AMERICAN DIPTERA. 181 



In the lower genera, as just stated, the pleura lie in their normal 

 position, one on each side of the segment, between the tergum and 

 the sternum. In other genera, however, they may be either exserted 

 upon the posterior rim of the segment, or form a small plate set into 

 the posterior rim, or they may be absent entirely. 



In the genus Tipula the pleura are never very conspicuous and 

 are in many species entirely absent. Westhoff, in his paper on the 

 hypopygium of Tipula,^ almost entirely overlooks them. He re 

 gards the hypopygium as being formed of both the eighth and the 

 ninth segments of the abdomen, and aj)plies the following names to 

 the different })arts : the eighth tergum he calls the lamella basalis 

 supera, the eighth sternum the lamella basalis infera, the ninth ter- 

 gum the lamella terminalis supera, and the ninth sternum the lamella 

 terminalis infera. But what is the use of employing such long and 

 cumbrous terms when we already have the shoiler names of eighth 

 tergum, eighth sternum, etc. ? 



In the lower genera the penis arises from the floor of the genital 

 chaml)er and projects straight backward as a slender rod. In the 

 genera of the Tipulina the base is carried up the anterior, and on to 

 the dorsal, wall of the chamber. The penis in this case extends in a 

 large curve down the anterior wall and then posteriorly along the 

 floor of the genital chamber, its terminal part retaining the original 

 position. The basal part in these forms is greatly swollen, forming 

 a hemispherical vesicle. This is the vesicula centralis of Dufour. 

 This name is used also by Westhoff, and in its anglicized form of 

 central vesicle is employed in this paper. 



The terminal part of the penis is in all cases protected by a guard. 

 This has typically the form of two longitudinal plates set close to- 

 gether and on edge, projecting caudally from their origin on the 

 floor of the genital chamber, and having their lower edges united 

 by membrane. Numerous modifications of, and departures from, 

 this typical form occur. The guard may be a solid cylindrical or 

 plate like structure, with simply a longitudinal groove above to ac- 

 commodate the penis, or it may be a hollow cylinder or tube entirely 

 inclosing the penis within it. It is called by Westhofl' the admini- 

 culum. 



In most of the lower species there project backward from the an- 



*■ Westhoff. Friedrich, Ueber den Bau des Hypopygiunis der Gattung Tijrula 

 Meigeii, luaug-Diss., Miinster, 1882. 



TRANS. AM. ENT. SOC. XXX. JUNE. 1904. 



