142 COI^ORATION IN LEPTINOTARSA. 



feeding has begun. They are not related to the azo-cuticula colors, but are 

 simply the derived pigments of the hsemolymph that have been absorbed into 

 the hypodermal cells. In every way they behave as do the subhypodermal 

 colors. They can be studied only in frozen sections. 



SUBHYPODERMAI, CGI^ORS. 



These colors were first recognized by Poulton in lepidopterous larvae. 

 Later I distinguished them in phytophagous Coleoptera. In Leptinotarsa 

 they are present in all larvae, and are directly derived from the pigments of 

 the food. They have been studied in the same manner as were the Lepidop- 

 tera by Poulton, and with the same results. As they are purely larval colors 

 and are modified by temporary conditions and in no permanent manner, they 

 may be passed by without further comment. 



PHYSICAI, COI.ORS. 



The only physical color in Leptinotarsa is white, and this is due to total 

 reflection from the granules in the fat body in larvae and the fatty lipochrome- 

 bearing bodies of the hypodermal cells. These colors are relatively unim- 

 portant in the evolution of color patterns and need no further mention. 



CHKMICO-PHYSICAI, COLORS. 



No complicated colors of this class are found in this genus of beetles. Me- 

 tallic colors — green, violet, copper, and blue — v/ith faint metallic reflections 

 over many dark areas, are all the chemico-physical colors that these beetles 

 display. Iridescent colors or those due to scales are lacking, as are also 

 those due to pits, striae, etc. 



The manner in which these colors are produced has been described in a 

 former paper. In forms like violescens the violet color is due to the differ- 

 entiation on the outer surface of a highly refractive portion of the primary 

 cuticula which is only slightly pigmented, and the development just beneath 

 this of dark azo colors. Light impinging upon the surface is acted upon by 

 refraction in such a manner that the violet and blue rays are reflected and the 

 rest are absorbed in the pigment beneath. The result is that the surface has a 

 dark violet color, and when to this is added the w^hite light reflected from the 

 surface a metallic appearance is produced. As far as I know, this is the only 

 method of chemico-physical color formation in this genus. 



This outer, modified portion of the primary cuticula appears only after the 

 beginning of the dark azo colors. As these develop, the chemxico-physical 

 colors grow stronger and stronger as the amount of pigment increases until it 

 reaches its full intensity. 



In the main the colors of these beetles are of chemical or pigmental origin. 

 They are due to cuticula or azo pigments or to hypodermal or lipochrome 

 pigments, and in larvae slightly to subhypodermal or derived pigments. Phys- 



