210 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



-airsccC/ 



.tracheal tube 



spiracle^ 1— 



ing vibrations of the so-called ventral diaphragm, a sheet of thin 

 tissue which is stretched across the ventral part of the abdominal 

 cavity, while on the dorsal side it is pumped by the heart toward the 

 anterior end of the body. The blood, which bathes all the tissues, 

 consists of a plasma, and colorless blood corpuscles or leucocytes. 

 The plasma is rich in food substances, but there are no oxygen- 

 carrying substances in it, so that insect blood only carries foods and 

 wastes. 



Oxygen is brought directly to the tissues by a very efficient type 

 of respiratory organ, the so-called tracheae and their branches. Along 

 the sides of the thorax and abdomen of insects are found paired 

 openings called spiracles. In the worker bee there are ten pairs of 



these openings, three pairs 

 in the thorax and seven 

 in the abdomen. The 

 spiracle is an oval open- 

 ing which can be opened 

 and closed by means of 

 a flat plate attached to 

 its rim. Each spiracle 

 leads into a tracheal tube, 

 the wall of which is 

 strengthened by a spiral 

 thread of chitin, thus 

 keeping the tube filled 

 with air. These tubes 

 branch again and again 

 until they finally end in 

 tiny tubules between the 

 body cells. Expansion 

 and contraction of the 

 muscles of the body wall 

 force air in and out 

 through the tracheae, thus securing circulation of oxygen to all body 

 cells. In addition to the tracheae, large air sacs are developed in 

 the thorax and abdomen, as are seen in the above diagram. Since 

 insects that fly rapidly usually have better developed air sacs than 

 those that are sedentary, it is evident that the air sacs must serve 

 to "lighten the load" of the body in its flight as a heavier-than-air 

 machine. 



Spiracle 3.. 



spiracle 4....^ 

 Spiracle 5... 

 Spiracle 6- 

 spiracle 7- 



-i|K — anrsac 



-,-tube5 join 

 ■ dorsal sclcs 



If. .commissure 



A portion of the tracheal system of the worker 

 bee. The dorsal trachea and air sacs have been 

 removed. Three spiracles are not shown. What 

 advantages are there in having this type of re- 

 spiratory system? (After Snodgrass.) 



