88 INSECT PHYSIOLOGY 



organism is again produced when the insect moults. In the fifth larval 

 stage the corpus allatum no longer secretes the juvenile hormone so 

 that when moulting is brought about by the 'growth and moulting 

 hormone' the latent imaginal organism is able to develop and meta- 

 morphosis takes place. If the corpus allatum is removed from the 

 young stages of insects, precocious metamorphosis occurs : diminu- 

 tive silkmoths weighing only a few milligrams have been produced in 

 this way. Or, conversely, if the corpus allatum from a young insect 

 is implanted into a late stage, giant larvae can be produced. The 

 juvenile hormone reacts directly with the epidermal cells, for if only 

 a little of the hormone is produced from an implanted gland, it may 

 result in a tiny patch of larval cuticle developing in an otherwise 

 normal adult. 



There is evidence that in such insects as Lepidoptera, with a pupal 

 stage, the larva develops when much juvenile hormone is present, the 

 pupa when only a little is present, the adult when the hormone is 

 absent altogether. In parts of some insects, such as the abdominal 

 integument in Rhodnius or in Lepidoptera, if the adult is caused to 

 moult once more by supplying it with the moulting hormone and if 

 at the same time it is supplied with abundant juvenile hormone, it 

 may show a partial reversal of metamorphosis with the partial de- 

 velopment of larval characters again. 



It would seem that the gradual changes in form that occur in the 

 successive stages of insects, as well as the more striking changes at 

 metamorphosis, are likewise controlled by a delicate balance in the 

 timing and concentration of the secretions from the corpus allatum 

 and thoracic gland. Sometimes this balance is upset within the intact 

 insect, particularly in species hybrids or in insects reared under ab- 

 normal conditions, and then monsters intermediate between larva 

 and pupa, or between pupa and adult, are produced. This pheno- 

 menon is termed 'prothetely' or 'metathetely'. 



Chemistry of insect growth hormones 



The secretion of the neurosecretory cells in the dorsum of the brain 

 is made up of minute spheres of protein (1000-3000 A in diameter) 

 which pass along the axons that end blindly in the 'corpus cardia- 

 cum* lying just behind the brain. The active material is believed to 



