224 THE COURTSHIP OF ANIMALS 



slightly moved away from the abdomen, and thereby 

 a wide fissure will be produced, displaying the mechanism 

 beneath. If the whole operculum be removed, three 

 membranes will be seen, an external, called the " timbal," 

 an anterior folded and soft membrane, and a posterior 

 " mirror." This last is a very beautiful object, tensely 

 stretched and pellucid, yet reflecting all the hues of the 

 rainbow. The sound is primarily produced by the vibra- 

 tions of the timbal, to which a muscle is attached ; the 

 other membranes are probably also set vibrating, and the 

 whole skeleton helps to increase or modify the sound, 

 which is probably also influenced by the position of the 

 operculum, which varies in different species. A further 

 control of the tension of the air is exerted by " stigmata " 

 or pores, and certain air-chambers connected therewith. 



Throughout these pages comment has been made on the 

 apparently " fortuitous " character of complex patterns 

 and structures. The " musical-box " of the Cicada affords 

 yet another instance. Nevertheless there is an impressive 

 harmony between the several parts ; an interdependence 

 which is not fortuitous. There is obviously a nexus of 

 growth-controlling factors preserving harmony between 

 each separate part which as yet has escaped all endeavour 

 to discover. 



While it is difficult to picture the initial stages of growth 

 of so complex an organ as that of the Cicada, the beginnings 

 of simpler structures such as the stridulating organs of 

 Beetles and Grasshoppers are more easily discernible. 

 " It is probable," remarks Darwin, " that the two sexes 

 of many kinds of Beetles were at first enabled to find 

 each other by the slight shuffling noise produced by the 

 rubbing together of the adjoining hard parts of their 

 bodies ; and that as those males or females which made 



