STRUCTURE OF EAR. 101 



Graber regards the covered tympana as a develop- 

 ment from the open ones, and suggests that in time 

 to come the species in which the tympana are now 

 exposed may develop a covering fold. 



If now we examine the interior of the hg, the trachea 

 or air-tube will be found to be remarkably modified. 

 Upon entering the tibia it immediately enlarges and 

 divides into two branches, which reunite lower down. 

 To supply air to this wide trachea the corresponding 

 spiracle, or breathing-hole, is considerably enlarged, 

 while in the dumb species it is only of the usual size. 

 An idea of the form of the trachea will be given by 

 Fig. 69, which, however, represents the anterior tibia 

 of an ant, where these tracheae are less considerably 

 enlarged, and where one of the branches is much smaller 

 than the other, while in locusts they are nearly equal 

 in width, and one lies against each tympanum. The 

 enlarged trachea occupies a considerable part of the 

 tibia, and its wall is closely applied to the tympanum, 

 which thus has air on both sides of it ; the open air on 

 the outer, the air of the trachea on its inner surface. 

 In fact, the trachea acts like the Eustachian tube in 

 our own ear ; it maintains an equilibrium of pressure 



no stridulating apparatus. For instance, in the following forms, both 

 the stridulating apparatus and the tympana are absent, viz. : — 



Among the CEcanthidsG : Phalangopsis and Gryllomorpha (both are 

 wingless). 

 »' PlatydactylidsB : Metrypa and Parametrypa (both wing- 



Tettigonidse : Trigonidium. 



Gryllidse: Gryllus apterus, Parabrachytrupes Australis, 



and Apiotarsus (all wingless). 

 GryllotalpidsB : Tridactylus apicalis. 

 Mogoplistidse : Mogoplistes, Myrmecophila, Physoblemma 



(all wingless), and Cacoplistes. 



