BY R. J. TILLYARD. 591 



when j) resent, i'rom the basal cell below it, has been named by 

 Tinner the chorda (32). 



The banal cell of the Lepidoptera is formed normally by the 

 abortion of the main stem of M. 1 _ i and its two branches M 1+2 

 and M 3+4 , as far as the limits of the median cell distally, i.e. 

 up to the cross-vein im. All stages in this reduction are to be 

 met with in existing types, as can be seen by a study of the 

 wing-venations ligured in Section xiv. The basal cell, therefore, 

 includes within itself the original median cell, and is separated 

 from the areole above it by the chorda. The veins bounding 

 the basal cell distally have been called collectively the disco- 

 cellulars. But it is better to retain their correct designations, 

 if we wish to preserve in our minds a true picture of what the 

 basal cell really is. The disco-cellulars, from above downwards 

 in order, are the cross-vein r-m, the basal piece of M x the basal 

 piece of M 2 , the cross-vein im, and a short basal piece of M 3+4 

 Below this, the cell is bounded by the stem of the trigamma. 



The basal cell may be enlarged, in the forewing only, by the 

 incorporation within it of the areole or radial cell. This is 

 brought about by the abortion of the chorda, or portion of 

 R 4+s ' separating them. The complete cell so formed has been 

 named by Turner the areocel (32). An areocel is normally 

 formed in those Tortricina in which no separate areole can be 

 seen, and in many Tineina, as, for example, in Wingia (Text-figs. 

 90-93) . An areocel of a somewhat specialised type is also form- 

 ed in the Butterflies, as may be seen from the account given in 

 Section xiv., pp. 079-687. It differs from the areocel of the 

 Tortricina in that, before it was evolved, the radial sector has 

 become split into two parts K 2+3 and K 4+s , arising separ- 

 arately from R. and wide apart. In Text-tig. 59, I have in- 

 dicated the more important stages in the evolution of the basal 

 cell and areole, culminating in the formation of the highly spe- 

 sialised areocel. 



In the hindwings of Heteroneura, the formation of an areocel 

 is not possible, owing to the reduction of Rs to a simple, un-. 

 branched vein. Thus there is never an areole present in this 

 wing, and the most complete formation of the basal cell is that 

 formed by the loss of M 1—4 and its two main branches, as 

 far as the cross-vein im, by the alignment of the discocellulars, 

 and by the strong formation of the trigamma. This type of 

 cell is to be found in all the higher groups, quite independently 

 of whether the forewing possesses an areocel or not. 



