1898.] NATURAL SCIENCES OF PHILADELPHIA. 167 



A little caudad, ventrad and laterad of the outer vertical, on the 

 occipital surface of the head, there is sometimes a small bristle 

 which curves dorsad, mesad and a trifle cephalad, apparently decus- 

 sating with the outer vertical. I call this the occipito- lateral. 



Parallel to and at a variable distance caudad from the occipital 

 margin of the eye, is a row of bristles called the cilia of the posterior 

 orbit. They begin dorsad very near the outer vertical bristle but 

 distinctly caudad of it and also very near but cephalad of the occi- 

 pito-lateral. The members of the dorsal part of this row are com- 

 monly larger and more perfectly aligned than the members of the 

 members of the ventral portion, and there is much difference in their 

 size and correctness of alignment in different species as well as in 

 the distance to which they can be traced ventrad as a distinct row. 

 Frequently there are one or more rows of exceedingly small bristles 

 intercalated between the cilia of the posterior orbit and the occi- 

 pital margin of the eye. The posterior orbit is the space between 

 the cilia of the posterior orbit and the occipital margin of the eye ; 

 it is usually narrower in the male than in the female, and may be 

 absent. 



I spoke above of a triangular area marked out at the dorso-cen- 

 tral part of the occipital surface of the head by the two little ridges 

 and their mesal branches. This triangle and the more or less dis- 

 tinct eminence on which the ocelli are situated, are continuous with 

 one another over the vertex and form a differentiated portion which 

 is occupied by a group of bristles which may be called the ocellar 

 group. Among them we must distinguish the greater ocellar (ocel- 

 lar pair of Baron Osten-Sacken), and the lesser ocellar. 



The greater ocellar is a pair of bristles usually easily distinguish- 

 able by their larger size, which are inserted between the anterior 

 ■and posterior ocelli. They vary in size, direction and curvature in 

 different species and in the two sexes of the same species, and in 

 different individuals (especially males) of the same species. They 

 are generally parallel to the lesser ocellar bristles and sometimes 

 differ from the latter in size by a very small amount. 



The lesser ocellar are more numerous, from three to twelve pairs, 

 and are usually inserted in two parallel lines (sometimes in four) 

 which begin very close to the insertion of the greater ocellar and 

 extend caudad a variable distance, sometimes about half way to an 

 imaginary line joining the insertions of the inner vertical bristles, 

 sometimes beyond this line to the occipital surface of the head. 



