1881. 225 



ON FEMALE DIMORPHISM OF JPALTOSTOMA TORRENTIUM. 



BY DR. FRITZ MTTLLER. 



As there seems to be some hesitation in accepting the female 

 dimorphism of JPaltostoma torrentiam (see Ent. Mo. Mag., xvii, p. 130), 

 I will here very briefly state the facts which seem to me to prove that 

 the two sets of females belong to the same species. 



First as to the sex of the three forms of JPaltostoma. Were it 

 not for Baron Osten-Sacken saying that " error may easily occur," I 

 should have thought it quite unnecessary explicitly to state, that I 

 ascertained the sex by examining the internal sexual organs ; the 

 females of either set have three dark brown pear-shaped receptacula 

 seminis ; the eggs, in nearly ripe pupae, are 0*5 mm. long, 0'13 mm. 

 thick, one side being more convex and one end a little more obtuse 

 than the other. 



Had the males and the two sets of females been caught at the 

 same locality, it would indeed have been rash to consider the females 

 (widely differing in the organs of the mouth, the size of the eyes, and 

 the structure of the last tarsal joints) as belonging to the same 

 species. But the case is quite different. In the rapids of some of 

 our rivulets the larvae and pupae of JPaltostoma are extremely frequent, 

 and may be collected in large numbers. Thus I have been able 

 carefully to compare and to dissect hundreds of them ; but I have not 

 discovered any differences corresponding to the three sets of flies. 

 From the pupae I have extracted repeatedly numerous flies, and have 

 always met with two sets of females, and never with more than one 

 set of males. The two sexes seem to occur in about equal numbers. 

 One day from 70 pupae I extracted 20 males and 20 females, and of 

 these 13 had small eyes, short claws, and no mandibles, whereas 7 were 

 provided with mandibles, and had large eyes and long claws. The 

 structure of the external sexual organs (as already stated in my article 

 in"Kosinos") is quite the same in the two sets of females, and this 

 would hardly be the case, if they belonged to different species. 



If the two sets of females belonged to two distinct species, 

 unavoidably one of the two following equally unacceptable assumptions 

 must be admitted : the males of one of the two species either must be 

 extremely rare, so that among very numerous females I never saw 

 them, or their larvae and pupae must live in different localities and 

 under quite different conditions ; the latter assumption is the more 

 improbable, as the larvae of Paltostoma are wonderfully adapted to 

 inhabiting rapids. 



