THE ANATOMY OF INSECTS. 33 



In many insects, instead of a rapid alternation of contraction and 

 expansion of the chambers, the heart begins to contract at its caudal 

 end, and a wave of contraction passes towards the head along the 

 entire length of the organ ; frequently one wave will pass the entire 

 length of the heart before another begins. 



The cephalic prolongation of the heart, which extends through 

 the thorax and into the head, is a simple tube, the aorta (Plate I., 



Fig. 40, c and a). The aorta ends in the head, near the brain, 

 where it is usually somewhat branched. The branches are very 

 short, and the blood passes from them directly into the body-cavity. 

 Here it bathes the viscera, receiving the products of digestion from 

 the alimentary canal, giving up to the various glands their secretions, 

 and carrying nourishment to all parts of the body. In its course 

 through the body the blood flows in regular channels, without walls, 

 like the currents of the ocean. 



The blood is usually colorless, or slightly tinged with green ; but 

 its circulation is made conspicuous by the movements of the large 

 corpuscles with which it abounds. In transparent insects it can be 

 seen pouring forth from the cephalic end of the aorta, bathing first 

 the brain, and then passing to all parts of the body, even out into 

 the appendages. By tracing the course of any one of these currents 

 it will be found to flow sooner or later to the sinus in which the 

 hear* nd from which it receives its blood. 



The Ptricdrdial Sinus, to which reference has just been made, is 

 separated from the general cavity of the body by a membrane, the 

 fericardial diapJiragin. This diaphragm is perforated by many open- 

 ings, through which the blood passes on its return to the h< 

 Into the diaphragm there are inserted a double series of triangular 

 musclo (Plate I.. 6; and Fig. 40, c). These meet on the mid- 

 dle line, and are attached by their smaller ends to the lateral walls 

 of the body. They were formerly supposed to be attached to the 

 ind to aid in the expansion of that organ ; they 

 . therefore . named by Lyonet thi ^f the / 



musdefl to tin- Ix.ut jfl shown in 

 4^. which . cross-section of the body. \j^. 



In this :i- r< the position of the wings of tin- heart. F|G 43 ._Dia. 



I do not think that tin- use of these muscles has yet been ifn^" 1 

 fully determined. One function, though probabl; 



>rdinate one, is doubtless to protect the heart from pi 

 On :ch the ; ic movements of the alimrir 



canal in a transparent larva to appro importance of this. 



