1921] Metcalf: Genitalia of Male Syrphide 205 
allow difficulties of this nature to be an ultimate bar to the 
utilization of the greatest single group of taxonomic characters 
that insects possess. The further hopeful feature is that, when 
these parts have been carefully worked out for a group, subse- 
quent students will often be able to make use of the characters, 
without clearing, by simply comparing the opaque parts of 
the fresh or relaxed specimen. 
It may be objected that the process of removing the gen- 
italia mutilates the specimen. But if the specimen is fresh or 
well-relaxed and one uses moderate care, the removal of the 
modified terminal segments is not a matter of any seriousness. 
All the parts generally used in taxonomy are left intact; anda 
specimen with the mounted genitalia unmistakably associated 
with it is in my opinion not damaged but greatly enhanced in 
value. 
Another criticism of the genitalic method is that, in the 
process of mounting, especially in treating with caustic potash, 
the parts are likely to be variously distorted and so lead to 
serious taxonomic errors. This criticism should be anticipated 
in every investigation and any possible variable effect of the 
method of preparation checked by examining the parts before 
clearing and by preparing in several different ways. It would 
seem that the fact that numerous mounts made at different 
times and by varying methods are identical to the most minute 
detail, is sufficient answer to this criticism, so far as the 
Syrphide are concerned. In fact the parts herein considered are 
almost all heavily chitinized and not likely to be distorted 
either by clearing or by the pressure of the cover glass. Of 
course one would not derive taxonomic characters from such 
parts as the delicate, inflatable membrane terminating the 
ejaculatory duct in Spherophoria, for example. 
One matter of genuine difficulty is that it is often hard to 
define the characters that give to these parts a peculiar and 
easily recognizable facies. The appendages are generally very 
irregular figures of three dimensions; and in the more complex 
forms nothing short of a figure of the parts in two or more 
aspects, together with a careful description is adequate to 
convey a real impression of their makeup. But however difficult 
the interpretation of these parts may be they have the essential 
points of intraspecific constancy and interspecific variability to 
a degree hardly equaled by any other set of organs. 
