Growth and Senescence 



type of senescence due to the suppression of development or to 

 imbalance between continued, divergent growth processes; or 

 it might nevertheless undergo senescence from the same cause, 

 whatever that cause may be, which limits the life of the imago. 

 To ask whether a larva or a nymph would senesce if it did not 

 metamorphose is not entirely idle speculation. The data which 

 we have on the developmental physiology of Rhodnius, chiefly 

 from the work of Wigglesworth, make it possible to contemplate 

 interfering with the development of nymphs. The pre-imaginal 

 phase of Rhodnius, during which growth takes place, is main- 

 tained by the so-called juvenile hormone. 'During larval life, 

 imaginal differentiation is suppressed because in the presence 

 of the juvenile hormone secreted by the corpus allatum the 

 intracellular system which leads to the production of larval 

 structures takes precedence over the system which leads to the 

 formation of adult structures' (Wigglesworth, 1953b). The 

 influence of temperature on larval development appears to act 

 through this system, high temperatures or low O a tensions 

 depressing the juvenile hormone and producing prothetely, low 

 temperatures enhancing its effect and producing metathetely. 

 This system lends itself particularly well to analysis in terms of 

 control-mechanisms. The tendency of the cellular system in its 

 Tree-running' state appears to be towards the imaginal form. 

 Moulting hormone from a fifth-stage larva will cause a first- 

 stage larva of Rhodnius to metamorphose (Wigglesworth, 1934). 

 Second-instar moth larvae will metamorphose to minute pupae 

 and adults if the corpora allata are removed (Bounhiol, 1938) 

 and isolated fragments from the integument of newly-hatched 

 moth larvae tend to pupate (Piepho, 1938). The function of 

 the juvenile hormone appears to be to moderate or prevent this 

 free-running tendency, though as a standing bias, not as a 

 negative feedback. The point to which the free-running system 

 tends, moreover, is an unstable one, ending in eventual sen- 

 escence. In the fifth stage Rhodnius nymph the thoracic gland 

 undergoes very rapid disintegration as soon as metamorphosis 

 takes place, and the possibility of moulting and cuticular 

 renewal is thereby lost, from lack of evocator, although the 

 power of the dermal cells to respond to injected moulting 

 hormone remains (Wigglesworth, 1953a). Long-term change 

 l 147 



