THE INTERNAL ANATOMY OF INSECTS 



121 



V. THE CIRCULATORY SYSTEM 



The general features of the circulatory system. — In insects the cir- 

 culatory system is not a closed one, the blood flowing in vessels during 

 only a part of its course. The greater part of the circulation of this 

 fluid takes place in the cavities of the body and of its appendages, 

 where it fills the space not occupied by the internal organs. 



Almost the only blood-vessel that exists in insects lies just beneath 

 the body-wall, above the alimentary canal (Fig. 105, h). It extends 

 from near the caudal end of the abdomen through the thorax into the 

 head. That part of it that lies in the abdomen is the heart; the more 

 slender portion, which traverses the thorax and extends into the head 

 is the aorta. 



On each side of the heart, there is a series 

 L %J^ of triangular muscles extending from the heart 



to the lateral wall of the body. These con- 

 stitute the dorsal diaphragm or the wings of the 

 heart. They are discussed later under the 

 head : Suspensoria of the Viscera. 



The heart. — The heart is a tube, which is 

 usually closed at its posterior end; at its 

 anterior end it is continuous with the aorta. 

 The heart is divided by constrictions into 

 chambers which are separated by valves (Fig. 

 139). The nimiber of these chambers varies 

 greatly in different insects; in some, as in 

 Phasma and in the larva of Corethra, there is 

 only one, in others, as in the cockroach, there 

 are as many as thirteen, but usually there are 

 not more than eight. The blood is admitted to 

 the heart through sKt-like openings, the ostia of 

 the heart; usually there is a pair of ostia in the 

 lateral walls of each chamber. Each ostitmi is 

 furnished with a valve which closes it when the 

 chamber contracts. 



The wall of the heart is composed of two dis- 



Fig. 139. — Heart of a 

 May -beetle; a, lateral 

 aspect of the aorta; b, 

 interior of the heart 

 showing valves; c, 

 ventral aspect of the 

 heart and wing-mus- 

 cles, the muscles are 



represented as cut away 

 from the caudal part of 

 the heart; d, dorsal tinct layers: an inner muscular layer ; and an 



outer, connective tissue or peritoneal layer. 



The muscular layer consists chiefly of annular 

 muscles; but longitudinal fibers have also been observed. 



aspect of the 

 (After Straus-Durck- 

 heim). 



