Endocrine Mechanisms 733 



heminietahola and largely confined to the pupal stage in the holometahola. 



The number of larval or nvmphal stages in development differs from 

 species to species, but is usually a constant for each species. The bug Rhod- 

 nius, for example, has five nymphal stages or instars, the orthopteran, Dixip- 

 -pus, has six, and the dipteran, Drosophila, has three. It is now well 

 established that the sequence of events in the growth and differentiation in 

 the post-embryonic insect is in good measure under the integrating action of 

 hormones derix'cd from glandular sources in the anterior region of the in- 

 sect.'" 



Experiments on the tropical bug, Rhodnius prolixus, have provided a 

 clear demonstration of the action of two developmental hormones,^^*^- ^^^' ^^" 

 one inducing molt accompanied by metamorphosis, and the other inhibiting 

 metamorphosis. The former has been called the molting hormone or the 

 growth and differentiation (GD) hormone, and the latter the inhibitory 

 or juvenile hormone. The roles of these hormones are readily investigated 

 in developing Rhodnius. This insect, at each nymphal instar, molts a defi- 

 nite number of days after a meal of blood. The distention of the abdomen 

 appears to serve as the adequate stimulus. Following decapitation these in- 

 sects will survive for 6 to 10 months. It has been shown by decapitation at 

 different interxals after feeding that there is a "critical period" occurring a 

 few days after the feeding. Nymphs decapitated before this period never un- 

 dergo molt; those decapitated afterward do. That the critical period consti- 

 tutes a time when a blood-borne molt-inducing hormone is being liberated 

 from some organ in the head is shown by experimental telobiosis (Fig. 

 280, A) of two decapitated individuals. Under such conditions, for example, 

 molting in a fourth stage nymph decapitated before the critical period will 

 be induced by a fourth stage nymph decapitated after the critical period. 

 Both thereby become fifth stage nymphs. 



The fifth nymphal stage, upon molt, typically metamorphoses at that time 

 into an adult. If a fifth stage nymph is decapitated after its critical period it 

 can be shown by telobiotic union with a first or second stage nymph de- 

 capitated before the critical period, that the fifth stage nymph possesses 

 within its blood factors which determine molt with metamorphosis. The at- 

 tached first or second stage nymph becomes in this instance a diminutive 

 adult. If a fourth stage nymph which would normally molt to become a fifth 

 stage nymph is decapitated after the critical period and united with a fifth 

 stage nymph decapitated before the critical period, the latter molts and 

 becomes a giant, supernumerary sixth stage nymph instead of an imago. 

 Furthermore, although the adult normally does not molt again, it can be 

 made to do so by telobiotic and parabiotic union to two or three fifth stage 

 nymphs decapitated after the critical period. If fourth stage instead of fifth 

 stage nymphs are used, the adult on molt shows a partial return to the 

 nymphal condition (Fig. 280, B). 



From such experiments it is readily seen that the blood of a nymph after 

 the critical period contains factors that determine molting either without or 

 with an accompanying extensive metamorphosis, the blood of the molting 

 fifth-stage nymph, alone, inducing metamorphosis. A careful study of the 

 results of decapitation of third and fourth instars at different times during 

 the critical period itself shows that, of those molting after decapitation, those 



