70 NATURE SKETCHES IX TEMPERATE AMERICA 



Wallace sums up the various modes in which color is produced 

 or modified in the animal kingdom as follows: "The various 

 causes of color in the animal world are molecular and chemical 

 change in the substance of their integuments, or the action on 

 it of heat, Hght, or moisture. It is also produced bj' inter- 

 ference of light in superimposed transparent lamellae, or by 

 excessively fine surface striae. These elementary conditions 

 for the production of color are found everywhere in the surface 

 structure of animals, so tiiat its i)resence must be looked upon 

 as normal, its absence as exceptional. Colors are fixed or 

 modified in animals liy natural selection for various purposes; 

 obscure or imitative colors for concealment; gaudy colors as 

 a warning; and special markings, cither for easy recognition 

 by strayed individuals, females or young, or to divert the 

 attack from a vital part, as in the large, brilliantly marked 

 wings of some butterflies and moths." 



Colors are jjroduced or iulonsified by processes of develop- 

 ment, either where the iiiteguinent or its appendages undergo 

 great extension or modification, or where there is a surplus 

 of vital energy, as in male animals generally and more esjiecially 

 at the breeding season. Colors are also more or less influenced 

 by a variety of causes, such as the nature of the food, the 

 phot()gra])liic or ])hysiological action of light, and also by 

 some unknown local action, probably dei;endent on chemical 

 peculiarities in the soil or vegetation. These various cau.ses 

 have acted and reacted in a variety of ways, and have been 

 modified by conditions dei)endent on age, or sex, on competi- 

 tion with new forms, or on geographical or climate changes.^ 



According to Tower, the colors of insects, when grouped 

 according to their causes, can be assigned to three categories: 

 namely, chemical or pigmental, j)hysical or structural, and 

 chemico-physiological or combination. The colors due to chem- 

 ical or pigmental changes are black, brown, orange, yellow, drab, 

 many reds, rarely blue, green, and white. The pigments which 

 produce them are soluble in various reagents. These compounds 

 nuiy be the product of the metabolism of the animal, or 

 derived from food, or they may be accidental inclusions. 



»" Natural Selection and Tropical Nature," pp. 391, 392. Of other 

 specific causes may be mentioned temperature and moisture. 



