HORMONES AND INSECTS — WIGGLESWORTH 315 



is, an insect wliicli does not show the extreme degree of transforma- 

 tion from caterpillar to butterfly, but one in which the young stages 

 are not very unlike the adult, and in which metamorphosis consists 

 in the development of wings and genital organs and other structural 

 changes, accomplished without the necessity for an intermediate pupal 

 stage. 



The blood-sucking South American bug Rhodnius, a creature 

 about 2 centimeters in length when fully grown, has proved a most 

 useful experimental animal (8, 9, 10, 11). All insects grow by under- 

 going a series of molts, during which the epidermis detaches itself 

 from the old cuticle, lays down a new and larger cuticle, and then 

 casts off the old. Like the small bedbug Cvmex^ Rhodnius molts five 

 times; in each of its molting stages it requires only one gigantic meal 

 of blood. It is at the fifth molt that it undergoes metamorphosis 

 and becomes adult (pi. 1, figs. 1-3). 



Molting in Rhodnius is preceded by growth, reorganization, and 

 the deposition of the cuticle for the next stage. This whole elaborate 

 process is set in motion by a secretion from certain large modified 

 nerve cells (neurosecretory cells) situated in the dorsal surface of 

 the brain. If the bug is decapitated within one day after its great 

 meal of blood, it fails to molt — although such headless bugs have 

 remained alive for more than a year (pi. 1, fig. 4). If, however, the 

 region of the brain containing the secretory cells is transplanted into 

 the abdomen of one of these decapitated insects it will duly molt; and 

 surprisingly enough, even if it is a young insect at an early stage of 

 development, it will undergo metamorphosis and develop into a 

 diminutive adult. We shall come back to this matter later. 



Thus the brain appears to secrete a molting hormone. In Rhodnius 

 this was believed supposed to act directly upon the growing organs, 

 but in the caterpillars and chrysalids of moths the process is more com- 

 plicated. The pupal brain of the silkmoth (14) contains two groups 

 of cells, apparently producing two different secretions, both of which 

 must be present if molting is to occur. These secretions do not act 

 directly upon the tissues but upon another secretory organ, the pro- 

 thoracic gland, which in turn produces the secretion that is neces- 

 sary for gTowth and molting. A similar gland activated by the secre- 

 tion from the brain has recently been found in Rhodnius (13). The 

 nature of these substances is not knoAvn. There are demonstrable 

 changes in the blood of insects at the time of molting and pupation, 

 e. g., increases in the amounts of cytochrome oxidase and cythochrome 

 C in the pupae of the large silkmoths (14), and the activation of 

 tyrosinase in the blood of pupating larve of blowflies (2). Whether 

 any of these substances is to be identified with the hormone itself, or 

 whether they are merely to be counted among the consequences of 

 its activity, remains to be decided. 



