TOO 



CRABS AND INSECTS. 



sects, into a second stomach, or gizzard, d, that is provided 

 with muscular walls and chitinous plates. From here it 

 passes to the true stomachy, and finally to the intestine h. 

 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. 



FIG. 113. S, spiracle or breathing-plate, with 

 the slit in the center which opens to take in 

 air. T, part of a breathing- tube, showing 

 the spiral thread which keeps it in its round 

 shape. 



w t 



FIG. 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 ; 

 h and m, where hind and middle legs were 

 attached ; s, spiracle on thorax ; ^, tym- 

 panum. (After Morse.) 



FIG. 115. Tracheal or 

 air-tube system of 

 a larval dragon-fly. 

 Tracheae 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 trachea, and are wound 

 with a minute thread (Fig. 113) that seems to preserve their 



