228 ROMANCE OF THE INSECT WORLD CHAP. 



conceal it more effectively. But assuredly on the 

 whole the change must be advantageous to the 



FIG. 43. A <Bird's Excreta mimicking Spider ; from Forbes. 



species, else variations conducing to it would not have 

 been preserved, but would have been neglected, and 

 finally lost. In caterpillars closely allied to Sphinx 

 ligustri, which feed near damp green soil, the 

 assumptionjof brown is far less marked. 



These alterations in colour naturally suggest 

 other differences of appearance known as di-* 

 or polymorphism. The larvae of certain insects 

 regularly appear under two conspicuously distinct 

 forms, chiefly a green and a brown. The cir- 



