644 



Metamorphosis 



Fraenkel ('35). Burtt ('38) identified the 

 active region as the ring gland by showing 

 that the removal of this gland or cutting the 

 nerves to it prevents pupation. 



In Drosophila there is a giant strain which 

 fails to pupate at normal size. Implantation 

 of ring glands (but not of various other tis- 

 sues) from normal larvae into individuals of 

 this strain leads to pupation at normal size 

 (Hadorn, '37). Precocious pupation in the 

 normal larvae is attained by ring gland im- 

 plantation (Hadorn and Neel, '38). Vogt 

 ('42) found that ring gland implants induce 

 the next normal molt in Drosophila tissues, 

 that is, third (last larval) instar abdomens 

 are induced to pupate by ring gland im- 

 plants whereas second instar eye-antenna 

 primordia merely molt to the third instar 

 stage when implanted into these same ab- 

 domens along with the ring gland. This in- 

 terpretation that ring glands induce the next 

 normal molt agrees with the results of Boden- 

 stein ('43, '44), who found that imaginal 

 discs from older larvae do not grow when 

 implanted into adult abdomens unless ring 

 glands are implanted along with them. It is 

 therefore possible to suppose that a hormone 

 or hormones produced by the ring gland reg- 

 ulate molting, pupation and imaginal disc 

 development in Drosophila, the effect in each 

 case depending on the degree of development 

 which has been attained by the reacting 

 system. The possibility that this effect of 

 the ring gland is hormonal is supported by 

 the work of Frew ('28), who found that 

 imaginal discs of mature blowfly larvae in- 

 vaginate when cultured in tissue fluids of 

 pupae but not of larvae. 



It was difficult at first to reconcile the 

 work on the ring gland of flies with the re- 

 sults on other insects, since the ring gland 

 by itself seemed capable of such varied func- 

 tions and apparently was not dependent upon 

 stimulation from the brain. But Possompes 

 ('50a,b), working with Calliphora, has re- 

 cently shown that the lateral lobes of the 

 gland, which histologically seem comparable 

 to the prothoracic glands, are the sources of 

 the hormonal stimulant to the tissues and 

 that these lobes are dependent for their acti- 

 vation upon stimulation by the brain, prob- 

 ably by the neurosecretory cells in that or- 

 gan. Like the prothoracic glands in other 

 insects, these lateral lobes degenerate after 

 metamorphosis (Vogt, '42). As mentioned 

 above, the dorsal cells of the ring gland ap- 

 pear to be homologous to the C.A. of other 

 orders of insects (Cazal, '48). The function- 

 ing of this region was also brought into line 



with the C.A. of other insects by the finding 

 of Vogt ('46) that this region causes at least 

 local inhibition of metamorphic transforma- 

 tion. There is reason to suppose, therefore, 

 that the apparent divergence of the Diptera 

 from other insects rests primarily upon the 

 complex nature of the ring gland. Wiggles- 

 worth ('48) further suggests that the prom- 

 inence of the pupation-inducing activity 

 among ring gland effects may rest on a great 

 capacity of the C.A. region in the late larva 

 for neutralizing any of the C.A. "inhibitory" 

 hormone left in the tissues from earlier 

 stages. According to this interpretation the 

 relationships of the hormones in Diptera 

 would differ only in minor respects from 

 those in Rhodnius as indicated in Figure 225. 



MODE OF ACTION OF THE METAMORPHIC 

 STIMULUS 



The striking similarity in pattern of the 

 mechanisms by which metamorphosis is reg- 

 ulated in insects and amphibians as discussed 

 above encourages us to think that a further 

 inquiry and detailed comparison may well 

 enable the information in one of these fields 

 to throw light upon the other. In that spirit 

 let us proceed to consider a number of fun- 

 damental questions in morphogenesis and tc 

 compare insects and amphibians with respect 

 to them. 



HORMONAL NATURE OF REGULATORY 

 MECHANISM 



Are the basic regulating mechanisms 

 hormonal in both groups? The formal dem- 

 onstration of the action of hormonal sub- 

 stances of thyroid and anterior pituitary 

 seem clear and fairly complete in regard to 

 amphibian metamorphosis. Evidence that 

 these glands act through hormones is varied, 

 but most important is the fact that pure or 

 relatively pure extracts are effective in re- 

 pairing the deficiencies created by removal 

 of these glands. The evidence is still incom- 

 plete in that changes in the level of hormone 

 concentration in the blood have not been 

 demonstrated. In contrast, the evidence from 

 insects is far less complete. Notably there is 

 a lack of purified extracts. The report of 

 Plagge and Becker ('38) appears to be the 

 only one in which positive results were ob- 

 tained with even crude extracts. Of course 

 the effectiveness of body fluids which has 

 been reported a number of times (see above 

 and also Schmidt and Williams, '49) is pre- 

 sumptive evidence of the presence of a hor- 



