38 - 



margin, more sinuate than it is in Eristalis, is indicated by the dotted line dz. 

 Its posterior ventral horns here project backward on each side of the oesophagus. 

 In Musca the fulcrum is not as much compressed laterally as it is in Eristalis, 

 as can be seen by comparing its sections in the two. (Compare pi. 3, fig. 1, 

 ', d' fpf, and pi. 4, fig. 4.) In both Musca and Eristalis the fulcrum contains 

 the pharynx and pharyngeal muscles, but in Musca the latter are divided into 

 two portions (pi. 4, fig. 4, pm~). Another peculiarity of the fulcrum in Musca, a 

 peculiarity not very evident in Eristalis, becomes very marked; that is, that its 

 walls fail on the upper side of its anterior extremity, so that a cross-section of 

 the fulcrum, as seen in pi. 4, fig. 1, /*', ///, is nearly U-shaped, and the upper 

 opening is closed over by the elastic membrane (<) which forms the outer walls 

 of that portion of the proboscis. In Eristalis the upper wall is really absent 

 from the anterior end of the fulcrum, altho its absence is not as marked as it 

 is in Musca. In pi. 3, fig, 1, <5', the chitinous portions and the muscles directly 

 around the pharynx (/>) are parts of the extreme anterior end of the fulcrum 

 and its contents, and are without upper covering-walls. The above-mentioned 

 absence of the upper wall on the anterior end of the fulcrum of Eristalis 

 and of Musca allows, when the proboscis is retracted, the labrum to sink its 

 convexity into the flexible membrane, which bridges from one side of the fulcrum 

 to the other, in the basal portion of the proboscis. Cross-sections of' the 

 fulcrum (pi. 3, fig. 1, e', pfd'p, and pi. 4, fig. 4) show that its ventral part 

 consists of two plates of chitin, the outer plate firmer than the inner, united 

 above on each side, that is, the inner plate is sprung into the concavity of the 

 outer one, so that, at rest, on account of the elasticity of the inner plate, but 

 little space remains between the plates ; this space is the pharynx. Upon the 

 upper surface of the chitiu plate which forms the upper wall of the pharynx are 

 inserted the pharyngeal muscles, their origin being in the dorsal portion of the 

 fulcrum. By their contraction the pharyngeal muscles enlarge the lumen of the 

 pharynx, which is again diminished, upon their relaxation, by the elasticity of 

 its upper wall. 



The function of the fulcrum is, in the first place and most essentially, to 

 furnish the frame-work of a suctorial organ, of which the motive power is the 

 enclosed pharyngeal muscles, as dilators of the pharynx, opposed by the elasticity 

 of the upper pharyngeal wall, as contractor of the pharynx. The function of 

 suction was attributed by Keaumur 12 to the proboscis of Musca; he regarded 

 the "trunk as a sort of suction-pump;"* but Gleichen 22 was the first to locate 



* "... trompe corame une sorte de pompe aspirante." (p. 268.) 



