i]tc colours of certain Lepidoptcra. 421 



throw further light upon the physiology of the process, 

 or, at any rate, to gain confirmation. 



Some small experiments had already been made, and 

 these had seemed to show that the freshly-formed pupa 

 is certainly not sensitive, and that the larva, if trans- 

 ferred during Stage III., may be susceptible (see Expts. 

 42, 47, 88, and 89 of this paper ; also Phil. Trans., 1887, 

 B, p. 318). 



In order to expose the anterior and posterior parts of 

 the larval body to conflicting colours for the whole of the 

 sensitive period, the case described on p. 393 and shown 

 in Plate XV., Fig. 5, was made use of. The strips of the 

 two upper rows of compartments were about half as wide 

 as the length (30"0 mm.) of a suspended larva of V. io. 

 In each of the 42 compartments a single larva was placed, 

 all belonging to the same company, taken near Oxford to- 

 wards the end of July. The pupse were compared August 

 11, and the results are given in the following table :^ 



{See Table, page 4^22.) 



The results in every way confirm those obtained in the 

 case of V. urtica (see pp. 394, 395), and support the same 

 conclusions as to the probable existence of a nervous 

 mechanism through which the cuticular colours are 

 created or dismissed in response to the stimulus pro- 

 vided by the light reflected from adjacent surfaces. The 

 pupae are intermediate, tending rather strongly towards 

 the dark side, very strongly in the lowest row of com- 

 partments where the black bands were much broader 

 than the gilt. There was not the shghtest tendency 

 towards a particoloured pupal surface corresponding to 

 the conflicting stimuli, nor was there any difference in the 

 effects when the head or the tail were exposed to either 

 colour. The amount of skin area receiving the reflected 

 light was evidently the decisive condition, the anterior or 

 posterior position of the area being of no importance. 



Eeflecting on these results, it occurred to me that the 

 dorsal or ventral position of the skin area exposed to 

 reflected light might be of more importance ; for when 

 the larvffi rest on some surface, during Stage II., the 

 dorsal half of the body is but slightly exposed to reflected 

 rays as compared with the ventral half. 



In order, therefore, to test the relative susceptibility 

 of dorsal and ventral surfaces, another form of case was 

 constructed. The larvfe wore placed separately in shallow 



