1 88 DARWINISM AND THE PROBLEMS OF LIFE 



their form, which protects them so well. Gradually 

 only two casts remained for the transformation into the 

 totally different imago. All the organs of the imago 

 had to be formed in this short period. But this meant 

 such a revolution in the animal's frame that the vital 

 functions, movement and nutrition, were impossible. 

 Hence we see that the animal remains, during the 

 stage between these two casts, in a state of immobility 

 that may be compared to that of the ovum. We call 

 this the pupa. In the last cast of skin, the pupa 

 covering is thrown off, and the wings, which were 

 formed under it, make their appearance fully developed. 



Thus bees, flies, beetles, and butterflies only grow in 

 the larva stage, as caterpillars or grubs. If we take two 

 beetles that seem absolutely like each other, and only 

 differ in size, they are not a younger and an older 

 animal, but different species. 



The material for studying the adaptation of larvae is 

 inexhaustible. When the butterfly-caterpillars leave the 

 ^^^ they are generally green, and difficult to distinguish 

 from the leaf they are on. Identical colour of this kind 

 can, however, only protect small animals ; larger ones 

 are sure to attract attention on the grass or the leaves, 

 because there is no purely green spot large enough to 

 cover them. As a matter of fact, the caterpillars of 

 the grass-butterfly (satyridae), when they are of a certain 

 size, have light and dark longitudinal stripes over their 

 whole body, and thus lie in the same direction as the 

 blades of grass and the shadows, which are always 

 vertical on the grass. The caterpillars of the hawk- 

 moth, which live on bushes and trees, have stripes 



