INSECT GROWTH AND DIFFERENTIATION 205 



above-mentioned prematurely metamorphosed Rhodnius nymph which 

 gave rise to a dwarf imago with adult integumental structures is another 

 example. Here again, we witness the same phenomenon which we already 

 encountered in the growth response — namely, that age is an important 

 factor in differentiation, and that different tissues possess different thresh- 

 olds of responsiveness. 



Differentiation and Hormone Titer 



Not all tissues respond alike to the same hormone titer. If in a last-stage 

 lepidopterous larva the production of the prothoracic-gland hormone is 

 cut short by the application of a ligature before much hormone has been 

 given off into the blood, the skin will respond with pupal differentiation 

 only at certain body regions. More pupal cuticle will be formed when the 

 ligature is applied later in development. Finally, if the hormone concen- 

 tration is high enough, the entire cuticle of the caterpillar will pupate 

 (Kiihn and Piepho, 1938). Or, when in Drosophila a leg and genital disc 

 are implanted together into the abdominal cavity of an adult host, and the 

 differentiation of these transplants is then initiated by the transplantation 

 of the ring glands, only the leg disc completes its imaginal differentiation, 

 while the genital disc remains pupal. Even the various regions of an organ 

 may differ in their competence to respond w-ith differentiation to the pro- 

 thoracic-gland hormone. In the presence of a rather low hormone titer, 

 only the tarsal segments of a Drosophila leg disc differentiate to imaginal 

 completion, while the rest of the leg structures remain pupal (Bodenstein, 

 1943). More examples could be added, all suggesting that certain cells 

 respond more easily with differentiation than others to the same hor- 

 mone concentration. 



Differentiation of Adult Structures 



The amazing plasticity of the target material is best illustrated in the 

 redifferentiation of adult structures. As before mentioned, adult insects 

 can be made to molt again by supplying them wath prothoracic-gland hor- 

 mone. Under these conditions, the adult individual is able to develop 

 almost all its imaginal characters anew. Not only is the entire imaginal 

 skin redifferentiated, but also the differentiation of such complicated 

 structures as the external genital apparatus, scales, sense organs, etc. is 

 completed to perfection. Not once, but several times is the adult skin able 

 to accomplish this task. Although the adult tissues still possess great de- 

 velopmental capacities, there are certain restrictions, and again the extent 

 of the restrictions seems to vary with the type of species and tissues in- 

 volved. For example, the mature cockroach wing is a structure which the 

 adult organism is unable to rebuild (Bodenstein, unpublished) ; and yet 



