104 ON THE ANATOMY OF THE FLY. 



I cannot help suspecting their outer coat is contractile. The 

 ejaculatory organ is evidently an elastic sac within a muscular 

 covering, and aided by the peristaltic action of the vas def erens 

 undoubtedly ejects the male fiuid through the ejaculatory duct 

 with considerable force, as the mass of muscles is very large 

 indeed in proportion to the magnitude of the organ. 



The four posterior abdominal segments (Plate IX., fig. 6 

 a. b. c. d.) form the external sexual apparatus of the male, and 

 are included within the genital fissure, which is formed by the 

 posterior margin of thejifth abdominal segment. This open- 

 ing is almost entirely closed by four valve-like pieces (Plate 

 IX., fig. 6 d), except during the sexual act, when the four 

 segments are partially protruded from the fissure, and the 

 valve-like pieces are directed backwards instead of forward. 



The homologies of the several parts of these segments are 

 very difficult to make out, and will require a lengthened study 

 of the male and female organs of other insects before they 

 can be determined with certainty. 



There is no difficulty in recognising the dorsal plates of 

 three segments, but the fourth seems to be represented only 

 above, by the inner pair of valve-like pieces already mentioned. 

 The ventral pieces are, however, considerably modified, and 

 two appear to be fused together, quite an exceptional condition, 

 for, as has already been remarked, the ventral are normally 

 more distinct than the dorsal plates. Another anomaly is 

 found in the unsymmetrical development of these plates, 

 affording the only example of the want of perfect lateral 

 symmetry in the integumental organs, and if the seminal 

 receptacles of the female are exCepted, in the structure of the 

 insect. 



The sixth abdominal segment presents only a very narrow thin dorsal 



