CABBAGE WHITE BUTTERFLIES 175 



reasons for a resting stage. Similarity of food in all 

 stages of growth, and abundance of food, which is 

 easily found and easily appropriated, are circum- 

 stances which render a resting stage less necessary. 



We must now turn back and study the formation 

 of the new organs beneath the larval skin. Many 

 larvae have to be sacrificed, and innumerable sections 

 examined to make out the whole story, but we shall 

 be satisfied here with learning the general plan of 

 development. At the time of the fourth moult there 

 is no indication of parts differing from those of the 

 larva. But as soon as the last larval skin but one 

 has been cast, a new skin, which we shall name the 

 pupal skin, begins to form beneath the last larval 

 skin. The new skin is not exactly moulded upon its 

 predecessor, but pushed inwards here and outwards 

 there. Where considerable prominences are to form, 

 the infoldings are deep, and from their innermost 

 extremities outward-directed folds project, which are 

 shaped in some cases like glove-fingers, in others like 

 pockets. These hollow folds are wings, legs, antennae 

 and other appendages, telescoped into the interior of 

 the body, because the corresponding parts of the larva 

 are not large enough to contain them. They are 

 often much bent and crumpled, but in a methodical 

 way, as the perfect symmetry of the two sides of the 

 body shows. Either from the first or after a short 

 interval, a second skin forms within the pupal skin ; 

 this is the imaginal skin. As the imaginal skin and 

 its complex folds develop, the pupal skin ceases to 

 grow. It is not cast or ruptured at present, but it 

 becomes a mere passive envelope, which takes accu- 



