156 
ANATOMY AND PHYSIOLOGY 
under two heads, viz. the external organs by which 
the air is admitted into the body and expelled from 
it, and the internal ones by which it is distributed. 
The former differ according as the place inhabited 
is laud or water ; in other words, according as the 
air is received directly, or through the medium of 
another element. 
Nearly all insects inhabiting the land and the air, 
as well as the amphibious species ( Dytiscidce 3 §c.), 
receive their supply immediately from the atmosphere 
which surrounds them on all sides, and it is admitted 
by apertures named spiracles or stigmata. These 
are small perforations, commonly of an oval or 
founded form, placed along the lateral margins of 
the body. In number, and almost every other re- 
spect, they vary greatly, but their arrangement is 
always symmetrical, one on the right side, and 
another on the left ; each segment, for the most part, 
being thus furnished with a pair. (PI. III. fig. 3.) 
They never exist in the head, and there are never 
more than two pair in the thorax, consequently the 
greater number are to be found in the abdomen. 
They are usually surrounded by a horny ring, and 
their aperture can be closed at the will of the insect 
by means of a muscular apparatus. To enable it to 
do this more effectually, the mouth is sometimes 
furnished with plates which close like shutters, or it 
is fringed with hairs, cilise, &c. Two of their most 
simple forms are represented on Plate III. figs. 4 & 5. 
A more complicated structure is exhibited by the 
hinder stigmata of Dytiscus marginalisj (PI. III. 
