160 Dr. T. A. Chapman on the 



may say to prove, that the membranous sheath (sinus) 

 is not a portion of the ductus, but is an invagination 

 of the floor of the genital cavity-* In many cases, I 

 cannot say in a majority, still less in all cases, though 

 it may be so, the sheath has a sparse armament of 

 hairs, such as are only found on external cutaneous sur- 

 faces, whence it follows that it must be a portion of the 

 surface of the 9th or 10th abdominal segment, or possibly 

 of the membrane between them ; the latter is very unlikely. 

 My own opinion is that it belongs to the 10th abdominal 

 segment, and that the opening of the ductus in the male 

 corresponds to that of the oviduct in the female on that 

 segment. There is never any trace of a hair or hairlike 

 structure on the aedeagus itself ; the aedeagus is therefore 

 an internal structure (a portion of the ductus). It is hardly 

 necessary to say that the cornuti are not in any way hairs, 

 though some of their forms are very similar to some forms 

 of hairs. 



I was for long ignorant of this armament of hairs on the 

 membranous sheath, due no doubt to several reasons ; that 

 they are often absent is probably one. If one removes the 

 aedeagus from the rest of the appendages, it is apt to 

 be separated at the zone, and the sheath is not seen with 

 it, and when examining the parts undisturbed the hairs 

 are obscured behind other parts, and, if seen, it is not at 

 all clear what their origin is. Nevertheless, when one is 

 aware of their existence, it is often possible to make them 

 out satisfactorily. In PI. XXIII are two specimens of the 

 aedeagus of Curetis bulls ; on one of them the sheath is left 

 uninjured, on the other only a portion of it remains. The 

 hairs on the sheath are in this instance very obvious. 



The wall of the internal portion of the aedeagus is not 

 a simple internal chitinous structure, but arises from the 

 fusion of two layers, which we may for the moment assume 

 to be an outer one belonging to the general surface, an 

 inner derived from the azygos tube ; the fusion being, of 

 course, of their opposed outer surfaces. It would, however, 

 equally meet the case if both the layers were of the ex- 

 ternal surface, or both of the azygos tube. The reflection 

 from the one layer to the other occurs at the opening in 

 the base of the aedeagus, frequently placed some way from 

 the extremity through which the azygos tube (" eversible 



* Diaphragm is a terra that has been applied to the membrane 

 forming this iloor. 



