170 Annals Entomological Society of America [Vol. XIV, 
Of such structural features the latest to be discovered and 
generally adopted as of vast importance in the taxonomy of 
insects are those of the genitalia. When one considers the great 
number of characters, many of them intricate and trivial, that 
students have invoked for use in this family, one who has studied 
the genitalia cannot but marvel at the extent to which these 
characters have been neglected by even the ,most careful 
students. 
Of course in those families of insects where the parts are, 
at least in part, normally exposed they could not be entirely 
neglected by systematists. But in the groups where their 
examination requires some preliminary preparation or manip- 
ulation, they have been most persistently ignored; and, indeed, 
the presence of any variable structures appears to have been 
altogether unsuspected. For example, for the Diptera, the only 
reference that Peytoureau® as late as 1895, gives to the gen- 
italia of this enormous order is to the work of Dziedzicki and 
Wagner on certain genera of the Siphonaptera! 
Many a writer has taxed his eyesight and racked his nerves 
trying to separate, by minute differences in color markings or 
vestiture, species that were provided with structural characters 
by which any novice could place them. The reason for it is 
that these parts are normally almost entirely concealed from 
view (See Figs. A to I, Plate IX), and require a little special 
preparation before they are available for use. One can almost 
fancy that Nature has indulged in a kind of practical joke in 
hiding these excellent structural features from the curious eye; 
for in the Syrphide the most beautifully variable appendages 
are often concealed beneath the most unpromising and dis- 
couragingly homely tergites of the postabdominal segments. 
(Note Figs. A to I and cf. Plates X to XIX). Onecan hardly 
restrain one’s enthusiasm upon relaxing and exposing from the 
exactly similar genital pouches of species after species, a complex 
of appendages that show an abundance of characters and furnish 
much ground for speculation regarding homologies, phylogeny 
and relationships. 
Since Peytoureau’s time, much work has been done on the 
genitaha of the Diptera, most noteworthy of which is that of 
Howard, Dyar and Knab* on the Culicide, Johannsen‘ on the 
Mycetophilide and Snodgrass’ on the Dolichopodide. Those 
