THE BUTTERFLY. 



165 



twisted or spiral thread, which runs all through them 

 like the spiral thread which keeps the air-vessels of the 

 bean-stalk open, thus protecting the life of the vine. 

 Indeed, they look very much like the spiral vessels of 

 plants (Fig. 138). This machinery for breathing, or 



Fig. 138 



Air-Tube, with one of its Branches. 



respiration, is entirely different from that of the mus- 

 sel or lobster. There is no special place in the body 

 where all the blood comes to get air. " Instead of the 

 blood coming to the air, the air goes to the blood" 

 through the little tubes that we find spreading them- 

 selves through the whole body (Fig. 138). The tubes 

 are called trachea, or wind-pipes — trachus means 

 rough, and these rings make the air-tubes quite rough. 

 When the air is to be expelled, the creature brings the 

 walls of his body together and forces the air out. 



The circulation of the caterpillar's blood is brought 



