242 Genetical Studies in Moths 



we have to demonstrate the existence in insects of such enzymes as 

 tyrosinase and of protein-like bodies capable of yielding humins or 

 melanins when hydrolysed, oxidised or submitted to enzyme action. 



Proof of the latter point is easy ; take a pupa of Oporabia dilutata 

 and puncture the wing cases carefully. The light coloured haemolymph 

 (yellowish in the males and greenish in the females) exudes; this on 

 simple exposure to air shows the phenomenon of melanosis, that is to 

 say it blackens. The haemolymph of insects is therefore endowed with 

 compounds able under enzyme oxidation to yield black pigments. 



It is equally easy to show that tyrosinase occurs in the haemolymph, 

 but in this case, to secure adequate quantities for experiment, it is best 

 to use large Sphingid or Saturniad pupae. Take say pupae of Platysamia 

 cecropia. Pierce their wing cases ; as before the pale haemolymph flows 

 but naturally in much greater quantities. To this add ammonium 

 sulphate and a precipitate is obtained. Dissolve this precipitate in a 

 •05 °/o solution of sodium hydroxide ; now add this tyrosinase solution 

 to one of tyrosin. Soon a violet colouration appears which gradually 

 darkens and finally a black flocculent precipitate is thrown down, to be 

 recognised as one of the humins. 



This insect-derived tyrosinase reacts similarly to yield pigmentary 

 compounds with many other benzene derivatives containing the hydroxy! 

 radicle. In the haemolymph the tyrosin does not exist as such but in 

 its place is a chromogen closely related chemically and conforming to 

 the latter condition ; it, likewise, when acted on by the enzyme tyro- 

 sinase yields coloured compounds. 



Now to return to our melanism ; it is perfectly clear that when we 

 consider the great difference between a typical Boarmia consonaria and 

 its melanic form var. nigra we are dealing with a profound derangement 

 of the ordinary metabolic scheme ; nor can it be denied that this dis- 

 turbance is inherited, and not ontogenetic. These melanic forms, whether 

 reared in the centre of affected areas or in some remote corner where 

 such are never heard of, always breed true. Whence it follows that the 

 original insect showii\g the new character, in this species, showed it by 

 a sudden change in germinal constitution — or had its somatoplasm and 

 germ plasm equally affected by some environmental impulse. The former 

 possibility in its simplest form may be dismissed when due allowance is 

 made for the fact that there is no reason whatever why melanism so 

 produced should be localised ; it should, on the contrary, be universal. 

 But if the altered metabolism gives its possessor great advantages in 

 dealing with unusual food the position would be greatly altered. And 



