406 JOHN H. GEROULD 



than the normal yellow-green. Possibly this will explain the 

 excess of blue-green individuals in brood 6, notwithstanding the 

 inroads upon one part of it by English sparrows, as described 

 in the previous section. A similar excess of blue-green cater- 

 pillars was noted in the small family of the second generation, 

 K, as shown in table 1. The small size of the brood is due to infer- 

 tility that accompanied inbreeding, rather than to disease. If 

 the blue-green caterpillars should prove to be stronger and more 

 resistant to disease (especially the virulent polyhedral disease 

 with which the breeder of lepidoptera constantly contends), 

 the conspicuousness of its color is a handicap that would never 

 allow it to succeed in the struggle for existence, unless it should 

 find and adapt itself to a bluish-green food plant of its favorite 

 botanic family, the Leguminosae. 



SIMILAR PIGMENTS 



The pronounced blue-green hue of the caterpillar, however, 

 is not exactly matched by any pure leaf color that I know. It 

 is physically comparable in the vegetable kingdom to the tint 

 of the blue-green algae, which is due to a special blue pigment 

 (phycocyanin) in addition to chlorophyl. Phycocyanin, accord- 

 ing to West ('04), is a "reserve albuminous substance containing 

 both nitrogen and phosphorus, and it occurs in small granules." 



The source of the blue-green color of the mutant caterpillar 

 is the blue-green component of chlorophyl from the food plant, 

 if, as I assume, the yellow component xanthophyl, or its deoxi- 

 dized derivative carotin, is destroyed, or at least changed 

 and decolorized, during digestion, leaving only the blue-green 

 component. The latter, formerly called chlorophyllin, is now 

 known through the researches of Willstatter ('13) and others to 

 be a mixture of two pigments: one of which, chlorophyl a, 

 C55H7205N4Mg, constituting about 72 per cent of the mixture 

 (with a variation of not over 10 per cent) is blue-green in alco- 

 holic solution as seen by transmitted light, blood-red by reflected 

 light; the remaining 26 per cent is called chlorophyl b, yellow- 

 green by transmitted light, differing from its associate in the 

 lack of two atoms of hydrogen and the addition of one of oxygen. 



