TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS 



83 



by function itself, but only by processes of selection which imparted to 

 each spot the thickness it required, in order to be effective in its 

 function, whether that be offering resistance to the strain of the 

 muscles, or giving suppleness to a joint, or affording the necessary 

 hardness for biting the prey, or for boring into wood or earth, or 

 merely for protecting the animal from external injuries. 



There are, however, many individual functions of the Arthropods 

 the exercise of which depends on the simultaneous change of several 

 skeletal parts ; as, for instance, many of the ' singing ' or vocal 

 apparatuses in insects. In quite recent times such vocal organs have 

 been discovered in ants, in which they consist of a small striated 

 region on the surface of the third abdominal segment, and a sharp 

 ridge on the segment in front ; the 

 latter is rubbed against the former 

 by the movements of the two seg- 

 ments. Quite a similar ' stridulating 

 organ' has long been known in 

 the bee-ant {Mutilla), and the 

 whistling sound produced by it is 

 easily heard by our ears ; moreover 

 August Forel has heard it in the large 

 wood-ant (Cainjxrnotus ligniperdus), 

 and has described it as an ' alarm- 

 signal,' which the animals give each 

 other on the approach of danger — 

 an observation which has recently 

 been confirmed by Wasmann and 

 extended by Robert Wroughton in 

 regard to Indian ants. All these 



arrangements for producing sound depend always on two organs, of 

 which one resembles the bow, the other the strings of a violin ; the 

 one is of no value without the other, and they must therefore have 

 developed simultaneously, yet they cannot have arisen through 

 use, and the inheritance of the results of use, because they are both 

 dead chitinous parts, which are never strengthened by rubbing against 

 each other with the movements of the abdomen, but are rather worn 

 away. 



The same is true of the chirping organs of grasshoppers, 

 beetles, and crickets ; in all cases they consist of two different parts, 

 which together produce a sound, and which therefore must have 

 arisen simultaneously, and the origin of which cannot be referred to 

 the inheritance of the results of exercise, but rather to selection. It 



G 2, 



Fig. 91 (repeated). Hind-leg of a 

 Grasshopper {Stenobothrus protorma), after 

 Graber. /e, femur. /», tibia, ta, tarsal 

 joints, schr, the stridulating ridge. 



