140 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



forward from one chamber to another until it issues from a 

 narrow anterior extension of the vessel, called the aorta. 



In many insects the dorsal vessel is not so long and slender as 

 in the caterpillar, nor composed of as many chambers. But in 

 all insects the circulatory system comprises nothing more than 

 a pulsating dorsal vessel and the blood lymph flowing freely 

 everywhere in the body cavity. 



Nervous System. Extending along the 

 middle of the floor of the caterpillar's body 

 will be seen a delicate white thread with 

 small expansions or knots in it arranged 

 segmentally, but wanting in the last two 

 abdominal segments. In the head there 



FIG. 57. FIG. 58. 



FIG. 57. Diagram of circulatory system of a young dragon-fly; in 

 middle is the chambered dorsal vessel, or heart, with single artery. Arrows 

 indicate direction of blood-currents. (After Kolbe.) 



FIG. 58. Diagram of ventral nerve-cord of locust, Dissosteira Carolina. 

 (After Snodgrass.) 



is a knot underneath the esophagus and from it a pair of 

 stout threads which run up and around the esophagus, 

 one on each side, and into a larger knot lying on top of the 

 esophagus. 



These knots and thread compose the main part of the central 

 nervous system of the caterpillar, the threads being the nerve- 



