HYPODDRMAIv COI.ORS. I4I 



MICROSCOPICAL EXAMINATION OF HYPODERMAI, PIGMENTS. 



The study of these colors by cytological methods is difficult, owing to their 

 solubility in alcohol and essential oils, but by freezing good results may be 

 obtained with ease. In frozen sections of the integument of decemlineata, 

 signaticollis, ruhicimda, and other species, these pigments exist as granules in 

 the cytoplasm of the cells. In such sections color reactions and changes like 

 those described above may be observed, and the color changes followed in the 

 case of individual granules. 



When frozen sections carrying the lipochrome pigments are treated with an 

 alkali (KOH), 1.5 per cent aqueous solution, the color is destroyed, but the 

 bodies or granules remain as pale yellow-white granules which stain strongly 

 in Soudan III. These are soluble in ether, benzol, or allied oils, leaving very 

 characteristic round vacuoles, which are often found in ordinary microscop- 

 ical preparations. 



DEVELOPMENT OF HYPODERMAL PIGMENTS. 



We may conclude that these hypodermal pigments found in granular form 

 are lipochromes, a conclusion based upon their color reactions, solubility, and 

 specific tests. They are present as complex molecules attached to fatty bodies 

 which are developed in the cells. These fatty bodies first appear in the 

 embryo, and are left behind on the extraction or destruction of the color- 

 producing molecules. They may be extracted by ether, oils, etc., leaving 

 characteristic vacuoles in the cytoplasm. It appears that the pigment-produc- 

 ing substances and the fat bodies are separate, the fat body acting merely as 

 a nucleus about which the color-producing molecules gather. From these 

 they may be dislodged by prolonged action of alkalis, when they show the 

 characteristic alternations of color found in lipochromes. This phenomenon 

 is due, I believe, to a change in the molecular arrangement of the color-pro- 

 ducing substance — isomerism — and not to the presence of two or more 

 associated colors, one of which appears in the acid state and the other in the 

 alkaline. In some species, undecimlineata, diversa, and their larvae, the only 

 hypodermal color present seems to be the pale yellow-white produced by the 

 lipochrome pigment nuclei; while others, as decemlineata, have developed 

 through stages of undecimlineata to a yellow and yellow-red condition of 

 these pigments, and still others, as ruhicunda, have reached a full red. The 

 same phenomenon is seen in other groups of the genus. 



DIFFUSE HYPODERMAL PIGMENTS 



In larvae and in some pupae and imagines the cytoplasm of the hypodermal 

 cells is seen to be uniformly yellow or yellow-red or rarely a dull red. These 

 colors fade quickly on exposure to light and are soluble instantly in water and 

 alcohol. They are not lipochromes, nor are they in any way related to them, 

 as they do not appear until after the development of the lipochromes and after 



