208 NATURAL HISTORY OF AQUATIC INSECTS CH. 



mentioned. Both chambers are subdivisions of one 

 cavity, and are only distinguished here for con- 

 venience of description. 



The partition or floor between the two chambers 

 is formed by nine ribs, which are long, narrow and 

 deep, resembling planks set at equal distances, upon 

 their edges. They are attached beneath to the wail 



FIG. 76. Cross-section of pharynx of Eristalis larva, showing below the fringes of 

 barbules, some open and some closed ; above these, the upper chamber ; and 

 towards the top of the figure the muscles in paired masses, with tracheal tubes. 



of the lower chamber, but their upper edges are free, 

 and carry fringes of numerous and close-set barbules. 

 The outermost rib on each side bears only one fringe, 

 which springs from the inner and upper border of the 

 rib ; each of the remaining seven ribs has a pair of 

 fringes, and in cross-section resembles the letter Y, 

 the two arms being the fringes, and the stem the 

 supporting rib. 



