56 MIMICRY, AND OTHER PROTECTIVE 
the bottom by their prehensile tails, and float about 
with the current, looking exactly like some simple 
cylindrical alge. 
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 Tapiribdé, 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. 
