352 



ELEMENTARY BIOLOGY 



In certain experiments with flatfish that are ordinarily pig- 

 mented on the upper surface and white on the lower surface, 

 the light was supplied from below by means of mirrors, with 

 the result that the fish developed pigments on the lower sur- 

 face and remained white above. 

 From these experiments and 

 from our own experience with 

 getting tanned, we may feel con- 

 fident that at least in many cases 

 the formation of the pigment is 

 due to the stimulation of the 

 light. But we know also that 

 there are many other pigments 

 that are formed without refer- 

 ence to the light, whether they 

 have any protective value or not. 

 401. Invisibility. In relation 

 to enemies that can see, one of 

 the most obvious means of protec- 

 tion is something to make one in- 

 visible. The jellyfish (Figs. 1 54, #, 

 and 155, b) is so nearly transpar- 

 ent that it is practically invisible 

 in the water. 



But transparency is not the 

 only means by which an object 

 may be made invisible. The see- 

 ing of objects depends upon the 

 contrasts in lights and shadows ; 



an object that is colored like the background becomes by 

 that fact invisible. This type of invisibility is so common 

 in nature that some men claim to be able to tell the kind of 

 surroundings an animal naturally occupies from the character 

 of its surface colorings. The green katydid among the green 

 leaves is a common example of so-called protective coloration 



FIG. 1 68. The underwing moth 

 (Catocala) 



When they are at rest, the moths of 

 this genus resemble the bark of trees, 

 so that they are no doubt often over- 

 looked by their enemies 



