Chap. 30 



ARTHROPODS INSECTS, SPIDERS, AND ALLIES 



607 



Alimentary 

 cana 



rtition 



Ner\/e 



THORAX 



ABDOMEN 



Fig. 30.15. Upper, diagram of the circulatory system of an insect. B, cross 

 section of the thorax of the same. C, cross section of the abdomen. Arrows indi- 

 cate the course of the circulation. The blood flows forward through the heart, a 

 tube extending along the middle of the back. It pours out of the open front end 

 of this and turns backward flowing through open spaces (sinuses) above and 

 below the digestive tube. As it does this some of it turns toward the back and 

 enters the heart through small openings. Some turns out into the legs and wings 

 where it bathes the tissues directly. 



It also transports hormones. Pressure upon the blood in one or another part of 

 the body is a part of the mechanics of molting and of moving the air in the 

 tracheae during breathing. 



Circulation of the Blood. The only blood vessel is the heart, a tubelike suc- 

 cession of connecting chambers extending along the mid-dorsal line of the body, 

 the ?ieart proper in the abdomen, the aorta in the thorax (Fig. 30.15). Peri- 

 staltic contractions move in waves over the tube from rear to front. In many 

 species, the movement is reversed in one or another phase of life, and the 

 blood flows backward. As each chamber dilates, blood is sucked into the heart 

 through slitlike openings along the sides. These close as a wave of contraction 

 passes them and pushes the blood before it. At the open end of the aorta it 

 floods out into an open space about the brain, circulates within the head and 

 turns backward through the spaces (hemocoels) surrounding the internal 

 organs, much of it passing into the wings and legs. Minute contractile pumps 

 in the thorax draw it through the wings and legs. In the wings it passes out- 

 ward beside the veins of the front part of the wing and inward again to the 

 body beside other veins as it does in cockroaches (Fig. 30.13). With a micro- 

 scope circulating blood can be clearly seen in the flattened legs of certain 



