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 antennse 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 /ace. This is usually 

 elevated to form a more or less nose-like carina (fig. 6, c), situated 

 between the antennse. 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 

 vibrissse or oral bristles at its anterior end, and has a row of bristles 

 or hairs along its lower edge. 



-— N 



Fig. 6. — Head of Drosophila robusta. Ao, 

 second antennal segment; As, third 

 antennal segment; ar, arista; C, 

 carina; E, eye; N, neck; oc, ocellar 

 bristle; ori, ori, or 3, orbital bristles; 

 Pr, proboscis; ve, vertical bristles; 

 vi, vibrissa or first oral bristle. 



In the systematic descriptions reference is frequentlj^ 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 antennae. 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. 



