204 INVERTEBRATE PHYSIOLOGY 



Perhaps a higher titer of hormone is needed to induce growth in the adult 

 than in the larval insect. Experiments on this point are not clear. It is 

 certain that regeneration is more easily induced in larval than in adult 

 animals (Bodenstein, 1955). However, mitosis in the epidermis of adult 

 Rhodnhis can be induced simply by wounding (Wigglesworth, 1937). 

 In this case, mitotic activity was brought about in the apparent absence 

 of prothoracic-gland hormone. This is evidence that under certain con- 

 ditions, mitotic activity may occur even in the absence of this hormone ; 

 there are other instances with like implication. These facts cannot be denied 

 and we have no intention of minimizing their importance. Yet the role 

 of the prothoracic-gland hormone for growth is a vital one, and it is this 

 aspect of the problem of growth which we have emphasized in the above 

 discussion. 



Differentiation 

 Age and Imaginal Differentiation 



It is a curious fact that growth and imaginal differentiation in insects 

 are controlled by the same hormone — the prothoracic-gland hormone. 

 Whether an organ responds with growth, or with differentiation, depends 

 on the age of the organ and on the hormone concentration to which the 

 organ is exposed. For instance, in the humoral environment of a late last- 

 stage Drosophila larva, an eye disc taken from a young last-stage larva will 

 begin its imaginal differentiation at the same time as do the host tissues — 

 that is, prematurely. But if this eye disc had been left in its own young 

 donor, it would have continued to grow. Still younger discs transplanted 

 into mature larvae are unable to differentiate synchronously with the host 

 (Bodenstein, 1939). Similarly, the genital anlage of a fourth-stage cock- 

 roach nymph transplanted into a last-stage nymph is unable to differentiate 

 its imaginal structures at the metamorphosis of the host (Bodenstein, 

 1953a). The same is true for the wing buds of a first-stage Rhodnius 

 nymph transplanted in the same manner (Wigglesworth, 1934). These 

 young Rhodnius can be made to metamorphose prematurely into minia- 

 ture adults, but their wing buds fail to differentiate to imaginal com- 

 pletion. 



On the other hand, other structures, as for example the integument, 

 appear ready at all times during the postembryonic period to differentiate 

 adult structures if supplied with the appropriate hormone stimulus. One 

 is able to make the skin of newly hatched caterpillars form imaginal skin, 

 bearing scales, by transplanting it into a mature caterpillar (Piepho, 

 1938). Thus, in synchrony with the differentiation of the host, the trans- 

 planted young skin will differentiate prematurely into adult skin. The 



