Hormone Balance and the Control of Metamorphosis in Rhodnius Prolixus (hemiptera)

The body cells of the developing insect carry the potentialities for both larval and imaginal differentiation. During larval life imaginal differentiation is suppressed because in the presence of the juvenile hormone (sometimes referred to as the 'inhibitory hormone'), secreted by the corpus allatum, the intracellular system which leads to the formation of larval structures takes precedence over the system which leads to the formation of adult structures (Wigglesworth, 1940). If the corpus allatum in Rhodnius is removed by decapitation at varying times in the moulting cycle of a young larva, when varying amounts of juvenile hormone have been secreted, all intermediate grades of metamorphosis between larva and adult can be produced (Wigglesworth, 1934). In the normal insect there is a nicely adjusted balance between the juvenile hormone and the moulting hormone;* in these experiments the balance has been grossly upset. The operation of this hormone balance has been well illustrated by the work of Piepho (1940, 1950 a) on Galleria. In the course of normal development in Rhodnius there is a small amount of differentiation towards the adult form in each larval instar. This is most striking at the moult from the 4th to the 5th instar, where the differentiation of the external genitalia, the enlargement of the wing pads, the sharp fall in the rate of increase in the number of abdominal plaques and bristles (Wigglesworth, 1940) are all indications of limited imaginal differentiation. These fine adjustments during growth are also the result of controlled hormone balance. In the present paper some of the ways in which this balance may be upset are described—with the object of throwing light upon the mechanisms by which the correct balance is normally maintained.