AMERICAN DIPTERA. 197 



forward ; and (3) the tliickening of the base of the penis to form a 

 central vesicle. Less distinctive characters are the disappearance of 

 the second gonapophyses, and the presence in most of the species of 

 three apical appendages on each side. 



The fusion of the pleura with the sternum is perfect in Ctenophora 

 and in many of the members of Tipala. In Pachyrrhina the condi- 

 tion is more primitive. In one species (P. polymera, PI. XI. fig. 60) 

 the suture between the pleuruni and the sternum on each side ex- 

 tends to tlie anterior margin of tlie segment, but near the anterior 

 end it is simply a groove. All the other species of Pachyrrhina ex- 

 amined have this suture ending some distance back of the anterior 

 margin of the segment. The suture may have a simple termination 

 as in P. lugens (PI. XI, fig. 51), or, as in more general, the anterior 

 end may be abruptly bent upward, as in P. incurva., P. ferrnginea 

 and P. pedunculata (PL XI, figs. 53, 56, 59). In many species of 

 Tipala this vertical arm of the suture is extended upward to the 

 tergal suture, thus cutting ofi;'a plate, generally having a triangular 

 shape, set into the posterior margin of the segment, between the ter- 

 gum and the sternum and carrying the apical appendages. In these 

 forms, then, the condition of the pleurum is intermediate between 

 that of Antocha, Dicranomyia, Bittacomorpha, etc., where the pleu- 

 rum has the normal position between the tergum and sternum, and 

 tliat of Erioptera, Limnophila, Amalopis, etc., where the pleurum is 

 an exserted lobe upon the posterior rim of the hypopygium. The 

 arrangement of the genera in such a sequence, however, is precluded 

 by the structure of the penis and other organs. 



In the lower genera of the Tipulidie the penis has the form of a 

 short straight rod, generally arising from two diverging root like 

 arms {e.g. see Antocha, fig. 3; Linmophila, fig. 21 ; Ptychoptera, 

 fig. 42). In the Tipulina the penis has a very different appearance. 

 It arises from a large, swollen, darkly chitinous vesicle, the central 

 vesicle situated on the roof of the genital chamber (see section of 

 Tipula angustipennis, PL XVII, fig. 149, c. v.). From the anterior 

 end of this vesicle the penis curves forward and downward close to 

 the anterior wall of the genital chamber, and then goes posteriorly 

 on its floor (fig. 149, p.). Often it goes far forward in the abdomen 

 within a special prolongation of the genital chamber, before turning 

 posteriorly. It may reach even into the first abdominal segment. 



The central vesicle usually has the form shown in figures 144 and 



TKANS. AM. ENT. 80C. XXX. JULY. 1904. 



