ANATOMY AND PHYSIOLOGY 



IOQ 



Parts of the fat-body may also be concerned in excretion; thus the 

 fat-body in Collembola and Orthoptera serves for the permanent storage 

 of urates. 



7. CIRCULATORY SYSTEM 



Insects, unlike vertebrates, have no system of closed blood-vessels, 

 but the blood wanders freely through the body 

 cavity to enter eventually the dorsal vessel, which 

 resembles a heart merely in being a propulsatory 

 organ. 



Dorsal Vessel. The dorsal vessel (Figs. 160, 

 164) is a delicate tube extending along the median 

 dorsal line immediately under the integument. 

 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. 161), which permits the ingress of blood but 

 opposes its egress; within the chambers occur 

 other valvular 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 se i O f beetle, LMOMMW. 



a, 



(FigT764) , each of which may again branch. In 



the head the blood leaves the aorta abruptly and DURCKHEIM. 



enters the general body cavity. 



Alary Muscles. Extending outward from the "heart," or pro- 

 pulsatory portion, and making with the dorsal wall of the body a pericar- 

 dial chamber, is a loose diaphragm, formed largely by paired fan-like 

 muscles the alary muscles (Figs. 160, 162). These are thought to assist 

 the heart in its propulsatory action. 



Structure of the Heart. The wall of the heart is remarkably thin, 

 and consists essentially of a muscular layer containing closely-set circular 

 or spiral fibers and separated longitudinal fibers, with scattered nucle- 

 ated cells among the fibers. This muscular tube is between two layers : 



