Chap. 30 



ARTHROPODS INSECTS, SPIDERS, AND ALLIES 



609 



Body wall 



External 

 opening 



or 

 spiracle 



Fig. 30.17. A trachea, with its external opening, branches, and the tracheoles 

 associated with muscle cells where the exchange of oxygen and carbon dioxide 

 mainly occurs. (Courtesy, Ross: Entomology. New York, John Wiley and Sons, 

 1948.) 



system is the maintenance of a good environment in the body, mainly by the 

 eUmination of unneeded substances from the blood. 



The kidneys of the grasshopper are the thread-sized Malpighian tubes 

 named after Marcello Malpighi (1628-1694), an Italian anatomist, who first 

 described them in the silkworm. In the grasshopper, each one extends through 

 the blood from its opening in the intestine to its free end, a blind pocket (Fig. 

 30.14). Metabolic wastes, destined to form uric acid, are diffused from the 

 body cells into the blood. The walls of the Malpighian tubes gradually absorb 

 the uric acid, discharge it in a watery solution into the tubes which in turn 

 empty it into the intestine from whence excess water is absorbed back into the 

 blood through the rectal wall. This is in hne with the small animal's usual 

 economy of water. 



Metabolism. Whether it is a grasshopper or a palm tree, the living or- 

 ganism is a result of chemical and physical reactions of which metabolism is 

 the sum total. Digestion, respiration, excretion, and other processes are parts 

 of metabolism. Grasshoppers become more active as surrounding temperatures 

 rise. With increased activity their bodily temperature and the rate of metabo- 

 lism also rise. Chemical reactions are increased. Heat is produced, and energy 

 is liberated. When grasshoppers are warm they jump, fly, and eat more. 



Chemical Regulation — Hormones. The hormoneUke substances in in- 

 sects are briefly discussed with the endocrines (Chap. 15). One endocrine 

 gland, the corpus allatum, is mentioned here because its endocrine nature was 

 established largely by experiments on grasshoppers. It is a double body near 

 the brain, often taken as two glands. During the growth of young grasshoppers 

 its secretion, the "juvenile hormone," checks the differentiation of adult char- 

 acters and stimulates the retention of nymphal ones. It gives the nymphs time 



