402 APPLIED BIOLOGY 



that the green color makes it harder for birds to find the 

 insects, and hence the color gives some protective advantage. 

 Likewise, warning colors are not always efficient danger 

 signals; and mimicry does not always deceive enemies. 

 However, there is probably a decided advantage, on the whole, 

 to species of insects which possess any of these forms of color- 

 ation. 



334. Colors of other Animals. Animals other than 

 insects may have useful coloration. Thus fishes, frogs, 

 lizards, and many other animals may have concealing colora- 

 tion which may work either protectively or aggressively. The 

 white of birds, hares, and other small animals on arctic snow- 

 fields is believed to be protective ; while the white of polar 

 bears may be aggressive, enabling them to lie in wait for 

 approach of prey. The stripes of tigers are supposed to be 

 aggressive because they harmonize so well with the lights and 

 shadows among the reeds of jungles. Likewise, the tawny 

 color of the lion resembles the sands of the desert. 



Some of the most remarkable cases are those of frogs 

 and lizards, which can quickly change colors when placed 

 on new objects, e.g., from green to brown when moved from a 

 leaf to bark of a tree. This is accomplished by a peculiar 

 arrangement of pigment-cells in the skin. Some cells contain 

 green pigment and some have darker colors. When the 

 green cells are expanded so as to expose their maximum 

 surface, and the dark cells are contracted, the animal's skin 

 appears green. Expanded dark cells and contracted green 

 cells make the skin brown in color. Partial expansion and 

 contraction of both kinds of pigment-cells give intermediate 

 shades. The cells are controlled through the nervous sys- 

 tem and the eyes. 



In the case of those birds and mammals which appear to have 

 concealing colors, it should be remembered that many of 

 the enemies have a keen sense of smell, and so colors cannot 

 always be useful For illustration, quails and other birds 



