316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



The implantation of the cells of the brain, which produce the molt- 

 ing hormone, into the abdomen of the decapitated Bhodnius, results, 

 as we have seen, in the occurrence of metamorphosis — even in the 

 young insect whose growth is far from complete. This result sug- 

 gests that the head produces a second factor which normally prevents 

 metamorphosis in the young stages. That such a factor exists has 

 been proved. It has been termed the juvenile hormone, and it is 

 secreted by a special gland of internal secretion named the corpus 

 allatum. The corpus allatum lies just behind the brain; it shows 

 some remarkable resemblances to the glandular part of the pituitary 

 gland in marmnals. 



Rhodnius, as we have seen, has five larval stages before it becomes 

 adult. If the corpus allatum is removed from one of the young 

 stages and implanted into the abdomen of a fifth-stage larva, when 

 this molts it turns into a giant or sixth-stage larva instead of under- 

 going metamorphosis to an adult (pi. 2, fig. 3). Even a seventh- 

 stage larva has been produced in this way, and some of the sixth- 

 stage larvae have transformed successfully into giant adults (pi. 2, 

 fig. 1, andcf.pl. 2, fig. 2). 



Conversely, removal of the head of a young Rhodnius when molt- 

 ing is just beginning causes the body to undergo a precocious meta- 

 morphosis. It has not been possible to remove the corpus allatum 

 in the living Rhodnius without undue injury to the head and brain. 

 This has, however, proved possible in other insects, and the results 

 obtained in Rhodnius have been amply confirmed. Stick insects, 

 Dixippus (6), treated in this way begin to lay eggs while still quite 

 small. Cockroaches (7) become prematurely adult. Silkworms 

 (1, 3) turn into tiny pupae which will give rise to tiny moths (pi. 3, 

 figs. 1-3). 



The juvenile hormone is secreted throughout the first four larval 

 stages of Rhodnius. During the fifth stage it is no longer secreted ; 

 the corpus allatum of the fifth stage implanted into the abdomen of 

 another fifth stage does not prevent metamorphosis. In the adult in- 

 sect, however, this hormone is once more produced. Here it is neces- 

 sary for the ripening of the eggs. If the adult female is decapitated 

 after feeding, no eggs are developed ; they are developed normally if 

 the corpus allatum is implanted in the abdomen. If the corpus allatum 

 of the mature adult is transferred to the abdomen of the fifth-stage 

 larva, metamorphosis is prevented and a sixth stage is produced. 



It is evident that these hormones serve only to control the manifesta- 

 tion of characters that are latent within the cells. It is therefore not 

 surprising to find that they are not limited in their action to the insect 

 species from which they have been derived. The blood of a molting 

 Rhodnius will induce molting in a decapitated larva of the related 



