198 R. E. SNODGRASS. 



146 on Plate XVII. It consists of a very convex, often hemispher- 

 ical, strongly chitinized body projecting into the genital chamber 

 from the dorsal wall of the latter. Posteriorly there project from 

 its base two arras (a) that diverge posteriorly, upward and outward 

 in the roof of the genital chamber. From the anterior angles of the 

 base two wider and shorter plates project anteriorly and outward (b). 

 From the dorsal surface there project dorsally two large apodemes 

 (c) into the space between the genital chamber and the tergum of 

 the hypopygium. It is evident that the posterior arms of the cen- 

 tral vesicle may be homologous with the anteriorly diverging roots 

 of the penis in the lower genera. If the base of the penis in any of 

 the latter forms were carried up the anterior wall of the genital 

 chamber and then forward upon the dorsal wall, the arms originally 

 projecting anteriorly would come to project posteriorly. 



Between the posterior arms of the central vesicle, in the dorsal 

 wall of the genital chamber, is a chitinous bar generally composed 

 of two arms diverging posteriorly outward to the side walls of the 

 genital chamber. This bar, on account of its usual shape and posi- 

 tion, will be called the ^-shaped brace (fig. 148). It sometimes does 

 not have this typical form, however, and may be absent. 



In some species of Pachyrrhina there are present rod-like append- 

 ages arising from the base of the guard of the penis that may be 

 second gonapophyses. In Tipula such appendages are generally 

 absent. When they are present they usually project posteriorly be- 

 low the guard, and arise either from the sides, or the lower part, of 

 its base. If these appendages are the homologues of the second 

 gonapophyses in the lower genera, they have become greatly dis- 

 placed. 



Throughout the entire group there is a strong tendency toward the 

 formation of three apical appendages. One of these is always situ- 

 ated dorsal to (which may mean either above or in front of) and on 

 the outer side of the others. This is the one called by other authoi-s 

 the upper appendage. The second appendage: is often concealed 

 within the first and has been called the middle appendage. These 

 two are almost invariably present and evidently are homologous with 

 the apical appendages of the lower genera. The third is developed 

 as a small lobe on the outer side of the base of the second. It is 

 rudimentary in Paehyrrhina, Gtenophora and in many species of 

 Tipula. In a large number of the species of Tipula, however, it is 



