Hormones and the Metamorphosis 

 of Insects^ 



By V. B. WiGGLESWORTH 



Department of Zoology, University of Cambridge, England 



[With 4 plates] 



"Those strange and mystical transmigrations tliat I 

 have observed in Silk-worms," wrote Sir Thomas 

 Browne, "turned my Philosophy into Divinity. There 

 is in these works of nature, which seem to puzzle 

 reason, something Divine, and hath more in it than the 

 eye of a common spectator doth discover," Who, 

 indeed, he says, can fail to wonder "at the operation 

 of two Souls in those little Bodies?" 



The contemplation of the metamorphosis of insects has always 

 evoked feelings of mystery. When regarded more closely through 

 the eyes of the anatomist and the experimental biologist, the super- 

 ficial mystery is dispelled — to be replaced by deeper mysteries. 



Even in that extreme example, metamorphosis in the Lepidoptera, 

 where the caterpillar is transformed into the chrysalis and the butter- 

 fly, the rudiments of the organs of the adult or imago — the wings and 

 legs and so forth — are already present in the young larva as clusters 

 of undifferentiated cells, the so-called imaginal disks. Throughout 

 the larval life of these insects (the "endopterygote" insects) the wing 

 germs grow inward and do not become apparent until they are everted 

 at pupation. 



The fact remains, however, that the strictly adult structures play 

 no functional part in larval life. Whereas the form of the caterpillar 

 becomes fully differentiated before it hatches from the egg, the adult 

 insect persists in an embryonic state until the growth of the caterpillar 

 is complete and metamorphosis takes place. Indeed, the caterpillar 

 is not a walking embryo, as some authors have contended, but a fully 

 differentiated organism which contains within it, in an embryonic 

 state, the adult butterfly. Metamorphosis consists in the dissolution of 



' Reprinted by permission from Endeavour, vol. 10, No. 37, January 1951. 



313 



