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 land 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, fyc.}, 

 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 cr 

 rounded 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, cilice, 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 marginalis, (PI. III. 



