INTRODUCTION. 
15 
fine long convoluted threads or tubes surround the 
lower part of the chyle-forming stomach, and pour a 
biliary secretion into it; the small intestine follows this 
stomach, 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 by minute spiral-formed filaments—which open 
out on each side of the insects body. Through these 
