1924] 
The Biology of Trichopoda pennipes Fah. 
67 
rather more heavily chitinized than elsewhere, resulting in the 
appearance of a chitinized box (chb) from which the sedeagus 
protrudes and on which the gonopophyses are borne. This 
chitinized box also bears a median dorsal hook-like projection, 
called by the writer the genital prong (gp). The styli of the 
ninth segment, which in some insects function as outer claspers, 
are here much reduced in size and are apparently non-functional, 
since when the genitalia are extruded they barely appear beyond 
the posterior edge of the eighth tergite. A peculiar structure, 
which the writer is at a loss to homologize with any genital 
appendage of generalized insects, appears in the ‘‘genital furca’^ 
(gf). This is a fork-like chitinized rod which lies between the 
sides of the ninth sternite, to which it is connected by muscles. 
It splits at the base -of the oedagus, one arm extending to either 
side of the latter organ. Its function is quite evidently that of 
guiding the movements of the oedagus. 
In the female the eighth segment is a narrow ring, bearing 
below the median ventral valve (vv) of the ovipositor and laterally 
the two inner valves (iv). Dorsally this segment seemed to 
bear a median dorsal valve (dv), but this may prove to be a 
modified portion of the ninth segment, which is supposed to 
bear the dorsal valve. This point could not be definitely de- 
termined from the dried material at the writer’s disposal, even 
after soaking in caustic potash and gently extending the ovi- 
positor by pushing from within by means of a blunt needle. 
Secondary Sexual Characters. The foregoing account of 
the external anatomy of Trichopoda pennipes contains scattered 
references to certain differences which were apparent between 
the two sexes. These differences were constant in a series of 
eight males and seven females. Scarcely any difference in size 
could be noticed, the males averaging 8.6 mm. in length, the 
females 8 mm. Both the largest and the smallest were males, 
the one 10 mm. long, the other measuring 7 mm. To a certain 
extent the size of the adult fly is affected by the abundance of 
food available to the larva which preceded it, and when con- 
tained in keys for the identification of species may be found 
misleading. 
