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, antenne, and 
mouth parts. In the thorax 
there are three more ganglia, 
Fic. 13.— Gizzard of a cricket, showing 
mouscles and grinding surfaces. Enlarged. 
Original. 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. 
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 stored up in them, especially 
