STRUCTURE OF INSECTS 43 



and rectum, and connected with tliis is the Malpighian 

 tubules which are excretory in function. 



The circulatory system is simple; it consists of a delicate 

 tube near the dorsal wall, and in this the blood current is 

 carried forward and blood circulated freely through the 

 various tissues. 



The respiratory system of insects is very different from that 

 of the vertebrates and in fact is of a type that occurs only 

 in part of the arthropods. It consists of a great number of 

 minute tubes, tracheae, which are distributed throughout the 

 tissues, so that the air contained in the tubes may be 

 brought in contact with tissues in all parts of the body. 

 Externally these tracheae open through the spiracles which 

 have been noted as located on the thorax and abdomen. 

 The tracheae arising from the abdominal spiracles in most 

 insects unite each side with a longitudinal tube running 

 through the abdomen into the thorax, and from this lon- 

 gitudinal one numerous smaller tracheae are given out, and 

 these in turn divide into smaller branches until they ter- 

 minate in minute parts called tracheoles, which are so deli- 

 cate that the air contained in the tubes is readily absorbed 

 into the surrounding tissues. The movements of respira- 

 tion are fairly rhythmetical in expansions and contractions 

 of the body, expecially of the abdomen, serving to force 

 the air in and out of the spiracles. Minute valves in the 

 spiracular openings permit air to enter and closely hold the 

 contained air, so that further contraction of the muscles 

 serves to force the new supply of air out into the minute 

 tracheoles. It is very evident from the structure of the 

 respiratory system that contact insecticides which serve 

 to close the spiracles or which may penetrate along the 

 trachea and be absorbed into the tissues must serve very 

 effectively for the destruction of the insect. It is for this 

 reason that contact poisons, and especially oily substances, 

 such as kerosene emulsion, are so efficient in the control of 

 suctorial insects. 



The nervous system of the insect consists of a ladder-like 

 arrangement of ganglia and fibers along the ventral wall 



