HORMONES AND INSECTS — WIGGLESWORTH 317 



genus Triatoma (pi. 4, fig. 1, and cf. pi. 2, fig. 4) , or even in the bedbug 

 Cimex. The fifth-stage larva of the bedbug can be prevented from 

 becoming adult if it is transfused with the blood from a young larva of 

 Rhodnius containing the juvenile hormone. 



Normal development of an insect, with the restraint of metamor- 

 phosis until growth is complete, and then, at the appropriate moment, 

 the activation of imaginal differentiation, clearly demands a very nice 

 timing of events. The hormones must be released at the correct time 

 and in the correct amounts. If these conditions are not satisfied, as 

 may happen, for example, in some experiments when a corpus allatum 

 from a young Rhodnius is transplanted into a fifth-stage larva, and the 

 amount of juvenile hormone present is too small or is produced too late, 

 metamorphosis is incomplete, and creatures intermediate between lar- 

 vae and adults are produced (pi. 4, figs. 2, 3). Errors of this kind 

 are not uncommon in nature. Caterpillars with wing lobes and an- 

 tennae like half-formed pupae may occur; or pupae may have parts 

 of the body resembling larvae. Such abnormalities are most liable to 

 appear in hybrids resulting from the crossing of different species. 

 There can be little doubt that they result from errors in the timing or 

 the concentration of hormone secretions. 



It would seem that the cells of the young insect contain two systems, 

 one capable of producing the adult insect, the other producing the lar- 

 val insect. In the presence of the molting hormone alone, the adult 

 system is activated and metamorphosis occurs. In the presence of both 

 molting hormone and juvenile hormone, the larval system is activated 

 and metamorphosis is suppressed. Or perhaps we have to do with a 

 single system whose activities are modified in alternative directions 

 depending on the presence or absence of the juvenile hormone. In the 

 last analysis, the nature of the constituent elements, and the mode of 

 interaction between the hormones and the potential organism latent in 

 the tissue cells, are biochemical problems. For the moment we can de- 

 fine them only in biological terms. 



Once the insect has reached the adult state it does not molt again, 

 save in the most primitive forms. The adult Rhodnius can be induced 

 to do so if it is joined to a young molting insect so that the blood flows 

 from one to the other. If at the same time it is provided with a supply 

 of juvenile hormone by the implantation of corpora allata, it develops 

 on its abdomen a type of cuticle which shows unmistakable larval 

 characters. There has been a partial reversal of metamorphosis, a 

 partial recovery of youth. 



The future of this subject, which is being actively studied in institu- 

 tions in many parts of the world, clearly lies with the chemist. It 

 should not prove an insuperable task to discover the nature of the 

 growth-controlling hormones of insects, and to define the conditions 



