tJic colours of certain Lepidoptera. 415 



T'. urticce (see pp. 365 — 369), that the latter pass through 

 a far shorter preparatory period. In this ease the two 

 conditions are probabl}^ intermixed, and there is no cri- 

 terion by which the one can be distinguished from the 

 other. It is therefore impossible to test by these figures 

 the conclusion indicated by the parallel investigation 

 upon V. urticcB, as to the protraction of the stages in the 

 formation of dark pupse. But in other respects the 

 results are extremely interesting, enabling us to contrast 

 the lengths of the stages with those of V. urtiae. Stage 

 II. appears to be very short : in 5 cases it lasted about 

 4 hrs., in 3 about 6, in 1 about 9. Its far greater length 

 in the remaining larvae was probably a result of distur- 

 bance. Stage III., on the other hand, is very long, — 

 about twice as long as its ordinary duration in V. urticce. 

 Stage III. was not subject to great fluctuations, with a 

 single exception of 50 hrs'. duration (probably due to 

 disturbance). On the other hand, it varied from 25 j to 

 32 hrs., and variations of a rather less extent were quite 

 common. Although there were many larvae in which 

 the length of this stage was accurately ascertained, they 

 produced (with one exception) dark or darkish pupae, so 

 that we cannot compare the lengths with those passed 

 through when light pupse are formed. There is nothing, 

 however, in the table to oppose the conclusions arrived 

 at in the case of V. urticce. 



The ai)parently normal moderate fluctuation in the 

 length of Stage III., as well as its great relative length, 

 suggest that it includes far more of the susceptible 

 period than is the case with V. urticce (and this is proved 

 to be the case further on; see Experiments 94 — 100). If 

 so, and the conclusions derived from the study of the 

 latter are sound, we must expect that Stage III. will be 

 shorter in light than in dark pupae of V. io, although 

 such a tendency is restricted to Stage II. in the former 

 species, corresponding to the inclusion of the chief sus- 

 ceptibility within its limits. 



We may conclude from these experiments that in the 

 production of dark pupse the normal length of Stage II. 

 is from 4 to 6 hrs., the shorter period being the 

 commoner ; while that of Stage III. varies from 25J to 

 32 hrs. : longer periods being commoner than shorter 

 ones. 



