206 INVERTEBRATE PHYSIOLOGY 



the imaginal scale-bearing cuticle of the adult wax moth can be made to 

 form scales again (Wiedbrauck, 1953). The scales of the new imaginal 

 skin develop from the original scale-forming cells. These secondary scales 

 are normal in size but simpler in form. Apparently, the formative cells have 

 gradually lost their ability to form normally shaped scales. On the con- 

 trary, this adult cockroach integument loses none of its complicated archi- 

 tecture even at successive extra-imaginal molts (Bodenstein, 1953a). 

 Even a small feature characteristic of the adult skin, namely the loss of 

 the molting sutures in the head and thoracic skin, is not restored in the 

 molting of the adult. The adult is thus unable to get out of its exuviae. 



Larval Versus Imaginal Differentiation 



The reader must recall an earlier statement of the utmost importance 

 for the understanding of the following comments. It has been said that 

 differentiation takes place not only in the development of the adult organ- 

 ism, but that complicated processes of differentiation also occur at each 

 larval molt, when, from instar to instar, larval characteristics of great 

 specificity are laid down anew. Larval differentiation is thus distinct from 

 imaginal differentiation. The presence not only of prothoracic-gland hor- 

 mone but also of corpus allatum (juvenile) hormone is necessary for 

 larval differentiation. The latter apparently alters the course of differen- 

 tiation, leading it toward larval characteristics. Imaginal differentiation 

 ensues at metamorphosis, because at the last larval stage the allatum hor- 

 mone titer in the organism is too low to support larval differentiation 

 effectively. The latter occurs at the larval molts, because at these stages 

 the allatum hormone titer is higher in relation to the prothoracic-gland 

 hormone titer than it is at the last larval stage. It is possible experimentally 

 to prevent the developmental system of the last larval stage from meta- 

 morphosis, i.e., from imaginal differentiation, by supplying the organism 

 anew with allatum hormone through transplantation of larval corpora 

 allata. Under these conditions the reacting tissues are forced to respond 

 with larval differentiation and an extra larval molt occurs. Many experi- 

 ments of this kind, by which supernumerary larval molts have been in- 

 duced, attest to the validity of this concept. They also prove that the tissues 

 of the final larval stage are still able to respond to the allatum hormone with 

 larval differentiation. 



The question now arises: Is the target material of all stages equally 

 responsive to the allatum hormone or is its responsive threshold to this 

 hormone gradually lowered as the tissues become older ? Transplantation 

 of adult cockroach skin into younger nymphal stages demonstrates that 

 the capacity of this tissue to respond to the allatum hormone has appar- 

 ently been lost, or considerably reduced. Although the grafted pieces of 



