126 



ENTOMOLOGY 



FIG. 162. 



within the chambers occur other valvular folds that allow the 

 blood to move forward only. With few exceptions (Ephe- 

 meridse) the dorsal vessel is blind behind and the blood can 

 enter it only through the lateral ostia. 



Aorta. The posterior, or 

 pulsating portion (heart) of 

 the dorsal vessel is confined 

 for the most part to the abdo- 

 men; the anterior portion, or 

 aorta, extends as a simple 

 attenuated tube through the 

 thorax and into the head, 

 where it passes under the 

 brain and usually divides into 

 two branches (Fig. 162), 

 each of which may again 

 branch. In the head the 

 blood leaves the aorta ab- 

 ruptly and enters the general 

 body cavity. 



Alary Muscles. Extend- 

 ing outward from the " heart," 

 or propulsatory portion, and 

 making with the dorsal wall 

 of the body a pericardial 

 chamber, is a loose diaphragm, 



formed largely by paired fan-like muscles the alary muscles 

 (Figs. 158, 1 60). These are thought to assist the heart in its 

 propulsatory action. 



Structure of the Heart. The dorsal vessel has a delicate 

 lining-membrane, or intima, and a thin enveloping membrane ; 

 between these, in the heart, is a layer of fine muscle fibers, cir- 

 cular or spiral in direction, which effect the contractions of the 

 organ. 



Ventral Sinus. In many if not most insects a pulsatory 

 septum (Fig. 177, v) extends across the floor of the body cav- 



Diagram to indicate the course of the 

 blood in the nymph of a dragon fly, 

 Epitheca. a, aorta; h, heart; the arrows 

 show directions taken by currents of 

 blood. After KOLBE. 



