30 INSECT PESTS OF FARM, GARDEN AND ORCHARD 



branous lips. These apertures are called spiracles or stigmata 

 (Fig. 21 sp.), and are the openings of the respiratory system. 

 Similar openings are to be found in all insects, though not so 

 easily seen in the adults. Connecting these spiracles is a pair of 

 tubes on each side of the body, throughout its length, from which 

 branch off smaller tubes to all of its organs and tissues. Fresh 



tr 



,-a.v 



FIG. 23. Internal anatomy of silk-worm, from photo of Azoux Model: 

 A, upper or dorsal bodywall seen from within; B, the back of the silk- 

 worm removed, showing alimentary canal; C, alimentary canal removed, 

 showing nervous system and trachea! trunks; tr., trachea; d.v., dorsal 

 vessel or heart; ph., pharynx or mouth; su., supra-oesophageal ganglion; 

 sp.sp., spiracles or breathing pores; n., nerve cord; tr.t., tracheal 

 trunk; oes., oesophagus or throat; cr., crop; s.g., silk gland; pro., 

 proventriculus ; st., stomach; h.i., hind intestine. 



air is thus inhaled to all parts of the body through these tubes 

 (Fig.21,*r). 



The blood of insects does not circulate through any system of 

 tubes as it does in the higher animals. Along the middle of the 

 back, above the alimentary canal, is a long tube popularly called 

 the heart (Fig. 23, dv). This heart is composed of a number of 

 chambers, each of which is furnished with side valves for 

 admitting blood from the body-cavity. The blood coming 

 into the heart from the body-cavity is propelled forward toward 



