INSECTA 



121 



of kidneys. There is no liver. The entire viscera are en- 

 veloped in the " fat body." The anal opening is in the last 

 segment of the abdomen. 



Insects feed upon the juices, leaves, or even the wood of 

 plants, or are parasitic or predaceous upon various forms of 

 insects, and upon other animals as well. Some live upon de- 

 caying organic matter. 



Fig. 94. Ideal transverse section of an insect: h, Dorsal vessel; i, 

 intestine; n, ventral nerve-cord; t, t, stigmata leading into the branched 

 tracheal tubes; w, w, wings; a, coxa of one leg; b, trochanter; c, femur; 

 d, tibia; e, tarsus. (After Packard.) 



The circulatory organs are extremely primitive in character. 

 The heart or dorsal vessel extends through the abdomen just 

 underneath the dorsal surface. It is partially divided by valves 

 into chambers, the number of which varies. The anterior 

 chamber extends into or near the head and is sometimes called 

 the aorta. The heart chambers pulsate rhythmically, from the 

 posterior one forward, and force the blood out into the body 

 cavity. There are no veins or arteries, so it flows through the 

 sinuses or open spaces between the organs, bathing the tissues, 

 and finally bathing the walls of the alimentary tube, where it 



