16 BULLETIN 329, U. S. DEPARTMENT OF AGRICULTURE. 



have the antennae 4- jointed, the first long joint described by Johann- 

 sen being divided by a very distinct suture. Though the first and 

 second joints are very closely articulated and rigid, the constant 

 occurrence of a distinct suture between them seems to indicate clearly 

 the existence of two segments. The third segment is slender cylin- 

 drical, and the fourth a short conical process. 



There are two conical processes situated at the apical end of the 

 second segment, which seem to be sense organs. The antennae of S. 

 vittatum (fig. 11) are 5-segmented, the second segment being again 

 divided. The sense organs are borne on the apical end of the third 



segment. In color the antennae vary with 

 the species from yellowish hyaline to dark 

 brown. 



The mandibles are situated just below 

 the fans and move horizontally. With 

 the exception of a slight difference in size, 

 according to the species, there are no con- 

 stant characters of real systematic value 

 in the mandibles. 



The following is a description of the 

 mandibles of S. venustum: 



The mandibles (fig. 12) are very chitinous, 



stout, somewhat elongate, with the outer lateral 



margin rounded. There are from two to four 



large teeth at the apex on the inner lateral 



margin, almost dense black in color. Situated 



Fig. 12.— Simulium venustum. immediate iy dorsad along the. inner margin of 



Left mandible of larva, ven- . ' " , , „ „ .-..■,. 



tral view. Greatly enlarged. the lar §' est tooth IS a TOW of from SIX to eight 



(Original.) smaller teeth of the same shape, but much 



lighter in color and gradually decreasing in size. 

 Behind this row, on the ventral surface of the mandible, arising generally from 

 the third or fourth tooth, is a second row of small, sharp-pointed teeth very pale 

 in color, about 10 to 14 in number, which continues along the basal portion of 

 the lateral margin of the largest apical tooth. Later ad of this second row of 

 teeth, on the inner margin of the mandibles, there is a large wide-angled flat- 

 tened tooth, very pale in color, followed by a smaller one of the same descrip- 

 tion. Near the apex of the mandible, a short distance from the outer lateral 

 margin of the ventral surface, are two large bristles arising from almost the 

 same point. At the base of the large apical teeth, on the ventral surface, are 

 two clumps of stout hairs. There is a fringe of long hairs on the dorsal sur- 

 face of the mandible, which extends halfway down, commencing at the apex, 

 where it almost obscures the apical teeth. Opposite the base of this fringe on 

 the inner lateral margin of the mandible there is a fan of long hairs, and a 

 fringe of very long hairs, which continues clown to the base, gradually devel- 

 oping into bristles explanate and divided at apex. 



The maxillae are situated immediately ventrad and mesad of the 

 mandibles. The maxillary palpus is short, cylindrical, and bears 

 on its distal end several small cone-shaped protuberances similar to 



