BOTH THE SEXES IN DIPTEEA. 355 



not being definite enough in view of its importance in other families, I propose to call it 

 the " ejaculatory apodeme." 



Bibionidce. — My preparations of Sclara and Bibio are not successful in showing these 

 organs, but in the minute Scatopse notata I have been able to make them out quite 

 clearly. The sac is an elastic globe, and the elaborate transverse levers, actuated by the 

 apodeme, compress it, and force the seminal fluid through the duct (PL 23. figs. 8, 9). 



Ti}}Hlid(V. — In Tipula oleracea the sac, as I said before, is hard and horny, with two 

 jn-ocesses, fitted for the attachment of muscles, fused on to it. The duct opens widely 

 but soon narrows, forming the hyaline flagellum. The base of the apodeme is blunt, 

 fitting into an opening in the sac, and is capable of closing the opening of the duct. 

 The apodeme has two arms, and it is obvious, on looking at the parts, that when the 

 right-hand or dorsal arm is drawn in that direction by its muscular attachments, the sac 

 is rotated, the duct is closed (owing to the changed position of the apodeme in its 

 containing cavity), and the flagellum is withdrawn into the theca. On the other hand, 

 when the left or ventral lever is pulled, and the sac rotated in the contrary direction, it 

 not only opens the duct, but protrudes the flagellum (PL 24. figs. 24, 26). 



In Pachyrrhina maculosa the same arrangement is found, but the apodeme is of the 

 more usual form, with a fan or disc instead of opposing arms (PL 24. figs. 30, 31). 



In Gi/nopUstiu bella the apodeme is seen in the centre, between tlie apodemes of the 

 penis, but my preparations do not clearly show the structure of the sac (PL 23. fig 15). 



In Ttyclioplera scutellaris an even more curious organ is found. Three flat plates, 

 Avith muscular attachments, are fixed on three sides of a hollow chitinized receptacle 

 which is situated immediately below the curious penis (PL 23. figs. 17, 18 ; PL 24. fig. 20). 



Straliomijiidce. — In Be^'is vallata the penis takes a curve, and is then bent on itself, 

 broadening out into a cavity which forms the sac. In this cavity the apodeme is 

 articulated in such a manner that when it is drawn towards the thorax it opens the sac 

 and extrudes the penis, and when towards tlie posterior, contrai-y motions take place, a 

 variation on the device in T. oleracea (PL 24. fig. 33). 



TabanidcB. — A very complicated mechanism exists in Tabamis hromius, and I only offer 

 the following as a hypothetical exjilanation of tlie action of the parts. 



The apodeme is placed in the centre of tlie tbeca, and reaches well up into that part ; 

 at the broadest part of the theca an articulation with another piece is apparent. Prom 

 this point two very fine rods curve outwards, and then approach each other till they 

 overlap and form a fine point, capable of easily passing through the opening in tlic 

 point of the theca. These rods are contained in a membrane, which is the sac. On the 

 apodeme being drawn forwards it would push the rods through the orifice of the theca. 

 On being still further drawn, or being drawn backward and forward, it would separate; 

 the points and, at the same time, compress the sac against the containing sides of the 

 theca, thus opening the orifice and expelling the spermatic fluid (PL 29. fig. 109). 



AsiUdce. — In the Asilidte a similar mechanism to that of Beris is seen, but with the 

 difference that the part is much thickened and diitinized (PL 24. fig. 38). 



EmpidcB. — In the Empida; we have another variation on the mechanism in Tipula. 

 The flagellum, after turning back on itself, widens out into a sac, into which an apodeme 



SECOND SERIES. — ZOOLOGY, VOL. IX. 52 



