12 THE INTERNAL STRUCTURE OF INSECTS 
Fig. 10.—Spiracles of a grasshopper. 
Enlarged. Original. 
higher animals, whose 
blood circulates in ar- 
teries, veins, and capil- 
laries. In insects the 
blood flows freely around 
the internal organs and 
through the tissues. 
There is a heart, how- 
ever, which keeps the 
blood moving. It is an 
elongated structure, situ- 
ated just beneath the 
upper surface of the in- 
sect’s body, and consists 
of a series of chambers, 
each with valves opening 
from the body cavity 
into the chamber, and 
with another valve open- 
ing into the next chamber 
toward the front. The 
trachez are extremely thin walled, 
and the oxygen that they contain 
is thus brought to the various 
tissues. Air circulates slowly in 
the trachee. The openings or 
spiracles are guarded by various 
devices, such as a fringe of hairs. 
The Circulatory System 
The entire body cavity of an 
insect is bathed in a yellowish or 
greenish fluid that we speak of as 
its blood. There is no closed 
system of blood vessels, as in the 
Vic. 11.—Aninsect’s heart. (Diagrammatic.) 
Original. 
