256 STUDIES IN AUStRALlAK XEt'ROPTERA, iv., 



downwards to Mj, we obtain what I shall call the radial formula, 

 which may be equated to the total number of branches passed. 

 Thus for D. hninilis, as figured in Plate xiii., fig. 2, we have 



Radial formula, 3 4-1 + 1 + 1 + 2 = 8 (the count stopping short 

 at the vein lying above the radio-median furrow, shown as a 

 dotted line). An equally common, radial formula for this species 

 is 3 + 1 + 1+3 = 8, representing the case shown in Text-fig. 6, h. 



One of the most remarkable structures to be seen in the wing 

 of Drepanepteryx phalceno'ides is the peculiar transparent patch, 

 at about the middle of the posterior border of the forewing, 

 resembling a split or tear in the wing. Such a structure I pro- 

 pose to call a fenestella (Lat. = a little opening). It occurs in a 

 slightly less conspicuous form in some species of Drepanacra. Its 

 effect is to give the insect, when at rest, the appearance of a 

 dead leaf, with a small tear in it. It is clearly a development 

 correlated with the falcate type of wing, for the eft'ect of a dead 

 leaf is obtained primarily by this latter formation, and there is 

 no record of a fenestella occurring in any but a falcate wing. 

 This is a nice little problem in protecti\e resemblance, which has 

 long puzzled entomologists. 



If we examine a fenestella (Plate xiii., figs. 11, 13), we shall 

 see that the apparent interruption of the veins is a very simple 

 deception brought about by the absence of pigment. The veins 

 are all present, and are all easily seen under a moderate power 

 of the lens, the actual condition being such as I have shown in 

 my di'awings, and not as figured by Sharp (13; p.4()S). The 

 "split" efl'ect is obtained (1) by the complete absence of pigment 

 on the wing-uiembrane within the fenestella itself, and (2) by a 

 certain amount of increase of the pigment surrounding it. To 

 understand how the fenestella arose, we must first of all realise 

 that it is foi-med around a small series of cross-veins which have 

 been brought into line; they are, in fact, a posterior extension 

 of the outer gradate series, originally, no doubt, arranged in step- 

 form, but later on combined to form a single line running trans- 

 versely in from tlie wing-border. Now we have many cases of 

 the enclosui-e of gradate cross-veins in small pigmentless areas, 



