CHAPTER III 
Tue INTERNAL STRUCTURE OF INSECTS 
How Insects Breathe 
ALL insects, even those living in water, need air. But their method 
of obtaining it is entirely different from that developed in higher 
animals. No insect has 
nostrils, or any opening in 
its head through which it 
breathes. Instead, there is 
a row of small apertures, 
called spiracles, down each 
side of its body, one on 
each segment, beginning 
with the second or third 
segment of the thorax and 
extending back along the 
rings of theabdomen. The 
spiracles of each side open 
into an air tube running 
lengthwise of the insect, 
just within the body wall. 
From these main tubes 
smaller tubes diverge, and 
these in turn branch and re- 
branch, growing constantly 
smaller, until the finer tubes fy¢. 9.—Tracheal system of an insect. (Dia- 
permeate every part of the grammatic.) Original. 
insect, even to the tips of 
the antenne and the joints of the feet. The tubes are known as 
traches and the entire group as the tracheal system. The smaller 
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