356 ME. WALTER WESCHE ON THE GENITALIA OF 



is inserted and acts in the same way, the muscles pnlling it one way opening the 

 passage and protruding the flagellum, the other way, closing the sac and withdrawing 

 the flagellum (PL 25. fig. 41). 



BoUchopodidw. — In Dolichopiis g^Hseipennis a beautiful mechanism is present; the 

 hase of the apodeme is pierced by a neat fitting for the vas deferens, which, when the 

 apodeme is in its normal position, fits against a pad and closes the duct. When it is 

 drawn back, as is seen in the drawings (PI. 25. figs. 50, 51), an elastic membrane is 

 stretched across the space between the theca and the apodeme, leaving the ejaculatory 

 duct free to the orifice. The changes in mechanism from the type of the Tipulidse are 

 brought about by the fixture of the flagellum to the theca ; consequently the apodeme 

 only does its proper work of containing or releasing the spermatic fluid, and does not 

 rotate or extrude the penis. 



Syrphidce. — In Catahomha pyrastrl the sac is a flexible hyaline membrane with a 

 minute apodeme (PI. 25. fig. 53). In Syritta p)ipie)is an unusually large apodeme is 

 found at the base of an enlargement of the ejaculatory duct which represents the sac ; 

 the head of the apodeme has a transverse plate and a swelling which appears to be 

 modified to fit this space, and closes or opens the duct by backward and forward 

 muscular action (PI. 26. figs. 56, 57). In Eristalis tenax the sac is represented by a 

 hyaline tube with a small apodeme, as in Catahomba, but all contained in the theca and 

 fairly close to the orifice. The chitinous box at the extremity of the theca is fitted with 

 a ciliated valve, allowing for a free passage ovitwards (PI. 26. fig. 61). 



Muscidcc. — In the greater proportion of the flies the sac and apodeme are of the same 

 type and situation as in Catabomba pyrastri. The action of this arrangement is by no 

 means obvious ; either the apodeme is pressed against some other part by the attached 

 muscles, so as to close the sac, or is drawn backwards and forwards, acting as a pump 

 and driving the fluid through the ejaculatory duct (PL 26. figs. 60, 62, 68). 



In Calliphorci, the sac is at a much greater distance from the theca than in Follenia 

 or Scatopliaya. This part is found in a curious situation in Gloss'ma, where the 

 apodeme rests in the fork of the double apodeme (PL 27. figs. 72, 73, 74, 75), practically 

 at the orifice of the duct, and it is a knowledge of the mechanism in this genus tliat has 

 suggested my explanation of the part in Tabanus. 



Lo)icha;ld(e. — In Toxoneura muliebrls the sac and small apodeme are at the base of 

 the long ciliated penis, below the forked apodeme (PL 30. fig. 122). 



Trypetidce. — In Acidia heraclei (Linn.) the sac is in a precisely similar position, but 

 the base of the apodeme is spatulate. 



Sepsidce. — In the Sepsidae the apodeme has a remarkably long stalk (PL 28. fig. 94) ; 

 this is also the case in the nearly related Diopsidae. 



Use. — This apparatus is arranged to control the flow of the seminal fluid, being so 

 situated and held by muscular attachments that when the penis is in its normal jiosition 

 the sac is closed ; a diff'erent process seems to be the rule in those Muscidaj where the 

 ovipositor is long, as in Ilusca domestica; in such flies the valve would probably not be 

 opened till after the elaborate interlocking of the parts shown in Berlesi's section had 

 taken place. 



