METAMORPHOSIS I 5 5 



able are to be found in literature : nevertheless the observations 

 that have been made as to metamorphosis, and the analysis that 

 has been commenced of the facts are at present little more than 

 sufficient to show us how vast and complex is the subject, and 

 how great are the difficulties it presents. 



There are three great fields of inquiry in regard to meta- 

 morphosis, viz. (1) the external form at the different stages ; 

 (2) the internal organs and their changes; (3) the physiological 

 processes. Of these only the first has yet received any extensive 

 attention, though it is the third that precedes or underlies the 

 other two, and is the most important. We will say a few words 

 about each of these departments of the inquiry. Taking first 

 the external form the instar. But before turning to this we 

 must point out that in limiting the inquiry to the post-embryonic 

 development, we are making one of those limitations that give rise 

 to much misconception, though they are necessary for the acquisi- 

 tion of knowledge as to any complex set of phenomena. If we 

 assume five well-marked stages as constituting the life of an Insect 

 with extreme metamorphosis, viz. (1) the formation and growth of 

 the egg ; (2) the changes in the egg culminating in its hatching 

 after fertilisation ; (3) the period of growth ; (4) the pupal 

 changes ; (5) the life of the perfect Insect ; and if we limit our 

 inquiry about development to the latter three, we are then 

 shutting out of view a great preliminary question, viz. whether 

 some Insects leave the egg in a different stage of development to 

 others, and we are consequently exposing ourselves to the risk 

 of forgetting that some of the distinctions we observe in the 

 subsequent metamorphosis may be consequential 011 differences in 

 the embryonic development. 



Instar and Stadium. 



Figs. 84 and 85 represent corresponding stages in the life 

 of two different Insects, Fig. 84 showing a locust (Acridiuni}, 

 and Fig. 85 a white butterfly. In each A represents the 

 newly-hatched individual ; B, the insect just before its perfect 

 state ; C, the perfect or imago stage. On comparing the two sets 

 of figures we see that the C stages correspond pretty well as 

 regards the most important features (the position of the wings 

 being unimportant), that the A stages are moderately different, 



