36 INTRODUCTION TO ZOOLOGY. 



other mouth parts may be found on the butterfly, but 

 these organs never become functional in the imago. 

 They are said to be vestigial. In the Samia cecropia, 

 and other moths of the family Bombycidse, all the 

 mouth parts are rudimentary or vestigial. They are 

 unable to eat anything while in the imago state. 

 They do not live ; they die. The Samia cecropia will 

 die within about five days after it comes from the 

 cocoon. Its eating has all been done by the caterpil- 

 lar. The only function of the imago is to lay eggs and 

 reproduce the species. We shall find many examples 

 of vestigial organs. They seem to be organs which are 

 retrograding, or having once been functional in the 

 history of the species, have ceased to be so. 



Anosia plexippus furnishes a good example of the 

 form of protective coloration that is called mimicry. 

 The butterfly itself is not concealed by its color. It is 

 very conspicuous. But it is quite distasteful to birds 

 and other insect-eating animals. Its conspicuousness 

 is an advantage to it, for when the birds know what 

 it is they will let it alone. 



But there is another butterfly, Basilarchia archip- 

 pus, which closely resembles the plexippus in color. 

 It can be distinguished by a black streak across the 

 hind wing. This butterfly is not at all distasteful to 

 birds ; but birds, being deceived by its resemblance to 

 plexippus, seldom catch it. The archippus is said to 

 resemble, or imitate, or mimic the plexippus. Struc- 

 turally, the two butterflies are very different, and no 

 other species of the genus Basilarchia is colored like 

 this one. Numerous examples of mimicry may be 

 found among insects. 



All silk of commerce is produced by the caterpillar 

 of the silk worm (Bombyx mori). The cocoon of the 

 Samia cecropia is larger and contains more silk, but 

 it cannot be reeled, in consequence of being fastened to 

 a twig or some other solid support. 



