still show the original arrange- 

 ment of the trichiation, preserved almost in full, yet reduction 

 set in very early, leading to the loss of one or other series oi 

 hairs in all the higher types of the Order. 



The wing-trichiation of the Archetype of this Order will be 

 taken to be similar to that of the archaic genus ArchLchaull(/dt'^, 

 but with hairs of somewhat larger size. 



Order PLANTPENNTA. (Text-figs. I'O, 2:), 24). 



The oldest known types of this Order show a wing-trichiation 

 already specialised in certain directions. Ko traces of the arche- 

 dictyon are to be found, and macrotrichia have, in every case, 

 been eliminated from the membrane of the wing (Text-fig. 2 3). 

 This is easily understood, when we remember that the wing- 

 venation of the Order early underwent a unique specialisation, 

 in the form of a ra})id proliferation of sectors of the main veins, 

 and especially of the branches of the radial sector. This pro- 

 duced the "Prohemerobiid" type (still to be seen in recent Psi/- 

 chupsid(r) in which the whole of the wing is covered with numer- 

 ous parallel longitudinal veins, with few or no cross-veins sup- 

 porting them. In the very narrow spaces between these veins, 

 it was clearly impossible for the archedictyon to exist, and the 

 macrotrichia upon it probably disappeared with it. 



►Starting, then, from a type having no macrotrichia upon the 

 membrane of the wing, we find a furtlier specialisation, in the 

 higher families of this Order, in the complete elimination of the 



