26 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 
Ocellars (fig. 6, oc): A single pair of bristles situated on the front, just in 
front of each lateral ocellus and just posterior to the level of the median 
ocellus. They are directed outwards (are “divergent’’). 
Postverticals: A single pair of bristles, just behind and lateral to the lateral 
ocelli. They point toward each other (are ‘‘convergent’’). 
Verticals (fig. 6, ve): Two bristles on each side of the head, at the upper 
lateral corners of the front. The outer pair is divergent, the inner is 
convergent. 
Vibrisse, or oral bristles (fig. 6, vi): Long bristles at the anterior edge of 
the bucca. These are the conspicuous bristles at the anterior lateral 
corners of the oral cavity. 
ANTENN-. 
Each antenna is composed of three joints or segments. The first 
(basal) one is very short, and bears a few short hairs. The second one 
(fig. 6, A2) is somewhat longer, and bears a few hairs, of which two to 
four on the upper surface are perhaps long enough to be called bristles. 
The third (terminal) joint (fig. 6, A3) is longer than the other two 
combined, and is completely covered with small, fine hairs. It bears 
no hairs or bristles of the ordinary type. Near the base of this third 
joint arises the arista (fig. 6, ar). This is a branched, two-jointed 
structure that probably represents the distal joints of the antenna, 
present in nematocerous flies. Its basal joint is quite short and 
somewhat thicker than the long-branched terminal one. The dorsal 
branches of the latter joint arise at intervals along its entire length; 
the ventral branches are never present near its base. In addition to 
these branches, the arista bears a few short, hair-like branches on its 
inner side. 
PROBOSCIS. 
The proboscis is of the same type as that of Musca (Hewett) or of 
Calliphora (Lowne). There is a basal portion, the rostrum, arising 
from the ventral surface of the head (the oral cavity), and bearing on 
its anterior portion a palpus on each side. The rostrum is shaped 
like a truncated cone, to the apex of which is attached the haustellum. 
This part, which is roughly cylindrical in shape, is directed forward or 
downward. It bears on its posterior side a strongly chitinized plate, 
the theca. The labrum is a slender, pointed process arising at the 
angle between the rostrum and the haustellum, and usually lying flat 
on the dorsal surface of the haustellum. Attached to the apex of the 
haustellum is a pair of oral lobes, one on each side. Within each oral 
lobe is a series of about ten pseudotrachew. These are tubes that 
contain numerous chitinized rings, so that they resemble large trachee. 
They probably serve as rasps for grinding particles of food fine enough 
so that they can be ingested. The palpi are one-jointed structures, 
covered with hairs similar to those on the third antennal joint; they 
also bear, distally, several hairs of the ordinary type. The size and 
shape of the palpi and of their larger hairs vary greatly from species 
to species. 
