Phenocopy and Norm of Reaction 265 



Araschnia (Vanessa) levana-prorsa. The spring form and the fall form 

 differ as much as if they were separate species. By treatment of pupae 

 in a sensitive period with temperature shocks, one form can be trans- 

 formed into the other with all intermediate conditions possible. The 

 spring generation is the form levana with a yellowish brown and black 

 pattern. The summer form prorsa is almost entirely black, with a 

 white band across both wings. Prorsa produces offspring which 

 develop up to the pupal stage when the diapause sets in for the 

 hibernating pupa. In the spring the imago hatches and its offspring 

 develop without diapause into prorsa. By action of cold upon the 

 pupae of the summer generation, which were to metamorphose into 

 prorsa, levana resulted; by action of different low temperatures a 

 series of intermediates can be produced. The summer form could also 

 be produced by action of heat upon the hibernating pupa. 



We know now that diapause in the silkworm is controlled by 

 hormones. Here it is not the pupa which hibernates, but embryonic 

 development stops in the so-called univoltine races with diapause, 

 while it goes on in the multivoltine races without diapause. Fukuda 

 (see 1953) showed that the subesophageal ganglion secretes a dia- 

 pause hormone, while the brain is able, in the multivoltine races, to 

 suppress this action. 



To return to prorsa and levana. Stiff ert (1924) found that the 

 results reported are not simply temperature effects, as Weismann had 

 believed in his pioneer work ( 1875 ) . Actually, two different causative 

 agents come into play: the alternative of diapause or no diapause, and 

 the action of temperature. Pupae with diapause always metamorphose 

 into levana, even if kept in warm temperature. Thus levana is linked 

 with the presence, and prorsa with the absence, of a diapause. If 

 diapause is prevented, invariably prorsa hatches. If after the first 

 diapause, producing levana, a second diapause is induced experi- 

 mentally, again levana. is the result. The result can be obtained by the 

 influence of cold upon the caterpillar in a temperature-effective 

 (critical) period, an effect which we must now ascribe to an effect 

 upon the hormonic function of the brain. Also, if partial diapause is 

 induced in this period, intermediates result, their grade being roughly 

 proportional to the time of diapause. Thus the existence or non- 

 existence of the resting period controls the wing pattern. 



Now there is a second causative chain leading to the same pheno- 

 typic effects. Another critical period for pattern change occurs in the 

 early pupa. Cold shocks applied in this period produce levana even 

 without diapause, and also the graduated series can be obtained by a 



