ENVIRONMENTAL EFFECTS 



have shown pigmental asymmetry are exceedingly 

 rare. Observation of specimens mosaic for var- 

 ious mutant factors and for sex (gynanders), 

 however, indicate greater or less asymmetry in 

 body color. A certain quantity of black pigment 

 in the given region is, of course, a prerequi- 

 site for visible asymmetry. Total absence of 

 color resulting from high temperature during 

 development or to other causes obviously renders 

 a genetic difference incapable of expressing 

 itself. Likewise, if environmental conditions 

 are such that the parts are very black, a rel- 

 atively slight difference determined genetical- 

 ly would not be apparent. Females are in gen- 

 eral less deeply pigmented than males in any 

 given strain, and consequently gynanders usual- 

 ly show striking asymmetry in color correlated 

 with asymmetry in sex. 



The pigments of the stemmaticum of Habrobracon 

 are granular and present in hypodermal cells^ 

 beneath clear cuticle. The other pigments , rang- 

 ing from pale yellow through brown to black, are 

 in the cuticle and appear to be non-granular. 

 Both granular and diffuse pigments are similar- 

 ly affected by temperature changes and undoubt- 

 edly belong to the melanins. The melanins are 

 formed from colorless substances or chromogens) 

 with the aid of enzymes in the presence of oxy- 

 gen. A. R. Whiting (1939b) points out that 

 Wright's theory of pigment formation in mamma- 

 lian hair is apparently applicable to the cu- 

 ticular pigments of Habrobracon. According to 

 Wright (1917) the colorless chromogen is formed 

 in the cytoplasm, and two enzymes or their fore- 

 runners are formed in the nucleus. The union 

 of these substances in the cytoplasm in differ- 



12J 



