14 



THE INTERNAL STRUCTURE OF INSECTS 



tion takes place. Thence it enters the intestine, where it is still 

 further digested. The waste is expelled from the hind end of the body. 

 Opening into the intestine near the stomach are tubes that prob- 

 ably serve as kidneys. They are known as Malpighian tubes. 



The Nervous System 



Most insects are extremely active creatures, and have a well-de- 

 veloped nervous system. 



Taking a simple type, we find that a pair of nerve cords begin in 



the upper part of the head, 

 encircle the esophagus, one 

 on each side, again come 

 close together or unite, and 

 extend back to the hind ex- 

 tremity of the body, lying 

 just above the lower body 

 wall throughout. 



In the upper part of the 

 head and in the lower part 

 are enlargements, called 

 ganglia, from which are 

 given off branch nerves to 

 the eyes, antennae, and 

 mouth parts. In the thorax 

 there are three more ganglia, 

 one for each segment, though 

 these may be more or less 

 united. In the abdomen are further ganglia, often somewhat concen- 

 trated toward the front end of the abdomen. Many branch nerves 

 arise from the thoracic and abdominal ganglia. 



Fig. 13. — Gizzard of a cricket, showing 

 muscles and grinding surfaces. Enlarged. 

 Original. 



The Fat Bodies 



Within the body cavity are many irregular masses of peculiar fatty 

 tissue. The functions of these masses are not fully understood. It 

 is known, however, that reserve food is stgred up in them, especially 



