PART III. THE METAMORPHOSES OF INSECTS 



WE have seen that the embryo rapidly passes through extraordi- 

 nary changes of form, and now, after hatching, especially in the 

 insects with a complete metamorphosis, the animal continues to 

 undergo striking changes in form, in adaptation to different modes 

 of life. 



The life of a winged insect, such as a butterfly, fly, or bee, may 

 be divided into four stages: the embryo, or egg state, the larva, 

 pupa, and imago, the term metamorphosis being applied to the 

 changes after birth, or post-embryonic stages of life. The trans- 

 formations of the more specialized orders of insects involve wonder- 

 ful changes of form, which are only paralleled in other types of 

 animals by the metamorphoses of the echinoderms, of certain worms, 

 and of the Crustacea, as well as by those of the frog. An insect, 

 such as a butterfly or bee, during its post-embryonic life lives, so to 

 speak, three different lives, having distinct bodily structures and 

 existing under quite dissimilar surroundings and habits ; so that a 

 caterpillar is practically a different animal from the pupa, and the 

 latter from the imago, with different organs, the appendages and 

 other structures being so modified as to be, so far as regards their 

 functions, radically different. These changes of functions or of 

 habits have also been plainly enough the exciting cause of the diver- 

 gency in structure of what fundamentally is one and the same organ, 

 the change having been brought about by adaptation of the s,ame 

 organs to quite different uses. 



The changes are not only observable in the body and its append- 

 ages, but also in the internal organs, and consequently are both 

 structural and physiological. The term larva, as applied to the 

 first stage of animals, is a very variable and indefinite one, that of 

 insects in general being a much more highly organized animal than 

 the larva of a worm, starfish, or crustacean. 



a. The nymph as distinguished from the larval stage 



As there is no marked difference between the different stages of 

 the young in the insects with an incomplete metamorphosis (Hetero- 

 2 Q 593 



