414 W. WESCHE ON THE MALE ORGANS OF THE FLIES 



{Musca domestica), and has succeeded in obtaining sections of the 

 insects in copula. By this means he proves the remarkable 

 fact, hitherto unsuspected, that the male, after seizing the female, 

 is passive. It is the ovipositor that is forced into the cavity 

 of the hypopygium, and its soft parts fit into and round the 

 complicated armature of the male. This applies to Scatophaga as 

 well ; but with families where the females have long jointed horny 

 ovipositors, this observation of Professor Berlese's does not apply, 

 and we find a different type of genitalia in the male. In the 

 Lonchaeid {Toxenewa muliehris) the claspers are more like palpi, 

 there are no inner hooks, and the penis is modified into a long 

 ciliated ribbon. In other genera of the same family this ribbon 

 has a chitinous bulbous head, with an organ which appears to 

 correspond with Fig. 9 [Palloptera ustulata). In the Trypetid, 

 (Acidia he7'aclei), the ribbon is not ciliated, and ends in a bulb with 

 complicated internal parts. 



Speaking generally, in those families in Diptera where the 

 female has a type of ovipositor approximating to that in Muscat 

 the arrangement of genitalia in the male will be found to 

 homologise fairly well with that in Scatophaga. 



The characteristic specific modifications under discussion are the 

 means by which the sterility between allied forms, so necessary for 

 the preservation of distinct species, is obtained. Judging from 

 S. stercoraria and S. lutaria, besides others that I have observed, 

 these modifications will be found to be most pronounced in the 

 penis, the other differences being matters of detail. 



By microscopic comparison I am able to prove that Mr. 

 Xjt. H. Yerrall * is right in relegating the species Scatophaga 

 merdaria to the list of varieties. I am unable to separate my 

 preparations of S. stercoraria and S. merdaria, as the bleaching 

 process they have gone through has removed all the colour, which 

 appears to be the only characteristic difference, the genitalia and 

 other parts being quite identical. 



Returning to the subject of sterility between species of insects, 

 I think this is mostly brought about by mechanical means, such 

 as obtain in L. stercoraria and S. lutaria. It is obvious that 

 sterility must be in a more or less degree a character of species, 

 for if a modification were to appear beneficial to an individual, 

 unless there came with it either an accidental isolation of the 



* "A List of Britisli Diptera," 1901. 



