734 



Comparative Animal Physiology 



decapitated early in the period show a significant tendency toward prema- 

 ture metamorphosis, while those decapitated later do not do so. This ob- 

 servation has been interpreted as indicating that during the critical period 

 of the first four instars a molting and metamorphosis hormone is first lib- 

 erated and that quickly thereafter a metamorphosis-inhibiting principle is 

 also secreted. The fifth instar alone in Rhodnins fails to produce the latter. 

 The source of the metamorphosis-inhibiting or juvenile hormone is the 

 corpus allatum, a median unpaired gland in Rhodnius located in the pos- 

 terior region of the head. The source of the hormone inducing molting and 

 differentiation is in the pars intercerebralis of the dorsal mid-region of the 

 brain, probably in certain neurosecretory cells shown histologically to be 

 located there. The elongated form of the head of Rhodnius renders it very 



Fig. 280. Experimental telobiosis and parabiosis in the bug, Rhodnius. A, Telobiosis 

 involving only nymphal instars. B, Fourth and fifth instar nymphs united telobiotically 

 and parabiotically with an imago. From Wigglesworth.^*^' "" 



simple to cut the head transversely in such a manner as to retain the corpus 

 allatum while the brain is removed. Utilizing this technique in conjunction 

 with telobiotic experiments, as well as by implanting corpora allata and pars 

 intercerebralis regions of brains into nymphs decapitated before the critical 

 period, one can obtain at will either a nymphal molt or a metamorphosis in 

 Rhodnius. 



The metamorphosis and the juvenile hormones are not species-specific, 

 and the hormones of Rhodnius will induce similar changes in other hemip- 

 terans, such as Triatoma and Cimex. 



Among the orthopterans, Dixippws,^-"- ^-^ Leucophaea,^-^^ and Melano- 

 pitis/-'^ which have been investigated at some length and which also show 

 hemimetabolous development, there is as yet no clear proof of the existence 

 or site of origin of a molting or GD hormone, although it is generally as- 

 sumed one is present. The corpora allata in all, however, produce a juvenile 

 hormone. In Dixippus, which normally has six nymphal stages, removal of 

 the corpora allata in the third nymphal stage results in a premature meta- 

 morphosis, although two nymphal molts usually intervene between the op- 



