38 
ANIMAL ACTIVITIES. 
that we often speak of the “ Breath of Life ”. If we 
cover the holes along the sides of the grasshopper s 
body, so that no air can enter, he dies, just as we 
should die if deprived of air. Throughout animal life 
this same necessity for air exists. 
The air used by the grasshopper for breathing pur¬ 
poses enters the body through little holes along the 
sides of its abdo¬ 
men and thorax. 
Eight of these 
openings may be 
easily seen on 
Fig. 40.— The Trachea of an Insect ( magni - each side of the 
fi ed Y abdomen, and 
two others o n 
each side of the thorax. These holes, or spiracles, 
open into air-tubes, called tracheae, which divide and 
subdivide in order to send branches to every part of 
the body, even into the wings. These tubes and their 
branches are surrounded by blood-vessels through 
which blood is constantly coursing. The oxygen of 
the air filters through the walls of the tracheae into 
the blood, and the carbon dioxide, water, and other 
waste substances in the blood pass through the same 
walls in the opposite direction. Such an interchange 
of gases through a membrane is called osmosis. Not 
only do these breathing-tubes carry oxygen to the 
blood and remove the waste products of respiration, 
but they also render the body very light, enabling the 
insect to rise easily in the air. In addition to excreting 
waste substances by breathing, the grasshopper pours 
urea into the food-tube and thence out of the body. 
Reproduction. But animals or plants never eat 
enough to make them grow or live forever. In many 
common plants a single cell, called an ovum, is set 
apart and fertilized by union with another cell to form 
a seed, which, under proper conditions, reproduces the 
plant. In the grasshopper the egg corresponds to the 
