Differentiation and Morphogenesis in Insects 255 



The jurenilo hormone 



This conservative force proves to be yet another hormone, the so- 

 called "juvenile hormone." As demonstrated by Wigglesworth ( 1936) , 

 the juvenile hormone is secreted by a tiny pair of cephalic glands, the 

 corpora allata. 



The action of juvenile hormone is to modify the cellular response 

 to ecdyson — to suppress new synthetic acts without interfering with 

 the use and reuse of the information already at the disposal of the 

 cells (Williams, 1959, 1961a, b). If this "brake" on progressive dif- 

 ferentiation is removed by the excision of its source in the corpora 

 allata, then the immature larva reacts to ecdyson by undergoing meta- 

 morphosis into a midget pupa and adult (Bounhiol, 1938; Fukuda. 

 1944; Williams, 1961a, b ) . What we witness, in short, is the precocious 

 acting out of the life plan. 



Under normal conditions and circumstances, pupation becomes 

 possible late in larval life when the corpora allata undergo a progres- 

 sive loss of their endocrine activity. Ecdyson now acts in the presence 

 of low concentrations of juvenile hormone to cause the de-repression 

 of the fresh genetic information for constructing the pupa (Williams, 

 1961b ) . But in many insects one can reapply the "brakes" by im- 

 planting active corpora allata obtained from younger larvae. When 

 juvenile hormone is supplied, the mature larva can postpone its 

 metamorphosis and continue to grow as a larva. The net result is that 

 the insect after one or more extra larval instars finally transforms 

 into a giant pupa and adult (Wigglesworth, 1954). 



Equally spectacular is the effect of juvenile hormone on the trans- 

 formation of a pupa into an adult moth, a phase of metamorphosis 

 which normally takes place in the presence of ecdyson and what ap- 

 pears to be the total absence of juvenile hormone. If juvenile hor- 

 mone is supplied by the implantation of active corpora allata, the 

 pupa is prevented from turning into an adult moth. To varying de- 

 grees, as dictated by the concentration of juvenile hormone, the 

 formation of the adult moth is inhibited. In the presence of a suffi- 

 ciently high titer of juvenile hormone, the pupa responds to ecdyson 

 by molting into a second pupal stage (Williams, 1959). 



By the use of the pupal assay, the juvenile hormone has been 

 extracted and extensively purified. It proves to be a water-insoluble 

 oil containing numerous methyl and methylene functions and what 

 appears to be a lactone ring (Williams, 1956a. 1961a). Meanwhile, 

 the German investigator, Schmialek (1961). has been able to mimic 

 the action of juvenile hormone by the injection of the 15-carbon 



