300 Mr. E. B. Poulton. Essential Nature of the [Apr. 23, 



than the 8. Rubra, &c, the food of the yellowest variety was changed, 

 and it was fed upon apple from the beginning of the fifth stage 

 (August 14th. The larvae were hatched July 17 — 18). 



By August 23rd most of the larvae were full-fed, and a careful com- 

 parison was made, with the following results : — 



Apple. — The five adult larvae are quite typical whitish-green 

 forms. 



Crab. — Only one of the five larvae is nearly adult, the others far 

 behind. The larvae were hatched a day earlier than any of the others 

 (July 15), and yet they have been backward all through. This is 

 probably an effect of the food. The larvae are almost as white as 

 the others (becoming quite so afterwards). 



8. Viminalis. — The four adult larvae are not so white as the apple 

 or crab, but are almost intermediate forms. 



8. Cinerea. — The four larvae may be called intermediate, and they 

 much resemble the 8. Viminalis forms, but are as yet younger and 

 rather smaller (the tendencies towards the two varieties slightly in- 

 creasing with growth) . 



8. Rubra. — The four adult larvae are far more different from the 

 apple varieties than any of the others, but they are not greatly beyond 

 intermediate forms. The one fed upon apple for nine days was the 

 brightest of all ; it is now rather whiter than the others, which were 

 still fed on 8. Rubra, although it does not approach the apple or 

 crab larvae. But any change at all (as proved by the experiments to 

 be described) in so short a time shows the very strong tendency of the 

 whole of these larvae towards the white variety. On August 26th 

 this larva was adult without further recognisable change in colour. 



This then was the result of the experiments. No one who com- 

 pared the 8. Rubra with the apple larvae could hesitate for a mo- 

 ment in coming to the conclusion that the ground colour is largely 

 affected by the food-plant. But at the same time there was no doubt 

 that this is not the whole explanation of the differences observed in 

 larvae in the field, for my 8. Rubra and 8. Cinerea larvae were not 

 yellow-green varieties, but only intermediate. A strong tendency was 

 ^manifested in the larvae all through, towards the white variety, and 

 the food-plant could only overcome this to the extent of producing an 

 intermediate form. 



16. Observation in the Field upon Larvoe of S. Ocellatus during 1884. 



I will now describe the results of my observations in the field during 

 the past summer, which throw great light upon this question. The 

 larvae of S. Ocellatus were abundant last year, and I will give a list of 

 all found to indicate the proportion of forms which were of excep- 

 tional colour (in l elation to their food-plants). 



