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. 



VibrissoB, 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/E. 



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, .42) 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 haustdlum. 

 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 lahrum 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 pseudoiracheoB. These are tubes that 

 contam numerous chitinized rings, so that they resemble large tracheae. 

 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 v^-ry greatly from species 

 to species. 



