20 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



ganglia, small masses of nerve cells and tissue, which are con- 

 nected with each other by a pair of white nerve cords. The 

 largest of these ganglia is situated in the head above the esoph- 

 agus and is called the brain. Nerve cords which unite this 

 ganglion with one below the esophagus pass on either side of 

 the esophagus. From this ganglion a pair of longitudinal cords, 

 very close together, pass backward along the floor of the body. 

 At intervals along these cords are ganglia from which fine 

 branching nerve fibers run to all parts of the body. 



The circulatory system consists of an elongate, thin-walled 

 dorsal vessel called the heart situated just under the dorsal 

 wall of the abdomen. It is not easily distinguished except in 

 fresh specimens. Its structure will be described in a later 

 chapter where the internal anatomy of a caterpillar is discussed. 

 (See Chapter XVI.) There are no arteries or veins. The 

 colorless blood of the insect fills all the space of the body-cavity 

 not occupied by organs and other tissues, and is in an enclosed 

 vessel only while passing through the heart. 



