INTRODUCTION. 1 5 



fine long convoluted tlireads or tubes surround the 

 lower part of the chyle-forming stomach, and pour a 

 biliary secretion into it ; the small intestine follows this 

 stomacli, then the coecum and the rectum. Near the end 

 of the intestinal tract are often found other secreting 

 organs which serve to elaborate certain fluids (as the 

 poison of the bee and wasp), which various kinds of 

 insects eject when disturbed, and which are often of an 

 intensely disagreeable odour. The intestinal canal of 

 insects varies considerably in length ; as a rule, in car- 

 nivorous and suctorial kinds it is about twice the 

 length of the body, in vegetable feeders it is very long, 

 sometimes being equal to six or eight times the length 

 of the body. 



Circulation in insects is carried on by means of a long 

 contractile tubular organ, which, from its position on 

 the back is usually called the "dorsal vessel." This 

 vessel represents the heart, which ordinarily consists of 

 eight segments or sacs, which open one into the other 

 from behind forwards, and which by contracting, drive 

 the blood collected from the body and received into the 

 heart by a series of valvular openings, forward to the 

 region of the head where it escapes apparently, for no 

 trace of arteries or veins have been discovered in 

 insects. As the blood is on its passage through the 

 viscera and other organs of the body, on its return to 

 the heart, it becomes oxygenated by contact with the 

 respiratory organs, which ramify in all directions through 

 the body. These consist of an immense number of deli- 

 cate tubes — the membranous coats of which are kept 

 distended b}^ minute spiral-formed filaments — which open 

 out on each side of the insect's body. Through these 



