ANATOMY. 25 
narrow ring surrounding each eye, known as the orbit. This bears the 
orbital bristles. That part of it bordering the lower portion of the 
eye is sometimes known as the gena. The portion between the upper 
parts of the orbits, above the bases of the antenne and below the 
vertex, is known as the front. There is a furrow at its lower edge. 
Below this furrow, between the anterior portions of the orbits, and 
above the anterior edge of the oral cavity, lies the face. This is usually 
elevated to form a more or less nose-like carina (fig. 6, c), situated 
between the antennze. Below the face, and separated from it by a 
furrow, is a narrow plate known as the clypeus, which forms the 
anterior edge of the oral cavity. The posterior surface of the head is 
called the occiput. This part extends somewhat forward on the 
lower lateral part of the head, to form the posterior portion of the 
“cheek.” In this region it is hairy, while the gena is bare. This 
peculiarity will serve to distinguish the two plates. The bucca is a 
small plate bounded anteriorly by the face, above by the gena, pos- 
teriorly by the occiput, and below by the oral cavity. It bears the 
vibrisse or oral bristles at its anterior end, and has a row of bristles 
or hairs along its lower edge. 
Fig. 6.—Head of Drosophila robusta. A», 
second antennal segment; A3, third 
antennal segment; ar, arista; C, 
carina; EH, eye; N, neck; oc, ocellar 
bristle; or1, or2, ors, orbital bristles; 
Pr, proboscis; ve, vertical bristles; 
vi, vibrissa or first oral bristle. 

In the systematic descriptions reference is frequently made to the 
“greatest width of the cheek.” This usually means the length of.a 
line drawn from the lower hind part of the eye to the lower hind corner 
of the head. Such a line crosses the gena and the occiput. In judging 
the length of this line it is important that the head be held in a hori- 
zontal plane, as the proportions given were all observed when the 
head was in such a position. 
The head bristles of taxonomic value are the following: 
Orbitals, or fronto-orbitals (fig. 6, or): Situated on the orbits, between the 
vertex and the level of the bases of the antenne. They are usually three in 
number: (1) an uppermost one, directed toward the vertex, or ‘‘reclinate”’; 
(2) a middle one, also reclinate and usually smaller than the other two; (3) a 
lowermost one, directed toward the oral cavity, or ‘‘proclinate.’”’ The 
third one is situated a little farther from the eye than are the other two. 
