586 Color and Pattern and their Uses 



Both the reasoning and the observed facts on which these hypotheses rest 

 are based on the usefulness of the colors and patterns to the animals in their 

 relation to the outside world. And the influence of advantage and natural 

 selection is given the chief credit for determining the present-day conditions 

 of these colors and patterns. 



Before, however, we take up these h)'potheses, defining them and looking 

 over some of the evidence adduced for their support, as well as some of the 

 criticism leveled at them, we may advisedly look to the actual physical causa- 

 tion of color in insects. Whatever the use or significance of color, our 

 understanding of this use must be based on a knowledge of the method or 

 modes of the actual production of color. 



Color in organisms is produced as color in inorganic Nature is. Certain 

 substances have the capacity of selective absorption of light-rays so that 

 when white light falls on them, certain colors (light-waves of certain length) 

 are absorbed, while certain others (light-waves of certain other lengths) are 

 reflected. An object is red because the substance of which it is (superficially) 

 composed reflects the red rays and absorbs the others. Certain other objects 

 or substances may produce color (be colored) because of their physical rather 

 than their chemical constitution: their surfaces may be so composed of 

 superposed lamelte, or so striated or scaled, that the various component 

 rays of white light are reflected, refracted, and diffracted in such varying 

 manner (at different angles and from different depths) that complex inter- 

 ference effects are produced, resulting in the practical extinguishing of cer- 

 tain colors (waves of certain length), or the reflection of some at angles so 

 as not to fall on the eye of the observer, and so on. Such colors will change 

 with changes in the angle of observation, and are the so-called metallic or 

 iridescent colors. These two categories of color have been aptly called 

 chemical and physical: chemical color depending on the chemical make-up 

 of the body, physical on its structural or ])hysical make-up. As a matter 

 of fact we shall find that most insect colors are due to a combination of these 

 two kinds. 



Substances that produce color by virtue of their capacity to absorb certain 

 colors and reflect only one or more others we may call, in our discussion of 

 color production, pigments, and pigmental may be used as practically synony- 

 mous with chemical in referring to colors thus produced, while structural 

 may be sometimes used as sTOonymous with physical in referring to colors 

 dependent on su[)erficial structural character of the insect body. For colors 

 produced by the co-operation of both pigment and structure, combination 

 or chemico-physical may be used as a defining name. In a recent valuable 

 paper by Tower * the history of and authority for the adoption of these 

 various names is given. 



* Tower, W. L. Colors and Color-patterns of Coleoptera. Decennial Pubs, of 

 Univ. of Chicago, 1903, vol. X, pp. 33-70. 



