14 THE EXTERNAL STRUCTURE OF MUSCA DOMESTICA 



The Haustellum. This forms the distal p()rti(jn of the pro- 

 boscis and is attached to the rostrum. Its distal portion, which 

 comprises the oral lobes, will be described separately. The pos- 

 terior side of the proximal portion is formed by a strongly convex 

 heart-shaped sclerite, the theca (figs. 3 and 29), which morpho- 

 logically represents a portion of the labium. The lower angle of 

 the theca is incised by a semicircular sinus. By means of this the 

 theca rests on a triradiate continuous sclerite, the furca, which 

 consists of a median, slightly convex rod (fig. 3, /), from the 

 anterior end of which two arms diverge and form the chief skeletal 

 structures of the oral lobes. The lower end of the theca rides on 

 the structure, the bottom of the sinus resting on the median 

 rod, and the two pointed lateral terminations of the theca rest 

 on the arms. In this manner these processes, in a state of 

 repose, keep the arms of the furca closely approximated. The 

 result of this arrangement will be seen later in studying the 

 musculature of the proboscis. 



The sides of the haustellum are membranous. On its anterior 

 face, in a gi'oove formed by the overlapping membranous sides, lie 

 the labrum-epipharynx and labium-hypopharynx. The labrum- 

 epipharynx (fig. 3, l.ep.) is attached at its proximal end to the 

 membranous rostrum, but is incapable of a labral-like movement 

 on account of its close connection with the labium-hypopharynx. 

 Two slightly-curved, hammer-shaped apodemes (fig. 3, ap.) are 

 attached to the proximal end of the labrum-epipharynx. They 

 assist in folding the proboscis during retraction, as will be sho^^^l 

 later. The labrum-epipharynx is shaped like a blunt arrow-head ; 

 the external surface is somewhat flattened. It is composed of 

 two pairs of sclerites, an outer pair enclosing an inner pan-, which 

 form the pharyngeal channel. The edges of the inner tube are 

 comiected by a gi'oove with the hypopharjmgeal portion of the 

 labium-hypopharynx, as shown. The labium-hypopharynx (fig. 3, 

 IJip.) represents the fusion of the hypopharynx with the gTeatly 

 modified and fused second maxillae or labium. It consists of a 

 sclerite, curved in section, having the chitinous hypopharyngeal 

 tube (fig. 29, Jip.) fused to it along the upper half of its length. 

 The edges of the hypopharpigeal tube engage with those of the 

 inner pair of sclerites of the labrum-epipharynx, as mentioned 



