640 



Metamorphosis 



Fig. 224. Parabiotic union in moth pupae. A,B, Brainless pupa of Platysamia cecropia grafted to a 

 chilled pupa of the same species. The brain of the chilled parabiont provides hormone necessary for the 

 development of both animals. C,D, Brainless diapausing pupa of Telea polyphemus grafted to a chilled 

 pupa of P. cecropia. The successful development of both pupae shows that the brain factor is not species 

 specific. This fact also shown by E, which is adult of P. cecropia developed from a brainless pupa which 

 had received the brain from a chilled pupa of T. polyphemus. (From C. D. Turner, General Endo- 

 crinology.) 



results from a cessation of brain activity in 

 the pupa and the consequent necessity of 

 reactivation of the brain by exposure to 

 cold. 



INSECTS IN GENERAL 



We may now consider whether the endo- 

 crine mechanism discussed above for Rhod- 

 nius and Platysamia is applicable to insects 

 in general. Before doing so, however, a note 

 on terminology is necessary. As experiment- 

 ers have uncovered various factors in insect 

 metamorphosis, names descriptive of their 

 actions have usually been given to these 

 factors. Such names are generally rather 

 unsatisfactory, often being vague, too gen- 



eral, too restricted or sometimes even imply- 

 ing a mode of action later found incorrect. 

 When the source of a factor has been traced 

 to a particular organ it is generally more 

 satisfactory to name it in terms of that organ, 

 and such terminology is favored in this 

 discussion. The term "hormone" will be used 

 for these factors since in general they seem 

 to be hormone-like in their activity. How- 

 ever, as will be seen below, the exact cor- 

 respondence to a hormone as understood in 

 vertebrate physiology is open to some ques- 

 tion. The following terms, their definitions 

 and synonomy are important. 



Brain Hormone. An agent produced by the 

 protocerebrum, presumably by the neuro- 

 secretory cells contained therein. It acts by 



