]9io.] 29 



But the aedeagus as a whole is capable of y^reat iiioveinent. lu 

 repose when contracted it is contained not only within the lumen of 

 the internal sei!;nient described at tlie beginning of this account, but is 

 also retracted within another part, which I will call the encasement ; 

 and it is, when fully extended, found to be connected with this encase- 

 ment only by a delicate and very short part of the genital tube forming 

 a connecting membrane. 



It has happened in two or three cases that, when preparing a 

 specimen for examination of these parts, we have found what I have 

 described as the ventral aspect of the aedeagus to be uppermost. This 

 I believe to be not a morphological deviation, but purely accidental, 

 and in all probability due to a half -twist of the connecting membrane 

 having occurred at some moment when the aedeagus was fully extended; 

 if then suddenly withdrawn the part would be found in the encase- 

 ment with the orientation reversed. This point is of some importance 

 as showing how easily a change can be made as to this feature of the 

 orientation, although extreme importance has been attached to it by 

 some morphologists. My colleague F. Muir and I in our paper on the 

 comparative anatomy of the genital tube (Tr. Ent. Soc. London, 1912) 

 had occasion to mention (I.e., pp. 549, etc.) cases of reversed orienta- 

 tion, more particularly in the Heteromera, which we could not very 

 well understand, and I therefore add that I (and I believe from a 

 letter I have had from him I may also say my colleague) now attach 

 but little importance to these cases. For even if some of them should 

 prove to be normal, they may be due to a half -twist of the genital tube 

 occurring during the metamorphosis, in which case their morphological 

 importance would be comparatively slight. 



The encasement consists of two plates, a dorsal and a ventral, each 

 with remarkable chitinisations — or better — darker areas. The two plates 

 liave no lateral continuity except by membrane, though they are in very 

 close correlation. The alimentary canal enters the encasement and 

 there terminates. The encasement is tlierefore a highly modified body 

 segment, not only because of its relation to the alimentary canal, but 

 also because of its definite division into tergum and sternum. The 

 parts of the genital tube in Colenptefa frequently exhibit a segmentation 

 (exen)plified in HeJo^iharufi liy the relation of the basal sclerite to the 

 parts beyond), l)ut this ])seudo-segnientation does not exhibit the dis- 

 tinction of individualised tergum and sternal parts that is invariably 

 the case with the chrootic somites. This is one of the reasons for 

 deciding that the parts considered by Muir and myself as phallic are 



