INSECT GROWTH AND DIFFERENTIATION 201 



served differences in reaction between these two species might be based 

 en differences in the prothoracic-gland hormone concentration during the 

 intermolt periods, for which we have no accurate measurement. If the 

 hormone titer in Rhodnius is relatively high, a rather slight change in the 

 reaction threshold of the Rhodnius epidermis cells could bring about 

 mitosis. 



Hormone Concentration and Growth Rate 



The above considerations bring up the question of whether there is a 

 definite relation between hormone concentration and growth rate. If so, 

 one would expect that the growth rate would be a function of the hormone 

 concentration, within the limits of the growth capacity of an organ. Infor- 

 mation on this is provided by experiments in Drosophila (Bodenstein, 

 1943). Larval organ discs were transplanted into the abdominal cavity of 

 adult male flies, together with a varied number of ring glands. It was 

 found that the growth of a disc was more pronounced in proportion to the 

 number of ring glands which were transplanted together with the disc. In 

 other words, the growth velocity of the discs increased in proportion to the 

 amount of prothoracic-gland hormone present in the system. The hormone 

 produced by four ring glands was sufficient for the expression of the disc's 

 maximal growth capacity, since eight ring glands did not cause more rapid 

 growth than four ring glands. The importance of the hormone concentra- 

 tion as a significant factor in the regulation of growth velocities is thus 

 evident. The dependence of the growth velocity on the concentration of 

 prothoracic-gland hormone is well illustrated in another set of experi- 

 ments. There is good evidence to suggest that the concentration of the 

 hormone is greater in late larval stages than in younger ones. If this is cor- 

 rect, one would suppose that organs taken from younger donors and trans- 

 planted into older hosts would grow better in their new environment. This 

 experiment has been performed on Drosophila, where organ discs of 

 younger larvae were transplanted into the body cavity of older larval hosts 

 (Bodenstein, 1940). As was anticipated, the young discs grew better 

 in the older hosts than in those of their own age group. And vice 

 versa, older discs transplanted into younger hosts were retarded in their 

 growth. 



Growth Regulation 



These experiments not only illuminate the specific subject under dis- 

 cussion ; they also shed some light on the complicated problem of growth 

 regulation. In their new surroundings, the young transplants grow faster 

 than they would have if left in their own environment; they also cease 

 their rapid growth once they have caught up in size with the corresponding 



