SLIME SLUGS, MYRIAPODS AND INSECTS 151 



It is in this stage that most of the radical changes in structure 

 are undergone which are necessary to make the moth or 

 butterfly out of the caterpillar, the house-fly out of the maggot, 

 or the beetle out of the grub. 



However, most of these changes have their beginnings during 

 the larval stage. For example, little wing buds have been 

 developing all through the larval life, but they have remained 

 very small and invisible underneath the chitinized cuticle of the 

 larva. Especially in the last few days of the larval stage are the 

 various changes going on rapidly. But they are so radical in 



FIG. 66. Metamorphosis, complete, of Monarch butterfly, Anosia 

 plcxippns. a, Egg (greatly magnified); b, caterpillar or larva; c, chrys- 

 alid or pupa; d, adult or imago. (| natural size; after Jordan and 

 Kellogg.) 



character that it is impossible for the insect to maintain an 

 active food-getting life and make the changes at the same time. 

 Hence comes the necessity for the quiescent pupal stage in 

 which the insect, living on food stored up as fat by the larva, 

 and safely inclosed in a hard protecting shell, can make the 

 great changes necessary to its becoming a fully developed 

 winged imago, different as to mouth-parts, eyes and antennae, 

 different as to body shape, different as to legs and abdominal 

 appendages, and, together with all these structural differences, 

 radically different as to habits and behavior. Many of the 



