APPARENT EXCEPTIONS. 195 



tion — that of the " nervation " of the leaf — is pro- 

 duced by a modification, not very violent, of a 

 structure which belongs to all flying- insects. 

 Their wings are constructed of a thin filmy mate- 

 rial stretched upon a framework of stronger sub- 

 stance, as the sails of a windmill are stretched 

 upon a trelliswork of spars. This framework is 

 designed in a great variety of patterns — more 

 elaborate and more beautiful than the tracery 

 of Gothic windows. In the Mantis this tracery, 

 instead of being drawn in a mere pattern, is drawn 

 in imitation of the nervature of a leaf. And imi- 

 tative colouring is added to imitative structure — 

 so that nothing should be wanting to its com- 

 pleteness and success. 



It must always be remembered, however, that 

 Contrivance in Nature can never be reduced to a 

 single purpose, and to that alone. Almost every 

 example of it is connected with a number of 

 effects which fit into each other in endless ramifi- 

 cations of adjustment. For example, this imitative 

 structure of the Mantidce serves as well for their 

 own protection from insectivorous birds as for the 

 procuring of their food in the capture of other 



