NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS IO3 



strongly-sclerotized marginal band (e) which, when the spiracle is 

 closed, fits into the groove of the anterior lip. 



Tlie cleft of the first spiracle opens into a shallow atrium from 

 which are given off two tracheae, a larger dorsal one (fig. 51 B, /) 

 and a smaller ventral one (g). From without, therefore, the first 

 spiracle appears to have a double opening (A, /, g,). In some of the 

 Orthoptera that have tympanal organs on the front legs, the trachea 

 of the ventral (or posterior) opening of the spiracle appears to have 

 become specialized as an " acoustic " trachea since it goes only to the 



51.— First thoracic spiracle of Dissosteira. 



A, outer view of left spiracle. B, inner view of right spiracle, a, ventral lobe 

 of peritreme ; b, process of peritreme protecting spiracle from covering flap of 

 pronotum ; c, anterior lip of spiracle ; d, posterior lip of spiracle ; e, hard edge 

 of posterior lip; /, dorsal trachea; g, ventral trachea; /;, internal lever of pos- 

 terior lip forming a septum between the tracheae; i, head of lever on which 

 closing muscle (/g) is inserted; /, ventral internal process of peritreme on 

 which spiracular muscles arise; ni, external pit forming internal process /; Ptr, 

 peritreme ; yg, opening muscle ; 80, closing muscle. 



front leg, where it branches into the two tracheae of the tympanal 

 organ. This fact led Graber to the conclusion that the double structure 

 of the first spiracle in Orthoptera originated from the separation of 

 an " acoustic " trachea from the general respiratory tracheae of the 

 prothorax. Carpentier (1924, 1925), however, has shown that the 

 double first spiracle is a character of Orthoptera in general, whether 

 tympanal organs are present in the front legs or not, and that in most 

 forms the tympanal trachea is not isolated from the rest of the 

 respiratory system. The specialization, he says, is carried to its highest 

 degree in the tettigoniid Phasgoncura viridissiuia, where the spirac- 

 ular orifice of the leg trachea is enormously enlarged. Here, ap- 



