Differentiation and Morphogenesis in Insects 251 



The completion of the cocoon signals the beginning of a new and 

 even more remarkable sequence of events. On the third day after the 

 cocoon is finished, a great wave of death and destruction sweeps over 

 the internal organs of the caterpillar. The specialized larval tissues 

 break down. But meanwhile, certain more or less discrete clusters of 

 cells, tucked away here and there within the body, begin to grow 

 rapidly, nourishing themselves on the breakdown products of the 

 dead and dying larval tissues. These are the "imaginal discs" which 

 throughout larval life have been slowly enlarging within the cater- 

 pillar. Their spurt of growth now shapes the organism according to 

 a new plan. The specialized structures of the caterpillar are swept 

 away and replaced by new organs arising from the imaginal discs. In 

 addition, some of the less specialized larval tissues, such as the epi- 

 dermal layer of the abdomen, are transformed directly into pupal 

 tissues. Finally, the old larval cuticle is shed to unmask an essentially 

 new organism, the pupa. All these charfges normally take place within 

 the snug confines of the cocoon. 



Once the pupa has formed, there is an abrupt halt to the events 

 which during a period of 8 weeks have transformed the egg into a 

 larva and the larva into a pupa. During the months that follow, the 

 insect persists in a kind of suspended development — an obligatory 

 dormancy termed "pupal diapause.'" The insect overwinters in this 

 state of developmental standstill. 



Endocrine control of metamorphosis 



The pupal diapause proves to be the result of an endocrine de- 

 ficiency, namely, a failure of the brain to secrete a "brain hormone" 

 required for the continuation of development (Williams, 1946). The 

 brain hormone is the synthetic product of 26 neurosecretory nerve 

 cells (Williams, 1952a). For reasons that are not understood, the 

 brain's neurosecretory cells are somehow turned off at the time of 

 pupation. In order to get turned on again, they require several months 

 of exposure to low temperatures — a need which is obviously met in 

 the overwintering pupa (Van der Kloot, 1955; Williams, 1956b). 

 When the chilled pupa is returned to a warmer temperature (as 

 normally happens in the spring of the year), the brain secretes its 

 hormone and adult development begins. 



By appropriate experiments, it is possible to show that the brain 

 hormone does not exert its effect throughout the insect as a whole. 

 Its primary function is to activate another endocrine organ, the "pro- 



