174 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



The glaucous green patches on the wings of Colaenis dido are 

 composed mainly of green and yellow, but there is also a faint develop- 

 ment of about half of the blue and a still fainter trace of red. 



The iridescent blue-green ground color of the upper surface of the 

 wings of Morpho menelaus, viewed in such a way that the light 

 makes an angle of about 20° with the normal to the surface of the 

 wing, gives a spectrum of green and blue about equally developed. 



The yellow ground color found on the upper side of the wings of 

 Papilio turnus shows a continuous spectrum, in which the yellow 

 seems to be rather more brilliant than in the normal spectrum of 

 white light. 



The sepia-brown ground color of the upper surface of the wings 

 of Cercyonis alope gives a spectrum which lacks only the blue-green 

 and blue. 



(6) Summary of Hesults. The researches of Coste ('90-91) and 

 Urech ('93) have demonstrated that the colors of butterflies and 

 moths may be produced by two causes : by the presence of an actual 

 pigment, or by some structural effect. Some colors are due entirely 

 to pigment, others to structural causes, and still others to a combina- 

 tion of the two. 



Black, brown, red, orange, and yellow are invariably due to 

 pigment. 



Green is usually due to a structural effect, but in a few cases there 

 is a green pigment present. 



White, blue, and violet are almost invariably due to structural 

 causes. 



In addition to these facts I have found that most of the colors 

 which are displayed by Lepidoptera contain a surprisingly large 

 percentage of black. Also they are usually not simple colors, but 

 composed of a mixture of several different colors. It is remarkable 

 that Natural Selection, w:hich is generally assumed to have been one 

 of the principal factors in bringing about the wonderful develop- 

 ment of colors in Lepidoptera, has not been potent enough to make 

 these colors purer than is the case in existing butterflies. 



II. The essential ^Nature of Pigmental Color in 

 Lepidoptera. 



(1) Pigments of Laroae. Poulton ('85) showed that the phy- 

 tophagous larvae of Lepidoptera " owe their colour and markings to 



