298 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxi. 



The ocelli are three in number, arranged to mark the corners of a 

 triangle, the anterior and median being the larger of the three, 

 while the others are posterior and lateral. The distance apart of the 

 lateral ocelli as compared with that between one of them and the com- 

 pound ey» is often useful in the determination of species. Behind the 

 ocellar triangle there is sometimes a transverse oval area slightly 

 raised abo\'e the surrounding surface and perhaps marking the real 

 vertex of the head. In this paper the vertex is considered as being 

 on a lino drawn through the posterior ocelli. No sutures separating 

 the vertex from the posterior portion of the head above or from the 

 cheeks at the sides behind the eyes are present, and the limits of 

 these parts are therefore somewhat indetinite. 



The portion of the head showing behind the compound eye is termed 

 the cheek in this paper. Its w^idth and fullness vary greatly. When 

 full it gives to the head, as viewed from al)ove, an almost quadrangular 

 outline with rounded corners; when retreating the eyes also seem less 

 full, giving to the head a more oval outline. The width of the cheeks 

 is usually greatest a short distance below the top of the e3^e. Below 

 this point they may narrow rapidly or remain quite broad for some 

 distance, narrowing suddenly nearly at the level of the bases of the 

 mandibles. 



The labrum is attached to the lower inner edge of the clypeus, leav- 

 ing the outer edge of the latter well defined. In preserved specimens 

 it is usually bent backward nearly at right angles to the cWpeus and 

 with the mandibles closed together over it so that it is not accessible 

 for study. For this reason it has not seemed best to make use of the 

 characters it possesses for analytical purposes, though studies of its 

 structure indicate that m some cases distinctive features may be found 

 there. 



None of the mouth parts save the mandibles seem to be useful for 

 the determination of species and their description, therefore, it is not 

 included here. 



The mandibles vary considerably within the limits of the group. 

 Ordinaril}^ they are quite long, somewhat curved, stout and decidedl}^ 

 rapacious in appearance, each reaching the base of the other when the 

 jaws are closed. The mandible may be considered as consisting of a 

 shank, a terminal tooth, and one, two, or three teeth on the inner or 

 upper face, these last being much shorter than the terminal one in 

 most cases. The proyjortions of the teeth to each other var}^ greatly, 

 however, not only in ditfcrent species but even in the same individual 

 at different ages, the digging, which the mandibles are used for, often 

 wearing them down to mere stubs. The posterior face and under 

 (outer) surface are smooth so far as teeth are concerned but particu- 

 larly on the anterior surface grooves or rows of indentations, termed 



