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 dosed 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 apxia. 



On each side of the heart, there is a series 

 of triangular muscles extending from the heart 

 to the lateral wall of the body. . These con- 

 stitute the dorsal diaphragm or the wing's of the 

 heart. They are discussed later under the 

 ff c l 13 head: Suspensoria of the Viscera. 



11 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 number 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 

 Fig. 139. Heart of a not more than eight. The blood is admitted to 



May -beetle; a, lateral the heart through slit-like openings, the ostia of 

 aspect 01 the aorta; o, . . . . 1 



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



showing valves; c, IntrrnJ wnlte r>f pa^ Hi am NT Each ostium is 



ventral aspect of the **~x*^*- ^ 



heart and wing-mus- furnished with a valve which closes it when the 



cles, the muscles are chamber contracts. 



represented as cut away . ... 



from the caudal part of The wall of the heart is composed of two dis- 



the heart; d, dorsal t i nct l av ers: an inner muscular layer ; and an 

 aspect of the heart * . -11 



(After Straus-Durck- outer, connective tissue or peritoneal layer. 



heim )- The muscular layer consists chiefly of annular 



muscles; but longitudinal fibers have also been observed. 



