204 Annals Entomological Society of America  [Vol. XIV, 
TAXONOMIC VALUE OF THE GENITALIA. 
A character to be really satisfactory for taxonomic purposes — 
must meet several requirements. First and most important, it 
must be constant intraspecifically; or, if it vary, at least its 
variations in a given species must not overlap the development 
of the structure in the related species. Second, it must show 
great variation interspecifically, the greater the better. The 
number of species is so great in the class Insecta, that a really 
intricate structure is demanded if we are to find characters for 
all sufficientlly different to permit of recognizable description. 
Third, the character should be one that is readily available. 
It is axiomatic that all parts of living organisms vary. But 
it appears that in the cases where they have been carefully 
tested for variability, the genitalia show a constancy greater 
than that of almost any other external part of the insect, possi- 
bly because they have assumed a position whereby they are 
practically internal and so protected from the stimulus of the 
external environment. I have not examined them by the 
hundreds or thousands, but in several common species of dif- 
ferent genera, I have mounted from a dozen to twenty individ- 
uals, collected from points as remote as Mississippi, British 
Columbia, North Carolina, California, Minnesota, Maine, 
Ohio, Montana, Arizona, Michigan and Colorado. While 
occasionally one may find a slight variation in the number of 
hairs or bristles on a given part or a slight variation in the 
shape of a margin, they are always so remarkably similar that 
any one would place them together without the least hesitancy. 
In regard to variation between species I do not believe any 
group of structures will be found, that so admirably fulfills this 
requirement. Nature seems to have run wild in developing 
many of the structures one finds in the terminal, abdominal 
appendages of insects. 
In regard to the availability of the characters of the gen- 
italia, it must be admitted that this is the chief, if not indeed the 
only, real objection to their use in taxonomy. The parts are in 
this family completely retracted except during mating. 
Undoubtedly the preliminary preparation, such as relaxing and 
clearing in potash, that is required will at once limit the number 
of students who will make use of these characters. But it is 
hardly complimentary to entomological perseverance, if we 
