Pairing of the Plebeiid Blue Butterflies. 161 



membrane," vesica) passes from the visceral cavity into 

 the interior of the aedeagus. 



We may assume the initial structure to be a membranous 

 tube opening on the surface (between segment 9 and 10 

 ventrally). A portion of this tube remains more or less in 

 its primary condition as the ductus ejaculatorius (Auct.). 

 Beyond this is the eversible membrane, which in some 

 species barely exists, the extremity of the aedeagus reach- 

 ing the bursa ; in other species, those on which our usual 

 conception of it is formed, it is contained within the 

 aedeagus. There are, however, many species in which it 

 is of great length and extends back beyond the aedeagus 

 into the abdominal cavity, and except that it is eversible 

 and usually armed with cornuti, it might be regarded as a 

 portion of the ductus ejaculatorius. The eversible mem- 

 brane is, in fact, a portion of the ductus. Beyond this I 

 regard the external portion of the aedeagus as simply a 

 portion of the same duct, permanently everted and chiti- 

 nised. The internal portion of the aedeagus, as I have 

 said above, being a double inversion from the margin of 

 the original orifice. 



What one may call the usual or average length of the 

 extrusible portion of the aedeagus is somewhere probably 

 between that in Thecla, illustrated in fig. 1 on PI. XVIII 

 and XIX, and that in Melitaea, PI. XXI. 



There are not a few cases and whole groups in which the 

 aedeagus is much longer than this, as. for instance, in 

 Apatura, and very markedly in many species of Acraea 

 (Eltringham, Trans. Ent. Soc. 1912, PI. XI-XII) ; m a whole 

 group of Tineae (Bankes, E.M.M. 1910, PI. V. and 1912, 

 PI. IV). See PI. XXII. In all these cases of great length 

 in the aedeagus, there is a corresponding lengthening of the 

 saccus, no doubt to give a proper origin to the retracting 

 muscles. 



In fig. 4 I have (with Thecla as a basis) made a diagram 

 of an arrangement which one would suppose possible, ami 

 which for all I know may actually occur, but which 1 

 believe does not exist, and of which I have met with no 

 trace in any preparation I have examined, though it is 

 possible by undue violence to cause the membranous 

 sheath to be more or less everted. It is supposed to show 

 that the aedeagus might be advanced by the eversion of 

 the sheath by a length approximate to that for which it 

 provides a retreat. I don't know of any case in which 



TRANS. ENT. SOC. LOND. 1916. — PART I. (AUG.) M 



