Metamorphosis 



639 



persisting in the tissues from previous stages. 

 Recently Wigglesworth ('51), following 

 the lead of Williams and other workers on 

 lepidopterans (see below), found that the 

 brain hormone does not act directly upon 

 the tissues but rather activates the protho- 

 racic glands to the production of their hor- 

 luone. His evidence consists of the fact that 

 transplantation of the strands of gland cells 



the diapause when implanted into a non- 

 activated pupa (Fig. 224). Removal or im- 

 plantation of the corpus allatum, on the other 

 hand, has no effect, showing that activity 

 of this gland plays no role in the adult 

 transformation. That the brain hormone acts 

 by stimulating the prothoracic glands and 

 not directly on the tissues was shown (Wil- 

 liams, '47) by the fact that implantation of an 



AFTER CRITICAL 

 PERIOD 



CAPILLARY TUBE 



HORMONE 



BEFORE CRITICAL 

 A PERIOD 



CAPILLARY TUBE 



—^ FIFTH NYMPH BECOMES 



6TH-STAGE (SUPERNUMERARY) NYMPH 



AFTER CRITICAL 

 PERIOD 



BEFORE CRITICAL 

 PERIOD 



Fig. 223. The union of nymphal instars of Rhodnius by capillary tubes. 



A, Fifth nymph, decapitated after the critical period and thus with active glands, is coupled with a 

 first nymph, decapitated before the critical period. Both individuals undergo imaginal molts, the first 

 nymph becoming a diminutive adult. The fifth nymph of Rhodnius, serving as the donor in this instance, 

 furnishes prothoracic hormone with little or no C.A. hormone. 



B, Second-stage nymph, decapitated after the critical period, is combined with a fifth nymph that had 

 been decapitated before the critical period. The second nymph, acting as the donor, delivers both C.A. 

 and prothoracic hormones into the body of the fifth nymph. The fifth nymph molts and, under the 

 influence of both hormones, becomes a sixth-stage (supernumerary) nymph. (From C. D. Turner, Gen- 

 eral Endocrinology.) 



from the thorax of activated individuals is 

 sufficient in itself to induce molt in non- 

 activated abdomens. Thus the picture of the 

 mechanism of metamorphosis in Rhodnius 

 may be diagrammed as in Figure 225. 



PLATYSAMIA 



The work of Williams on Platysamia 

 began with an analysis of the mechanism 

 of diapause in this lepidopteran. The animal 

 normally spends the winter as a pupa, and 

 experiment has shown that the freshly col- 

 lected pupa will not develop into an adult 

 unless it is first exposed to a period of at 

 least two weeks of cold. Williams ('46) was 

 able to show that the influence of cold was 

 exerted through activation of hormone pro- 

 duction in the animal's brain. An activated 

 animal induces imaginal development in a 

 non-activated parabiont. The brain from a 

 cold-treated animal is effective in breaking 



activated brain into a diapausing abdomen 

 is not effective unless a prothoracic gland 

 is implanted along with it. At first Williams 

 ('49) was of the opinion that the mechanism 

 of post-diapause development involved a dif- 

 ferent brain hormone from that found by 

 others (see below) to be concerned with lar- 

 val and pupal molts. Later, however, Wil- 

 liams ('52) was able to show by ligation 

 experiments and transplants of the protho- 

 racic organs and brains from various stages 

 that the same hormonal mechanism is in- 

 volved in the pupal and larval molt as in 

 the breaking of diapause. Since the role of 

 the C.A. in larval molts was established by 

 others in Lepidoptera (see below), the mech- 

 anism of hormonal control of development 

 in Platysamia may be diagrammed as in 

 Figure 225. The correspondence of the mech- 

 anism controlling metamorphosis in Rhod- 

 nius and Platysamia is seen to be quite 

 complete. In this interpretation diapause 



