216 



NATURAL HISTORV. 



[CH. XII 



that the abdomen contains a moveable piston, and 

 this piston is composed of the air tubes. There are 

 four of these longitudinal trunks, although two only 



are represented, they terminate in innumerable 

 smaller ones, and, according to Reaumur, perform 

 the functions of respiration, as well as locomotion, 

 in. the ways detailed. 



After the voracious creature has lain in ambus- 

 cade, devouring the larvae of the gnat and other 

 aquatic insects, till its appointed hour of change, it 

 leaves its natal element for the shore, to undergo its 

 last metamorphosis: for this purpose it usually 

 fastens itself to some friendly plant, and begins the 

 important process which is to convert an aquatic ani- 

 mal into an inhabitant of the air. 



Any person who should at this period choose to 

 seize a number of them, and, taking them into his 

 chamber, fix them to a bit of tapestry, would be re- 

 warded for his trouble by witnessing the conversion 

 Of an aquatic into an aerial insect. 



It may easily be seen by the eyes of the nymph 

 whether it is about to change its form; for, instead 



