128 



RESPIRATORY MECHANISMS 





tween 15 and 20% renewal at each breath, was evidently 

 insufficient to sustain exertions for more than a couple of 

 minutes, and this again illustrates the size limits imposed on 

 insects. 



In some large insects, especially among the Orthoptera, a 

 unidirectional ventilation of the tracheal trunks has been 



noticed and in several cases 

 closely studied (Du Buisson, 

 1924; Lee, 1925; McGovran, 

 1931; Fraenkel, 1932; Kitchel 

 and Hoskins, 1935). It is 

 brought about by opening 

 and closing of spiracles in 

 conformity with the rhythm 

 of the respiratory move- 

 ments. In most cases thor- 

 acic spiracles open during 

 inspiration, and abdominal 

 during expiration, produc- 

 ing an intermittent flow in 

 the antero-posterior direc- 

 tion, but a reversal is occa- 

 sionally observed (McArthur, 

 1929). Fraenkel made the 

 most careful study of the re- 

 lation between the respira- 

 tory movements and the 

 opening and closing of spir- 

 acles, as illustrated in Fig. 

 74, while Kitchel and Hoskins showed for the grasshopper 

 Chortophaga viridifasciata that a pressure difference of over 

 20 cm water could be set up by the respiration of an animal 

 mounted as in Fig. 74. The biological significance of the 

 directed flow of air through a considerable part of the 

 tracheal system lies in the fact that it reduces the dead space 

 and therefore allows a better ventilation economy. This may 

 be of great importance for the ventilation during flight. 



Fig. 74. Gas chamber with rubber 

 partition enclosing grasshopper with 

 thorax in anterior, abdomen in posterior 

 compartment. A respiratory move- 

 ments, B and C respectively fall in 

 volume of thoracic compartment and 

 increase in abdominal, i inspiratory 

 phase, thoracic spiracles opened, ab- 

 dominal closed; e\ initial expiratory 

 phase, all spiracles closed, e 2 final 

 phase, abdominal spiracles opened. 

 (Fraenkel.) 



