Pairing of the Plebeiid Blue Butterflies. 159 



I have, for convenience, applied the name zone to that 

 line where the internal and external portions of the aedeagus 

 meet, a line by which the aedeagus is attached to the floor 

 of the genital cavity — the circumstance which renders the 

 outer portion " external " and the inner one " internal." 

 I shall return immediately to a fuller discussion of the zone, 

 since it is of importance in the homologies and morphology 

 of the aedeagus, and an important item in the specialisation 

 of the Plebeiid aedeagus. 



In most Lepidoptera the aedeagus is more or less retrac- 

 tile, or, conversely, one may say exsertible. In text fig. 1, 

 a diagrammatic view of the aedeagus in Thecla, only a small 

 portion of the aedeagus projects beyond the floor of the 

 genital cavity when the parts are at rest, the greater part is 

 retracted into a membranous tube (sinus *) extending from 

 the opening in the floor of the genital hollow to the zone. 

 In action the aedeagus can be exserted as in fig. 2, the tube 

 in which it rested being shortened and its walls gathered 

 together. 



The external portion of the aedeagus, that beyond the 

 zone, is almost evidently a portion of the azygos tube, 

 or ductus, everted and chitinised ; but the internal portion, 

 that is, that that is basal to the zone, consists not of a 

 simple internal prolongation of the external portion, but 

 has also fused to its external surface a prolongation of the 

 sheathing tube, these being reflected into each other at 

 the internal aperture of the aedeagus. This interpretation 

 of the actual structure is so nearly self-evident, that it 

 seems in the highest degree likely to be correct. But a 

 further speculation as to which is the point at which the 

 floor of the genital cavity meets the extremity of the 

 ductus has very few data on which to reach a conclusion. 

 Is it the zone, is it the internal opening of the aedeagus, 

 or is it the external opening of the membranous sheath ? 

 There is here a possibility of confusion from the use of the 

 term sheath; penis-sheath has been used as a name for 

 the aedeagus, and penis-sheath (potis-tasche) has been the 

 name applied to structures of the floor of the genital cavity 

 Burrounding the opening of what I have called the mem- 

 branous sheath (sinus). There is in the Plebeiids no penis- 

 sheath as a structure immediately surrounding the aedeagus. 



There is one fact that goes a long way to show, one 



* I find this term used by Kusnezov, whether <>n hia own or 

 some other authority, I don't know. 



