100 CRABS AND INSECTS. 
sects, into a second stomach, or gizzard, ¢, that is provided 
with muscular walls and chitinous plates. From here it 
passes to the true stomach /, and finally to the intestine 2, 
Circulation.—The heart, H, is tubular in shape, ex- 
tending along the back, and composed of numerous sacs, 
separated by valves that allow the blood to flow toward 
the head, where it branches out, re- 
turning through the tissues, there 
being no true veins or arteries. 
Fic. 113.—S, spiracle or breathing-plate, with 
the slit in the center which opens to take in 
air. 7, part of a breathing-tube, showing 
the spiral thread which feeps it in its round 
shape. 
Fic. 114.—Insect showing the spiracles, or open- 
ings in the sides of the body which commu- 
nicate with the air-tubes within the body: 
w, showing where the wings were attached ; Fic. 115.—Tracheal or 
4and m, where hind and middle legs were air-tube system of 
attached ; 5, spiracle on thorax; 7, tym- a larval dragon-fly. 
panum. (After Morse.) Trachez are shaded. 
Respiration.—The insects all breathe by a system of 
air-channels or tubes, and some, as the spiders, by lungs 
as well. The air-tubes are called ¢rachez, and are wound 
with a minute thread (Fig. 113) that seems ta preserve their 
