Endocrine Mechanisms 735 



eration and the metamorphosis. This latter is interpreted to mean that the 

 hormone from the gland is stored somewhere in the body over this period 

 Transplantation of corpora allata from third or fourth instar nymphs into 

 sixth instar nymphs of Dixi-pfus may produce giant imagos which have un- 

 dergone as many as three or four additional nymphal molts. In Leucofhaea 

 which normally possess eight nymphal instars allatectomy of a seventh in- 

 star before the critical period results in an adult-like stage following the 

 molt. Such an individual has been termed an "adultoid" and differs from 

 adults in having a smaller size and shorter wings. Allatectomy in fifth or 

 sixth instars results on molt in "pre-adultoids" which require one additional 

 molt to produce "adultoids." 



In the holometabolous dipterans the ring gland of the larva, located dors- 

 ally to the brain and between the brain lobes, is the source of a hormone 

 inducing pupation.^''- ^^ The role of this gland is readily demonstrated by 

 ligating the larva to constrict it into two portions, one containing the brain, 

 and the other without it."^ If this operation is performed before a certain 

 "critical period," only that portion containing the ring gland pupates; if 

 constriction is produced after the critical period, both portions pupate. 

 Transplantation of a ring gland from a last larval instar into a first larval 

 instar produces premature pupation in the latter. Ring gland implants will 

 also induce pupation in a portion of a larva cut off by ligation from its own 

 ring gland before the critical period. This hormone appears also to be the 

 effective one operating in metamorphosis as well as in pupation itself.^^ 

 Imaginal discs implanted into the hemocoele of adult flies will not differenti- 

 ate unless ring glands of late larvae are also implanted.^*^ Furthermore, the 

 metamorphosis hormone of Lepidoptera will induce pupation in dipterans.-^ 



The corpora allata of the Lepidoptera, like those of the hemipterans and 

 orthopterans, are the source of a juvenile hormone. Allatectomy in caterpil- 

 lars of younger instars is followed by a premature pupation, and, conversely, 

 implantation of corpora allata from early instars into caterpillars ready to 

 pupate will significandy delay the pupation.^'* 



From the time of the original experiments of Kopec^^ on lepidopteran pu- 

 pation and metamorphosis it has been known that at a certain "critical period" 

 prior to pupation a hormone is liberated from the anterior portion of the 

 larva and that this is essential for pupation and metamorphosis. In the silk- 

 worm moth, Bomhyx, this hormone is produced in the prothoracic gland 

 located in the prothoracic segment.^^ jf j^g prothoracic segment is cut off by 

 ligation from the more posterior regions of the body immediately after pupa- 

 tion^ the posterior portion fails to develop further. If the constriction is made 

 12 to 18 hours after pupation the posterior portion metamorphoses normally. 

 If prothoracic gland is implanted into the posterior portion of a pupa ligated 

 before the 12 to 18 hour "critical period" metamorphosis of this part also 

 proceeds normally; or an abdomen cut off by constriction may be induced to 

 metamorphose by connection of its hemolymph cavity with that of a normal 

 metamorphosing specimen, even when the connection is made by way of 

 a glass capillary tube. 



In the giant silkworm moth, Platysamia, after pupation, a dormant period of 

 diapause exists for some 5 to 6 months in pupa maintained at 25° C. If the 

 brain is removed from such diapausing pupa the insect never rnetamorphoses 



