28 ROMANCE OF THE INSECT WORLD CHAP. 



are extended outwards from the sides. The insect 

 possesses the voluntary power of moving them at 

 pleasure, and thus effects constant renewal of the 

 water in contact with the organ. They are abun- 

 dantly supplied with minute ramifications * of the 

 tracheal vessels, which are believed to convey the air 

 that the branchiae absorb into the main tubes, to be 

 distributed over the whole interior of the body, as in 

 insects that live in the open atmosphere. Branchiae 

 are of various kinds, but the most common one 

 consists of slender hair-like structures, growing 

 together in tufts, as in the young of the gnat. 



All perfect insects, whether they are inhabitants of 

 the air or water, breathe air alone. By the time that 

 this stage is reached, the branchiae, or other curious 

 adaptations for the larvae, where they were present, 

 are no longer distinguishable, and spiracles are 

 always developed at a part of the body at which the 

 branchiae were attached. After metamorphoses, the 

 water-beetle (Dytiscus), for example, which leads an 

 amphibious life, has the ordinary spiracles for res- 

 piratory purposes ; but they are situated on the 

 back, enabling the insect to breathe readily by 

 coming to the surface. 



In the larvae of aH insects, the internal respiratory 

 organs, or tracheae, are simply elongated and ramified 

 tubes. But in adults these tubes suffer certain modi- 

 fications, becoming dilated into an immense number of 

 minute vesicles, or sacs,* a development well adapted 

 to meet the exigencies of increased force and activity 

 of habit. It not only allows of most extensive respi- 

 ration, it enables the insect to alter its specific 



