14 PEEFACE. 



rations for shedding their skins to pass into the 

 chrysalis stage of growth. Eor this they spin a few- 

 loose threads of silk on the plant itself, which they 

 rarely forsake for that purpose. Within this light 

 weh the last larva-skin is thrown off, and the brown 

 chrysalis-skin is exposed. In this state the worm 

 passes from a week to ten days. During this time, 

 although appearing quiet outwardly, and without 

 exterior organs of locomotion, growth takes place 

 within the shell of the chrysalis. At last it has 

 progressed so far that it arrives at maturity. 

 Through an opening of the head and thorax of the 

 chrysalis the full-grown cotton-fly or moth appears, 

 its wings merely little pads at the sides. These are 

 quickly expanded by a muscular action, and by a 

 circulation in the veins of the wing, which ceases so 

 soon as the wings are dried in the sun. The body 

 is now found to be covered with scales ; the wings 

 cover the body so much, that at first we cannot see 

 that it is, after all, the same animal which we knew 

 first as a larva. But the three portions of the body 

 may be seen. The head has two long jointed an- 

 tennae or feelers. The jointed maxillse have become 

 a spiral tongue. The thorax supports its six legs as 

 before, while the fleshy or false legs of the abdomen 

 have disappeared, as being of no further use to the 

 animal. The insect is now mature, and in a con- 

 dition to commence its work of propagating its 

 young. 



The migrations of animals are among the most 

 important circumstances affecting tbe forms of life. 

 Wallace and Wagner have already shown how the 



