MELANIN 191 



be derivable from certain amino-acids, which result as the out- 

 come of katabolic changes in proteins. True melanin is produced 

 by the oxidation of an amino-acid, functioning as a colourless 

 chromogen, through the action of an enzyme. It is well known that 

 tyrosin is converted into melanin by the ferment tyrosinase and, 

 more recently, it has been shown that "dopa " (dioxyphenylalanine) 

 is also a source of this same pigment, under the influence of 

 tyrosinase or of dopaoxidase. Whether these two enzymes are 

 actually distinct appears to be doubtful, and opinions differ on the 

 subject. 



Melanin is probably a very widely spread pigment among 

 insects, and the existence of tyrosinase in the blood, or other 

 tissues, has been detected by a number of investigators. This bare 

 fact, however, without evidence of the presence of a chromogen 

 susceptible to its influence, has only a partial bearing upon the 

 problem of melanisation. Evidence of the presence of tyrosin 

 has been found by Van Fiirth and Schneider in the blood of 

 Lepidoptera ; by Gessard in the larva of Lucilia ; and by Gortner 

 in the larva of Tenehrio. The findings of these earUer observers 

 have been confirmed by Onslow (1916), who has shown that the 

 black markings on the wings of Pieris hrassicce are due to the 

 presence of this pigment. By grinding pupae of this species, 

 before the black markings began to appear on the wings, and 

 extracting the haemolymph, it was shown that the latter blackened 

 at the surface when in contact with the air. When the immature 

 fore wings were dissected off the insects, and immersed in the 

 haemolymph, they became blackened, but more so at the seat of 

 the usual markings. A dense blackening effect was produced 

 all over when the wings were treated with a solution of tyrosin, 

 but when placed in tyrosinase solution the blacking was confined 

 to the actual markings. These experiments show that, although 

 tyrosinase is generally distributed, the black markings and their 

 position are determined by the localisation of the tyrosin in 

 these areas, and that the process of natural melanisation occurs 

 as soon as atmospheric oxygen has access to the developing 

 wings. 



Subsequent researches by Hasebroek, published in numerous 



