ANATOMY AND PHYSIOLOGY rs 
Or 
g. RESPIRATORY SYSTEM 
In insects, as contrasted with vertebrates, the air itself is 
conveyed to the remotest tissues by means of an elaborate sys- 
tem of branching air-tubes, or trachee, which receive air 
through paired segmentally-arranged spiracles. ach spiracle 
is commonly the mouth of a short tube which opens into a 
maim tracheal trunk (Fig. 167) extending along the side of 
Fic. 168. 
Diagrammatic cross section of the thorax of an insect. a, alimentary canal; d, 
dorsal vessel; g, ganglion; s, spiracle; w, wing; rz, dorsal tracheal branch; 2, visceral 
branch; 3, ventral branch. 
the body. From the two main trunks branches are sent which 
divide and subdivide until they become extremely delicate 
tubes, which penetrate even between muscle fibers, between the 
ommatidia of the compound eyes and possibly enter cells. In 
most cases each main longitudinal trunk gives off in each seg- 
ment (Fig..168) three large branches: (1) an upper, or dor- 
sal, branch, which goes to the dorsal muscles; (2) a middle, 
or visceral, branch, which supplies the alimentary tract and the 
reproductive organs; (3) a lower, or ventral, branch, which 
pertains to the ventral ganglia and muscles. 
In many swiftly-flying insects (dragon flies, beetles, moths, 
flies and bees) there occur tracheal pockets, or air-sacs, which 
