INSECT GROWTH AND DIFFERENTIATION 207 



skin may molt as many as three times with their nymphal host, they retain 

 their adult characteristics regardless of the presence of allatum hormone 

 in the nymphal host (Bodenstein, 1953a). Adult structures of most in- 

 sects tested in this manner show a similar behavior. In order to cause adult 

 skin to revert into larval skin, a much higher titer of allatum hormone 

 is apparently necessary than that prevailing in normal stages. Such a high 

 titer was provided for the adult tissues in an experiment on Rhodnins 

 (Wiggles worth, 1940). When the adult insect was supplied with a great 

 number of corpora allata, the adult skin could at least partially revert to 

 nymphal skin. 



However, the skin of not all adult insects has this low responsive thresh- 

 old. The skin of the adult wax moth was exposed to humoral systems con- 

 taining different amounts of allatum hormone (Weidbrauck, 1953) . When 

 exposed to a larval system with its high allatum hormone titer, adult skin 

 occasionally reverted to larval skin. In a lower allatum hormone titer, 

 pupal cuticle was often formed, while the adult structures were repeated 

 most frequently in an even lower allatum hormone titer. Since larval skin 

 always responds with larval differentiation in a larval humoral environ- 

 ment, w^hile in the same environment adult skin shows this response only 

 occasionally, it follows that the target gradually loses its responsive capa- 

 city to the allatum hormone with age. But adult tissues may not entirely 

 lose their capacity to respond with larval differentiation. They merely 

 need a higher allatum hormone titer than the younger ones to elicit this 

 response. We have seen that the responsive capacity of the targets to the 

 prothoracic-gland hormone increases with age. This implies that the tar- 

 get systems change their responsive competence in the course of their de- 

 velopment. Younger tissues respond with greater ease to the allatum hor- 

 mone, but older tissues with greater ease to the prothoracic-gland hor- 

 mone. 



Furthermore, it must be noted that, even within the same individual, 

 dift'erent body regions vary in their reaction thresholds to the allatum 

 hormone. This point may be illustrated by some recent experiments on 

 moth larvae (Piepho and Heims, 1952). The removal of the corpora allata 

 from young caterpillars at the beginning of the intermolt period results at 

 the subsequent molt in the formation of pupal skin. Obviously this oper- 

 ation has not left enough allatum hormone in the organism to support 

 larval differentiation. If the allata are removed somewhat later in the in- 

 termolt period, when the allatum hormone titer is expected to be higher, 

 then the postoperative molt is a partially pupal molt. Only certain regions 

 of the cuticle show pupal characters, while the greater part of the skin has 

 differentiated larval structures. Thus not all regions of the cuticle exhibit 

 the same response to a given allatum hormone concentration. Essen- 



