1881. 
225 
ON FEMALE DIMORPHISM OF P ALTO STOMA TO RRJENTI UM. 
BY DR. FRITZ MULLER. 
As tlieie seems to be some hesitation in accepting the female 
dimorphism of JPaltostoma torrentium (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 Paltostoma. 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, 013 mm. 
thick, one side being more convex and one end a little more obtuse 
than the other. 
Ilad 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 stiucture of the last tarsal joints) as belonging to the same 
species. But the case is quite different. In the rapids of some of 
oiii uvulets the larvae and pupae of Paltostoma 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 “Kosrnos”) 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. 
