HUMORAL DEPENDENCE OF GROWTH AND 

 DIFFERENTIATION IN INSECTS 



Dietrich Bodenstein 

 Medical Laboratories, Army Chemical Center, Marj'land 



The postembryonic life of insects presents a series of developmental 

 steps interrupted by molts, by means of which the immature organism 

 gradually attains its adult form. The insect undergoes a larval or nymphal 

 molt when it retains its juvenile characteristics. It metamorphoses when 

 adult structures occur after the molt. Molting is usually accompanied by 

 growth, but it also always involves differentiation. Whenever the animal 

 passes from one stage to the next, morphogenetic events of great com- 

 plexity take place. These lead in the case of the skin, for instance, to the 

 deposition of an entirely new cuticle with all its often very complicated 

 structural elements. These processes of growth and differentiation are 

 under the control of hormones. The humoral situation prevailing at any 

 given stage regulates and guides the expression of the developmental char- 

 acters. Obviously the manifestation of these developmental events depends 

 not only on hormones but also on the target organs that respond to these 

 humoral stimuli. In the present account, special emphasis will be given to 

 a discussion of the responsive behavior of the reacting tissue. Those in- 

 terested in other aspects and further details of insect endocrinology may 

 consult the recent reviews by Wigglesworth (1954) and by Bodenstein 

 (1953b and 1954). 



The Humoral Cycle 



Although this paper will concern itself mainly with an analysis of the 

 target material, it is necessary for background information to give a short 

 account of the humoral cycle that triggers and controls the target re- 

 sponses. The humoral mechanism which causes the insect to undergo a 

 larval molt or to metamorphose can be briefly outlined as follows. Prior 

 to each molt, a humoral cycle is set into motion by impulses which in cer- 

 tain cases are known to be nervous in nature. They provoke the secretion 

 of a hormone from special cells in the brain, the so-called neurosecretory 

 cells. Under the influence of this brain hormone, the prothoracic glands 

 become activated and produce hormone. This hormone apparently acts 

 directly on the target organs. Its initial activity causes a wave of prolifera- 

 tion in the epidermal cells and thus sets the stage for molting. Because of 

 this growth-promoting ability, the prothoracic-gland hormone has been 



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