496 



INVERTEBRATE MORPHOLO&T. 



of the cuticle, recalling the tympanic membrane of the human 

 ear, beneath and in connection with which is a chordotonal 

 apparatus, further improved by the occurrence in close prox- 

 imity to it of a saclike enlargement of a trachea which serves 

 as a resonator. Similar organs occur in pairs on opposite 

 sides of the tibiae of the first pair of legs in the Crickets 

 (GryllidEe), and seem from their structure to be auditory 



bk 



-tr 



Fig. 229. — Subgbnual Chordotonal Organ op the Tibia of the Second 

 Thoracic Appendage of Isopteryx (after Graber from Lang). 

 bk = blood-coi-puscles. gz = nerve-cells, 



c = integment. tr = trachea. 



es = terminal ligament. sc = terminal hairs and sheaths. 



organs, whence the conclusion that the more simple chordo- 

 tonal organs also subserve this function. 



It is interesting to note that the males of the forms provided with a 

 tympanal organ possess the power of making a harsh or sharp chirping 

 noise, produced in the Grasshopper by rubbing the femora of the hind legs, 

 which are furnished on their inner surfaces with a row of fine teeth, over 

 the strong marginal veins of the anterior pair of wings ; and in the Crickets 

 and Looustidse by rubbing together the two anterior wings, a row of teeth 

 upon a vein of one wing working upon a projecting smooth vein of the 

 other. The male Cicadas also make a similar noise, the stridulating appa- 

 ratus resembling that of the Grasshoppers, and in all cases it seems to be a 

 sexual characteristic serving to attract the females. 



The sexes are separate, and a more or less distinct sexual 

 dimorphism occurs, the males being usually smaller and more 

 slender than the females. In some cases, as in the Tussock- 

 moth (Orgyia), the female lacks wings and has a very different 



