4io PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



culation this is impossible in any localized organ. In a 

 pipe system the whole of the blood may be made to go 

 through an organ localized in some part, but in a lacu- 

 nary system a local organ can receive only its small 

 share of the blood. Evidently, then, if the blood can 

 not go to the air, there is nothing left but that the air 

 must go to the blood. This is exactly what it does. The 

 air, by means of ramifying tubes, is carried to every 

 part of the body and aerates the blood on the spot 

 everywhere. 



Air Tubes; Trachea. — On the margins of the dorsal 

 part of the chitinous shell are openings, one on each side 

 of each movable joint. These openings (spiracles) con- 

 nect by a short 

 tube with a long 

 lateral tube extend- 

 ing on each side 

 the whole length 

 of the body (Fig. 

 288). The lat- 

 eral tubes connect 

 across the body 

 with one another 

 by several trans- 

 verse tubes; and 

 from the lateral tubes and their transverse connections 

 go branches and sub-branches until the minute capil- 

 lary branches touch every cell of every tissue. All these 

 tubes are kept open by a spiral thread in their interior 

 (Fig. 289). 



Now, as in vertebrates, in proportion to the vitality 

 of a part is the minuteness of the distribution of the 

 capillary blood vessels, so in insects, in proportion to 

 the vitality of any part is the minuteness of the distri- 

 bution of the air tubes. We have given the most com- 



Fig. 2; 



-Tracheae enlarged to show the spiral 

 structure. 



