PIGMENTAL COLORS. 120. 



At present the explanation of color phenomena is most difficult. Warning 

 coloration, protective resemblance, and mimicry are, I believe, adequately and 

 only explainable on the basis of natural selection. With other phenomena it is 

 not so. The myriad hues and color combinations and patterns of insects and 

 the brilliant colors of the males of many forms can not, I believe, be explained 

 by natural selection, inheritance of acquired characters, orthogenesis, or 

 mutation ; at least no known explanation seems to be satisfactory to any 

 considerable body of investigators. Nowhere else in the animal kingdom has 

 evolution, as it were, so run riot as among insect colors ; and yet its study has 

 made comparatively little progress in recent years. The evolution of insect 

 coloration in general seems at present to be practically inexplicable. A por- 

 tion here and there is elucidated by one or another of the theories advanced ; 

 a few facts, a few laws, are known ; but, on the whole, our knowledge of these 

 intricate and interesting phenomena is truly meager. 



THE COLORS OF THE GENUS LEPTINOTARSA. 



In the beetles which form the basis of this research, all of the three cate- 

 gories of color are found, either in the imagines or in the larvae. These will 

 be considered under their natural divisions. 



CHEMICAL OR PIGMENTAL COLORS. 



It is to the colors of this category that the genus owes its characteristic 

 coloration ; in fact, physical or chemico-physical colors enter but little into its 

 color phenomena. In general the colors of the adults are cuticula and hypo- 

 dermal, and those of the larvae cuticula, hypodermal, and sub-hypodermal. 



The cuticula colors, black, brown, and yellow, are at once recognizable by 

 the fact that they are insoluble in water, alcohol, oils, ether, weak acids, or 

 alkalis. In L. undecimlineata and decemiineata the black spots and stripes 

 are not pure black, but a deep brown that owes its black appearance to the 

 density of the deposit of pigment. If an elytron or a pronotum of one of these 

 beetles be boiled in alcohol, water, or one of the essential oils at one atmos- 

 phere of pressure, even for several hours, no perceptible change in color is 

 noted. Long standing in water or alcohol, or boiling in these reagents under 

 two or three atmospheres pressure, will in a few hours result in complete 

 extraction of the pigment. The same treatment with dilute HN0 3 , HCL, 

 H,80 4 , NH 4 OH, or KOH will produce solution after a sufficient period of 

 boiling two or three hours. These cuticula colors are absolutely permanent 

 in these beetles, five years of constant exposure to light not effecting any 

 perceptible change in them. 



The black cuticula colors are uniform throughout the genus, i. e., they are 

 due to dense deposits of a deep-brown pigment having the solubility and per- 

 manency given above. From the black we can pass through a graded series 

 to the browns, such as those of the brown areas in /,. setterstedti, or the dark- 

 brown spots and stripes of dilecta, belti, and others. The colors of these spe- 



