11 



FLIES WITH AQUATIC LARV^ 



203 



sheath ' by Batelli ; and an inner ' respiratory ' tube, 

 which encloses the tracheae. The outer sheath is 

 much wrinkled transversely, especially when con- 

 tracted. It bears great numbers of minute hair-like 

 setse. Batelli says that it is also provided with setae 

 shaped like Roman swords, and he figures these 

 (Tav. I, Fig. 11). I have found similar objects on both 

 the outer and the channelled sheath, but by examina- 

 tion with a high power could see 

 that they were diatoms, and not 

 setai. Four pairs of tubercles bearing 

 bunches of setae project from the 

 outer sheath. There is one pair at 

 the base, where it joins the body, one 

 pair at the tip, and two intermediate 

 pairs. These are set at equal dis- 

 tances, and mark out the surface of 

 the sheath into three equal tracts. 

 The intermediate or channelled sheath 

 is exposed when the tail is com- 

 pletely protruded, but during retrac- 

 tion it disappears more or less within 

 the outer sheath. Between the furrows are regular 

 longitudinal rows of small booklets. The inner or re- 

 spiratory tube is marked with innumerable transverse 

 folds or wrinkles. Its extremity is firmer and more 

 solid than the rest ; here are the external openings of 

 the two tracheal trunks, surrounded by five plumose 

 setae, which spread out and cling to the surface-film, 

 thus keeping the tip of the tail above water ; when 

 forcibly submerged, they carry down a bubble of air. 

 Two tracheal trunks, prolongations of the great longi- 



FiG. 72. — Extremity 

 of respiratory tube 

 of larva of Eristalis. 



