Metamorphosis 



643 



tion in isolated abdomens. This report pre- 

 sented the first evidence on the importance 

 of the prothoracic glands. A number of work- 

 ers have found correlations between molting 

 activity and cytological evidences of secre- 

 tion in the prothoracic glands (Kaiser, '49; 

 Rahm, '52). In contrast to the mass of evi- 

 dence linking the prothoracic glands with 

 pupation factor is the finding of Piepho ('48) 

 that these glands can be removed at any time 

 in the last instar of the moth Galleria with- 

 out preventing pupation. 



The nature of the hormonal factors in 

 molting and metamorphosis could not be 

 clarified until the role of the corpora allata 

 had been worked out. Following the lead of 

 Wigglesworth a number of investigators were 

 successful in demonstrating that the C.A. 

 played much the same role in many other 

 insects as in Rhodnius. Bounhiol ('37, '38) 

 extirpated the corpus allatum (leaving the 

 brain intact) from the caterpillars of a num- 

 ber of lepidopterans and showed that preco- 

 cious pupation ensued. Conversely he found 

 that grafts of the corpus allatum inhibit pu- 

 pation and permit supernvunerary molts. 

 Bounhiol concluded that the corpus allatimi 

 produces a hormone inhibiting pupation but 

 permitting molting. Pflugfelder ('37) like- 

 wise found premature metamorphosis result- 

 ing from corpus allatum removal in the stick 

 insect (Dixippus morosus — Orthoptera). Pfeif- 

 fer ('39, '45a) reported that transplants of 

 corpus allatum from young to older larvae 

 in the grasshopper suppressed metamorpho- 

 sis, thereby permitting extra molts and the 

 growth of nymphs to twice normal length. 

 Evidence for the existence of an inhibitory 

 factor in the corpus allatum was also found 

 by Fukuda ('40a) in Bombyx (Lepidoptera), 

 by Piepho ('40, '50) in Galleria (Lepidoptera) 

 and by Scharrer ('46) in Leucophaea (Or- 

 thoptera). In her cytological study of the 

 corpora allata of the grasshopper Melanoplus, 

 Mendes ('48) foimd evidence of secretory 

 activity throughovit the intermolt period in 

 all stages except the last. In this stage secre- 

 tion apparently stopped in the middle of the 

 intermolt. Comparable results on other in- 

 sects were obtained by Pflugfelder ('38) and 

 Rahm ('52), corresponding to those expected 

 from the experimental evidence. The inhibi- 

 tory action (in the sense of Wigglesworth, 

 '34) of the corpus allatum on insect meta- 

 morphosis may thus be said to be well estab- 

 lished for a variety of insects. 



DifFerentiation of Imaginal Primordia. A 

 number of early reports had indicated the 

 dependence of imaginal discs upon some 



humoral factors for their development. In 

 Vanessa (Lepidoptera) Bodenstein ('35) 

 found yovmg imaginal discs capable of re- 

 sponding to a differentiating factor when 

 transplanted to pupae. Fukuda ('39) reported 

 the accelerated development of larval ovaries 

 of the silkworm when transplanted into 

 pupae. This was independent of the sex of 

 the host and without effect on it. These re- 

 sults indicate that some condition, pre- 

 sumably the presence of hormonal factors in 

 the tissue fluids of pupae, stimulates the dif- 

 ferentiation of the primordia of the adult 

 structures. 



In the moth Phryganida, Bodenstein ('38b) 

 reported that abdominal ectoderm is unable 

 to differentiate without a factor from the 

 thorax which is normally produced in pupae 

 1 to 2 days old. However, he thought that 

 this effect was not transmitted through the 

 blood, since it did not pass through a glass 

 tube connecting two insect regions. As men- 

 tioned above his later work ('39) on Drosoph- 

 ila led him to conclude that the thorax 

 factor was tracheal development. However, 

 in view of the work of Fukuda and Williams 

 discussed below this interpretation does not 

 appear to apply to Lepidoptera. In the silk- 

 worm Fukuda ('41b) showed that the pro- 

 thoracic glands are necessary for the differ- 

 entiation of the imaginal discs of the pupae. 



It is apparent from the above discussion 

 of non-dipterous insects that, except for the 

 report of Piepho ('48) on the waxmoth, all 

 the data permit interpretation in terms of the 

 Wigglesworth-Williams concept, namely, 

 that the brain stimulates the prothoracic 

 glands to secrete a hormone which induces 

 metamorphic transformation in the tissues. 

 At larval molts the activity of the C.A. so 

 changes the response of the tissues that a 

 larval molt rather than metamorphosis en- 

 sues. 



The study of metamorphosis in Diptera 

 has centered around the functioning of the 

 ring gland of Weismann. In the larvae of 

 Drosophila this gland has been identified as 

 the source of a molting hormone by Boden- 

 stein ('44), who found that a larval head 

 deprived of its nervous system and ring 

 gland would not molt when transplanted into 

 the abdomen of the adult fly. But if two lar- 

 val ring glands were included with the trans- 

 planted head, one or two molts followed. In 

 these, as in Wigglesworth's experiments with 

 Rhodnius, the molting stimulus is found to 

 be non-specific for the species. A pupation 

 factor was localized in the anterior part of 

 the body of Calliphora by the work of 



