ANATOMY AND PHYSIOLOGY 



123 



At expansion inspiration takes place, and at contraction expiration 

 occurs. In the grasshopper, the thoracic spiracles open almost simul- 

 taneously with the expansion of the abdomen. Contraction is effected 

 by special vertical expiratory muscles (Fig. 180), 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, however, in Locustidae, Tri- 

 choptera 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 pro- 

 ject 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 PlG . r go. Diagrammatic cross-section 



the finest tracheal branches is not f abdomen of a grasshopper, Tropidacris. 



. . a, dorsal septum, or diaphragm ; ex, expira- 



Clearly ascertained, but it IS thought tory muscle; /, fat-body; g, ganglion; h, 



tW air iQ fnrrf>H into tri^f* tiihpc; hv heart; in ' ins P irator y muscle; *, ventral 



y septum, below which is the ventral sinus. 



pressure from the abdominal muscles, The dorsal and ventral septa rise and fail 



, .. . . . , periodically. After GRABER. 



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 suboesophageal ganglion, for they continue after decapi- 

 tation and even in the detached abdomen of a grasshopper or dragon 

 fly. Each ventral ganglion of the body is an independent respiratory 

 center for its particular segment. 



ex 



10. REPRODUCTIVE SYSTEM 



The sexes are always separate in insects, hermaphroditism occurring 

 only as an abnormal condition. The sexual organs, situated in the ab- 

 domen, consist essentially of a pair of ovaries or testes and a pair of ducts 

 (oviducts or seminal ducts, respectively). Primitively, the ducts open 

 separately, as they still do in Ephemeridae, but in almost all other insects 



