56 MIMICRY, AND OTHER PROTECTIVE 



the bottom by their prehensile tails, and float about 

 with the current, looking exactly like some simple 

 cylindrical algae. 



It is, however, in the insect world that this 

 principle of the adaptation of animals to their en- 

 vironment is most fully and strikingly developed. 

 In order to understand how general this is, it is 

 necessary to enter somewhat into details, as we shall 

 thereby be better able to appreciate the significance 

 of the still more remarkable phenomena we shall 

 presently have to discuss. It seems to be in pro- 

 portion to their sluggish motions or the absence of 

 other means of defence, that insects possess the pro- 

 tective colouring. In the tropics there are thousands 

 of species of insects which rest during the day cling- 

 ing to the bark of dead or fallen trees ; and the 

 greater portion of these are delicately mottled with 

 gray and brown tints, which though symmetrically 

 disposed and infinitely varied, yet blend so completely 

 with the usual colours of the bark, that at two or 

 three feet distance they are quite undistinguishable. 

 In some cases a species is known to frequent only 

 one species of tree. This is the case with the com- 

 mon South American long -horned beetle (Onychocerus 

 scorpio) which, Mr. Bates informed me, is found 

 only on a rough-barked tree, called Tapiriba, on the 

 Amazon. It is very abundant, but so exactly does 

 it resemble the bark in colour and rugosity, and so 

 closely does it cling to the branches, that until it 

 moves it is absolutely invisible ! An allied species (0. 



