II FLIES WITH AQUATIC LARV^ 201 



the depth of another half inch, when the tails were 

 lengthened to the same amount. Again and again 

 water was added, and the tails became lengthened in 

 proportion. When however the depth grew to five 

 and a half or six inches, the larvai could no longer 

 reach the surface from the bottom. Some crept up 

 the side of the vessel, while others floated up to such 

 a height that they could maintain their communi- 

 cation with the air. 



The organ, by means of which the larva is enabled 

 to breathe when its body is several inches under 

 water, deserves close examination. So transparent is 

 the tail, and indeed the rest of the body, especially in 

 young larvai, that the internal parts can be studied 

 almost as well as if their integument were a tube 

 of glass. It is easy to see that the tail consists of 

 two tubes, one sliding within the other. The outer 

 and wider tube looks like a prolongation of the body- 

 wall ; it is thrown when at rest into innumerable 

 transverse wrinkles. Within it is a second tube, a 

 considerable part of which is brown or nearly black. 

 The smaller tube is the special respiratory organ. 

 The tail can be lengthened by protruding this inner 

 tube from the outer one. But besides this mode of 

 adjustment, the outer tube is capable of elongation 

 and contraction, being furnished with innumerable 

 annular fibres and increasing in length as it diminishes 

 in diameter. When most extended it is no thicker 

 than a stout thread, and the inner tube resembles 

 a horse-hair. 



Mr. J. J. Wilkinson furnishes me with the following 

 additional information respecting the tail of the 



