EXPLANATIONS OF PLATE IV. 



Fig. 1. Side view of the head of Musca vomitoria, with extended proboscis. 



Fig. 2. Longitudinal section of head of M. vomitoria, with extended proboscis. 

 sd, salivary duct, d, dorsal posterior process of the fulcrum, z, ven- 

 tral posterior process of the fulcrum. [In this and the next figure 

 the section of the frontal sac is omitted to avoid complicating the 

 figures.] 



Fig. 3. Longitudinal section of the head of M. vomitoria, with partly retracted 

 proboscis. Lettering as in fig. 2. 



Fig. 4. Section of the fulcrum of M. vomitoria. 

 Fig. 5. Pseudotrachea of M. vomitoria. 



Fig. 6.- Pseudotrachea of M. domestica; d, one of the apparent segments of 

 the pseudotrachea- 



Fig. 7. A portion of a pseudotrachea of M. domestica spread out to show 

 its real structure; toward the right it is abnormal. 



Fig. 8. Cross-section of three pseudotracheae of M. domestica to illustrate their 

 position in the surface of the labella. 



