100 



ENTOMOLOGY 



ment. A simple tube in some larvae, it consists in most adults chiefly 



of a series of chambers, each of which has on each 

 side a valvular opening, or ostium (Fig. 159), 

 which permits the ingress of blood but opposes 

 its egress; within the chambers occur other val- 

 vular folds that allow the blood to move forward 

 only. With few exceptions (Ephemeridae) 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 abdomen; 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 abruptly and enters the general body cavity. 

 Alary Muscles. Extending 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 mem- 



FIG. 158. Dorsal ves- 

 sel of beetle, Lucanus. a, 

 aorta; a/, alary muscle; o, 

 ostium. After STRAUS- 

 DURCKHEIM. 



FIG. 159. Diagram of a portion of the 

 heart of a dragon fly nymph, Epitheca. 

 o, ostium; v, valve; the arrows indicate 

 the course of the blood. After KOLBE. 



FIG. 160. Diagrammatic cross-section of 

 pericardial region of a grasshopper, (Edipoda. 

 a, alary muscle; d, dorsal vessel; s, suspensory 

 muscles; sp, septum. After GRABER. 



