K. Jordan and N. C. Rothschild 
31 
less according to species. The external portion of the organ also exhibits 
sometimes notable specific differences, for instance, the brush-like 
structure in L. pallidus being a very striking characteristic for that 
species (PL IV, fig. 9). 
The differences found in the genitalia of both sexes of insects are of 
special importance to the systematist, since they enable him generally to 
recognise the species when other organs fail to exhibit sufficiently 
striking characteristics. In the study of Siphonaptera we lay great stress 
on the differences existing in these organs, conforming thereby to the 
general experience of entomologists in other groups of insects. The 
minute study of the genitalia of Siphonaptera is a great necessity, since 
these insects do not present so many and so varied external differences 
as is usually the case with the species of the orders of winged 
Arthropods. 
As the senior author of the present essay has made special researches 
bearing on the question (1896, p. 426 ; 1905, p. 163), a short resume of 
what is known of the constancy and variability of the external genitalia 
will perhaps be serviceable for those who are not acquainted with the 
taxonomic value of these organs. The discovery of the existence of 
differences in the genitalia of male insects is due to Leon Dufour (1844, 
p. 253). For a long time after entomologists held the opinion that the 
differences had been created or had developed for the purpose of 
preventing the species from intercrossing. These differences were 
considered constant within each species, and it was generally thought 
that specifically distinct insects exhibited in the genitalia some morpho¬ 
logical distinction from their nearest allies. Inversely it was accepted 
as a fact that all forms of insects which had some characteristic in the 
genitalia were specifically distinct. These notions of the origin and 
significance of the genital differences in forms of insects were somewhat 
crude, the differences having since been reduced to their proper value. 
We know now (1) that the majority of species of insects show some 
morphological distinction in the genitalia from their congeners, there 
being no obvious difference of this kind in a small minority; (2) that 
the genitalia vary to a generally slight extent individually; (3) that 
the succeeding broods of an insect, though often very different in other 
organs, have the same genitalia, the only exception so far known being 
a butterfly (Papilio xutlius ) in the spring and summer broods of which 
the genitalia exhibit some slight and not quite constant differences 
between; (4) lastly, that the geographical forms of a species are very 
often different in respect to their genitalia (cf. Jordan, 1905, p. 151). 
