222 



Comparative Animal Physiology 



gas bladder of primitive bony fish, have very satisfactorily met the respiratory 

 requirements imposed on animals in passing from aquatic to aerial media. 



Tracheae. The phylum Arthropoda has monopolized the evolution of the 

 tracheal mechanism, a distinctive system of air tubules, opening to the outside 

 through spiracles and ramifying to all parts of the body to penetrate tissues 

 and even cells. The system is adapted to aquatic as well as terrestrial condi- 

 tions, permits rapid oxygen uptake, and allows a high degree of animal organi- 

 zation; the main limitation seems to be the size attainable by tracheates. As 

 with lung mechanisms, the tracheal system may be dependent either on simple 

 diffusion for air or, in more active and larger forms, on mechanical ventilation. 



Respiratory movements are generally absent from the arthropods Onycho- 



Fig. 43. Tracheal response to osmotic change during rest (A) and after activity (B). 

 1, trachea; 2, tracheal cell; 3, parts of tracheoles containing air; 4, parts of tracheoles 

 containing fluid; 5, muscle. From Wigglesworth.'"'" 



phora, Arachnida, Myriapoda, and Chilopoda, from most terrestrial larvae, 

 from all pupae, and from many small winged adult insects.-""' Size and metab- 

 olic rate are important factors in regard to the adequacy of simple diffusion, 

 and although some insects (Periplaneta, Dixipp^is) rely on diflfusion when at 

 rest, they call upon \'entilation movements during higher levels of metabolic 

 activity. 



The innermost ends of the tubes, the tracheoles, arc fluid-Iilled in some 

 insects and are certainly permeable to water. They may respond to changes 

 in muscular acti\'ity by shifting the fluid reversibly between the tracheoles 

 and the tissue. According to Wigglesworth, •"'•'' in mosquito (Aedes) and 

 dragonfly (Aeschna) larvae water is withdrawn from the tubules by the 

 osmotic effect of increased acid metabolites produced during activity, and air 

 in the tracheoles then moves closer to the cells, the process being reversed 



