204 TEXT-BOOK OF ENTOMOLOGY 



they fade, change, and disappear after death. But where these 

 colors are preserved after death and enclosed in air-tight sacs, as in 

 the elytra and scales and hairs of the body, they persist, though, as 

 we well know, they may fade after exposure to light. 



The hypodermal colors are mostly brighter and lighter than the 

 dermal ones, being light blue or green in different shades, yellow to 

 orange, and the numerous shades of these colors combined with 

 white ; exceptionally they are metallic, as in Cassida, and are then 

 obliterated after death. 



"The fact that such metallic colors can be retained in dead specimens by 

 putting a drop of glycerine under the elytra, leads us to conclude that those 

 colors are based upon fat substances. The hypodermal colors are never glossy, 

 as far as I know ; the dermal colors frequently. 



"As the wings, elytra, and hairs all possess a cuticula, dermal colors are fre- 

 quently to be found, together with hypodermal ones, chiefly in metallic colors. 

 In the same place both colors may be present, or one of them alone. So we 

 find hypodermal colors in the elytra of Lampyridse. In the elytra of the 

 Cicindelidae the main metallic color is dermal, the white lines or spots are hypo- 

 dermal, by which arrangement the variability in size and shape of those spots is 

 explained. 



"There occur in a number of insects external colors, that is, colors upon the 

 cuticula, which I consider to be in fact displaced hypodermal colors : the mealy 

 pale blue or white upon the abdomen of some Odonata, the white on many 

 Hemiptera, the pale gray on the elytra and on the thorax of the Goliath beetle, 

 and the yellowish powder on Lixus. Some of these colors dissolve easily by 

 ether or melt in heat, and some of them are a kind of wax. I believe that those 

 colors are produced in the hypodermis, and are exuded through the pore-canals." 

 (Hagen.) 



The white colors are simply for the most part due to the inclusion 

 of air in scales. The white mother-of-pearl spots of Argynnis are 

 produced by a system of fine transverse pore-canals filled with air ; in 

 Hydrometra the white ventral marks have the same origin. (Leydig.) 



The further statements and criticisms of Hagen regarding the 

 relation of color to mimicry, sexual selection, and the origin of pat- 

 terns are of much weight and will be referred to under those heads. 

 Indeed, these subjects cannot well be discussed without reference to 

 the fundamental facts stated in the masterly papers of Leydig and 

 of Hagen, and much of the theorizing of these latter days is ill- 

 fouiided, because the colors of insects and animals are attributed to 

 natural selection, when they seem really the result of the action of 

 the primary factors of organic evolution, such as changes of light, 

 heat, cold, and chemical processes dependent on the former. 



As to the chemical nature of color, Hagen, after quoting the results 

 of Krukenberg and others, thinks that the colors of insects are 

 chemically produced by a combination of fats or fat-acids with other 



