ANATOMY AND PHYSIOLOGY 



139 



FIG. 177. 



The rate of respiration increases or diminishes with the 

 activity of the insect and with temperature and other condi- 

 tions. In six specimens of AJ clan o plus differ entialis, held be- 

 tween the fingers, the thoracic spiracles opened and closed 

 respectively 34, 43, 45, 54, 60 and 61 times per minute. Four 

 individuals of M. fcuutr- 

 rubruin under the same cir- 

 cumstances gave 70, 78, 90 

 and 92. 



At expansion inspiration 

 takes place, and at contrac- 

 tion expiration occurs. In 

 the grasshopper, the thoracic 

 spiracles open almost simul- 

 taneously with the expan- 

 sion of the abdomen. Con- 

 traction is effected by special 

 vertical expiratory muscles 

 (Fig. 177), but expansion 

 is due to the elasticity of 

 the abdominal wall, as a 

 rule; this is the reverse 

 of what occurs in mam- 

 mals, where expiration is 

 passive and inspiration ac- 

 tive. Inspiratory muscles 



are found, however, in Acridiidse, Trichoptera and Hymen- 

 optera. 



Though the respiratory movements of an insect may be 

 studied with a hand-lens, a more precise method is that of 

 Plateau the chief authority on insect physiology who made 

 use of the stereopticon to project an enlarged profile of the 

 insect upon a screen, on which could be marked the different 

 contours of the abdomen at its phases of inspiration and 

 expiration. 



The way in which the air reaches the finest tracheal branches 



Diagrammatic cross section of abdomen 

 of a grasshopper, Acridium. d, dorsal 

 septum, or diaphragm; ex, expiratory mus- 

 cle; f, fat-body; g, ganglion; h, heart; in, 

 inspiratory muscle; "c, ventral septum, be- 

 low which is the ventral sinus. The dorsal 

 and ventral septa rise and fall periodically. 

 After GRABER. 



