INTRODUCTORY. 21 



Where the metamorphosis is considerable, as for example 

 among the Lepidoptera and Amphibia, the structural differ- 

 ences are so great between the " larva " and the " imago " that 

 they are not fitted to lead a precisely similar life. The food 

 may be perfectly different : thus, the larvae or caterpillars of 

 Lepidoptera usually feed upon leaves, while the imago the 

 butterfly or moth can only suck juices through its long 

 proboscis. The tadpole of the common frog feeds upon decay- 

 ing vegetable and animal matter, while the frog itself is 

 insectivorous. Seeing that pigment has been proved in so 

 many cases to be alterable by changes in the food, it is not 

 surprising to find that as a rule the colours of larva? are totally 

 different from those of the adult form. There is no indication 

 of the gorgeous coloration of the Peacock or Red Admiral 

 butterfly in the dusky greenish larvre of these insects ; here, of 

 course, the change of colour is related to active internal changes 

 combined with a cessation from feeding. 



Indeed, other causes besides food may contribute to these 

 differences of colour and coloration ; but it is sufficient for the 

 present purpose to mention the fact that the differences exist, 

 furiously enough, the rule is not without exceptions. The 

 common Magpie moth has a coloration which is very similar 

 in both the larva condition and in the perfect state ; even the 

 chrysalis is not unlike the caterpillar : spots and blotches of 

 black and yellow on a whitish ground characterise the moth 

 and the caterpillar ; the pupa is dark brown with yellowish 

 rings. 



Some green moths such as, for example, Halias quercana, 



-have green larvas. These cases are less striking than that 



afforded by the Magpie moth, since green is so common a 



colour in nature. But resemblances of the kind shown by the 



caterpillar and imago of the Magpie moth are so very rare 



