THE ANIMAL KINGDOM 



427 



body wall (Fig. 250). Air enters and leaves through the spira- 

 cles. In the grasshopper it has been shown that the two 

 thoracic spiracles and the first two abdominal spiracles are 

 inspiratory and that the last six pairs of abdominal spiracles are 

 expiratory. Oxygen is carried directly to the tissues by the 

 tracheal system and carbon dioxide is removed by the same 

 system. The blood plays a minor role 

 in the respiratory exchange, but since the 

 tissues are bathed in blood, the latter 

 serves as a medium through which 

 oxygen passes from the trachioles to the 

 tissues and carbon dioxide in the opposite 

 direction. 



Blood vessels, poorly developed in 

 insects, are practically absent except near 

 the heart. The heart of the grasshopper 

 is a delicate muscular tube lying directly 

 in the mid-line of the dorsal region of the 

 hemocoel. It is closed at its posterior 

 end, open at its anterior end, and per- 

 forated along its sides by apertures called 

 ostia, guarded by valves (Fig. 251). 

 Anteriorly, blood vessels extend a short 

 distance from the heart. When the heart 



,i i i ,i , i sheet lying ventral to the 



contracts, the ostia are closed so that the heart; h< arrangeme nt of 



Wave of Constriction, beginning at the fibers. {From Packard, 

 , j i r i r Textbook of Entomology, 



posterior end and passing forward, forces copyright< The Macmillan 



the blood OUt of the anterior end. As the Company, after Grober. 



heart relaxes, blood reenters the heart v P ermisslon -> 

 through the ostia from the pericardium, a space about the 

 heart, incompletely separated from the rest of the hemocoel. 

 In some insects the heart reverses the direction of its beat at 

 regular intervals, propelling the blood first in one direction and 

 then in the other. The blood distributes nourishment to the 

 tissues and collects waste material other than carbon dioxide. 

 There are two general types of metamorphosis: complete 

 (holometabolous) and incomplete (hemimetabolous) . When 

 metamorphosis is complete, the egg hatches as a worm-shaped 

 larva, such as the maggot of flies, the grub of the beetle, or the 

 caterpillar of moths and butterflies. The larval period is 



Fig. 251.— Part of 

 heart of a beetle, Dytis- 

 cus marginalis, showing 

 spiral arrangement of 

 fibers, c, closed valve; 

 e, open valve; a, muscu- 

 lar and connective tissue 



