112 



ENTOMOLOGY 



optera and Heteroptera, and vice versa in Acridiidae, Odonata, Diptera 

 and aculeate Hymenoptera. The width of the abdomen usually changes 

 but little during respiration, for the tergal and sternal movements are 

 taken up by the pleural membranes which, as in the grasshopper, infold at 

 contraction and straighten out at expansion. Other respiratory move- 

 ments occur, but they are of minor importance. 



The rate of respiration increases or diminishes with the activity of the 

 insect and with temperature and other conditions. In six specimens of 

 Melanoplus differentiates, held between the fingers, the thoracic spiracles 



opened and closed respectively 34, 43, 

 45, 54, 60 and 6 1 times per minute. 

 Four individuals of M. femur-rubrum 

 under the same circumstances gave 

 70, 78, 90 and 92. 



At expansion inspiration takes 

 place, and at contraction expiration 

 occurs. In the grasshopper, the tho- 

 racic spiracles open almost simulta- 

 neously with the expansion of the ab- 

 domen. Contraction is effected by 

 special vertical expiratory muscles 

 (Fig. 178), but expansion is due to 

 the elasticity of the abdominal wall, 

 as a rule; this is the reverse of what 

 occurs in mammals, where expiration 

 is passive and inspiration active. 

 Inspiratory muscles are found, how- 

 ever, in Acridiidae, Trichoptera and 

 Hymenoptera. 



Though the respiratory move- 

 ments 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 is not 

 clearly ascertained, but it is thought that air is forced into these tubes 

 by pressure from the abdominal muscles, while its escape through the 

 spiracles is being prevented by the compression of the stigmatal tracheae. 

 The respiratory movements are entirely reflex and are independent of 

 the brain or subcesophageal ganglion, for they continue after decapitation 



FIG. 178. Diagrammatic cross-sec- 

 tion of abdomen of a grasshopper, Acrid- 

 ium. d, dorsal septum, or diaphragm; 

 ex, expiratory muscle; /, fat-body; g, 

 ganglion; h, heart; in, inspiratory mus- 

 cle; v, ventral =eptum, below which is 

 the ventral sinus. The dorsal and 

 ventral septa rise and fall periodically. 

 After GRABER. 



