198 INVERTEBRATE PHYSIOLOGY 



called a growth hormone. As the titer of the hormone gradually rises, 

 under the influence of this hormone alone, the target organs respond by 

 differentiation of imaginal structures. For example, the epidermis will lay 

 down the complicated imaginal cuticular pattern under the influence of a 

 sufficient hormone titer. Since it inaugurates imaginal differentiation, 

 the prothoracic-gland hormone has also been called a differentiation hor- 

 mone. Thus growth and imaginal differentiation are caused by the same 

 hormone. 



The sequence of events is different when the production of the pro- 

 thoracic-gland hormone is followed by the release of a second hormone 

 from the corpus allatum. In the presence of the allatum hormone, the re- 

 sponse of the targets is modified. After the mitotic wave induced by the 

 prothoracic-gland hormone has run its course, the allatum hormone causes 

 the targets to form larval structures ; thus the allatum hormone is respon- 

 sible for larval differentiation and in its presence a larval molt ensues. 

 Since the juvenile, or larval, characters are maintained by the presence 

 of the allatum hormone, the latter has been named the juvenile hormone. 

 During the entire larval life of the insect, the allatum hormone is present 

 in sufficient titer to cause larval molts. Only in the last larval stage is the 

 allatum hormone titer too low in relation to that of the prothoracic-gland 

 hormone to make its effects felt ; under these conditions, the animal meta- 

 morphoses, that is, the targets differentiate into imaginal structures. One 

 will notice that the development of larval features is controlled by the com- 

 bined action of these two hormones. As a matter of fact, the allatum hor- 

 mone can make its effects felt only in the presence of the prothoracic-gland 

 hormone, for it is the latter that sets the molting process into motion. The 

 special humoral balance prevailing at any time and the state of responsive- 

 ness of the target organ to this balanced hormone system together deter- 

 mine the features of the insect characteristic of any stage. 



Growth 



Growth in insects is often cyclic. At definite time intervals the immature 

 insect molts. Molting can be regarded as the visible expression of growth. 

 By the term "growth" we mean cell multiplication, unless otherwise indi- 

 cated. Now it has been known for a long time that the first perceptible indi- 

 cation of molting is the occurrence of a mitotic wave in the epidermal tar- 

 get, which is followed by differentiation. At each molt, therefore, the target 

 exhibits two main developmental reactions, namely growth and differen- 

 tiation. Only for didactic reasons will these two developmental events be 

 treated separately. Actually, both are closely related and often occur 

 simultaneously. We assume that cell multiplication can only occur in a 

 hormone-conditioned environment, which at the molting time reaches its 



