210 AMERICAN HYMENOPTERA. 



widest behind the upper halves of the eyes and are much 

 narrowed toward the mouth ; they have little taxonomic 

 value. 



The appendages of the head will be considered in detail 

 separately. 



Ajitenncz (figs. 19, 21, 23, 25 and 27). — These organs are 

 inserted upon the front part of the head opposite the middle 

 of the eyes. These insertions are usually a little nearer to 

 each other than they are to the eye margins, but in some 

 males {Bombias) they are nearer to the eyes than to each 

 other. The basal portion or bulb of the proximal segment 

 or scape articulates with the head in a socket. This bulb 

 has a slightly smaller diameter than does the main portion 

 of the scape where it is thickest. There is a strong con- 

 striction between the bulb and the rest of the scape, the most 

 of the narrowing being on one side (figs. 21 and 27). The 

 scape is much the longest segment of the antenna. The com- 

 parative lengths of the scape and the flagellum (the rest of 

 the antenna without the second segment) do not appear to 

 vary greatly among the females of different species, but, 

 among the males, they do vary very considerably. The scape 

 is fully two-thirds as long as the eye. It is thickest toward 

 either end. The fiagellum is usually carried at an angle 

 with the scape, thus forming what is known as a geniculate 

 antenna. The second antennal segment is known as the 

 pedicel. This segment is by far the shortest of all the an- 

 tennal segments. Its apical portion is usually sub-globular, 

 but it is more or less constricted basad. The fiagellum con- 

 sists of ten segments in the females and eleven in the males. 

 These segments vary considerably in length between the 

 males of the various species and between the two sexes. 

 The first segment always gradually increases in diameter 

 more or less from the proximal to the distal end. The rela- 

 tive lengths of the first three segments are of considerable 

 value in classification. The terminal segment (fig. 58) is 

 always compressed or flattened more or less apically so as 

 to appear wedge-shaped when viewed laterally. The middle 

 segments are usually straight and cylindrical, but in the an- 



