HEXAPODA: RESPIRATORY SYSTEM, ETC. 293 



palpi are slender, jointed organs, very flexible and sensi- 

 tive. 



A highly developed circulatory system is wanting ; but 

 just under the covering of the back there is a long tube 

 which is called the heart, and this organ performs regular 

 alternate movements of contraction and dilatation. The 

 blood enters this tubular organ by openings along its sides, 

 the openings being furnished with valves which prevent 

 its return, and the blood escapes at the forward end as 

 the organ contracts, and thus the blood is kept in motion 

 throughout the interior of the animal. The blood of In- 

 sects is colorless, and it nils all the interior of the animal 



FIG. 364. 



Cross section of an insect Bee - showing position of stigmata, air tubes, etc. 



st, stigmata opeuing into the trachese; dn, dorsal vessel or heart; ac, alimentary 

 canal; ng, nevous ganglia. 



not occupied by internal organs, and it permeates the tis- 

 sues of the organs themselves. 



The respiratory system is very different from that of the 



