642 



Metamorphosis 



the blood have not, in the opinion of the 

 writer, proved conclusive. Buddenbrock ('31) 

 leported an acceleration of ecdysis in various 

 caterpillars after the injection of blood from 

 molting larvae as compared to controls in- 

 jected with blood from non-molting indi- 

 viduals. His data, however, are not convinc- 

 ing and further doubt is thrown on the 

 conclusion by the careful work of Schiirfeld 

 ('35), who found control procedures such 

 as mere sticking with a needle as effective 

 as the injection of blood. 



Pupation, or the Metamorphic Molt. Much 

 evidence has accumulated to show that these 

 metamorphic molts are, like larval molts, 

 hormonally controlled. The first experi- 

 mental results in this field, however, were 

 complicated and obscured by the secondary 

 effects upon pupation of interference with 

 nutritution and oxygen supply. 



Kopec ('22) reported that the removal of 

 brain before a definite "critical" period pre- 

 vents metamorphosis in Lymantria (Lep- 

 idoptera). Initially he interpreted his results 

 as indicative of the activity of a metamor- 

 phosis hormone derived from the brain (this 

 appears to have been the first time evidence 

 for hormonal control of metamorphosis in 

 insects was adduced). The subsequent study 

 of Kopec ('24) led him to conclude that the 

 mechanism of brain action lay in the in- 

 hibition of feeding which follows its removal, 

 rather than in hormone production. Starvation 

 had the same inhibitory effect on metamor- 

 phosis as did brain removal, and the critical 

 period for starvation and brain removal 

 coincided. Boimhiol ('38), in experiments 

 on various lepidopterans, obtained extensive 

 confirmation of this viewpoint. The conclu- 

 sion of these authors that brain removal 

 before critical period inhibits feeding, and 

 that it is the consequent starvation which 

 prevents metamorphosis, is acceptable. How- 

 ever, this interpretation does not preclude 

 the possibility that the brain also produces 

 a hormone necessary for metamorphosis. 



The discovery of the importance of the 

 oxygen supply, and tracheal growth for pu- 

 pation, is another factor which complicates 

 the interpretation of experiments on isolated 

 insect parts. Bodenstein ('39) found that, 

 whereas the abdomen of Drosophila could 

 not pupate when isolated from the thoracic 

 region before a certain critical period, it 

 could do so if kept in circulating oxygen. 

 This he interpreted as showing that the 

 "thoracic" factor in pupation to which his 

 previous experiments on isolated abdomens 

 had pointed was the downgrowth of tracheal 



connections. This interpretation was sup- 

 ported by histological studies on the source 

 and time of formation of the tracheal supply 

 of the abdomen. The necessity of an adequate 

 respiratory mechanism for the development 

 of insect structures had also been indicated 

 in the work of Fraenkel ('35) on Calliphora 

 (Diptera) and Fukuda ('39, '40a,b) on the 

 silkworm. 



The uncertainties that arise in the inter- 

 pretation of experiments on the ablation and 

 isolation of parts because of the interference 

 by the non-specific factors of nutrition and 

 respiration make it difficult to interpret much 

 of the early evidence purporting to show the 

 existence of specific hormonal factors in pu- 

 pation. Fraenkel ('35), working on the fly 

 Calliphora, found that a posterior part of a 

 grub separated from the anterior portion by 

 a ligature would pupate only if the ligature 

 were made within 16 hours of presumptive 

 pupation time. The anterior part could al- 

 ways pupate. Blood from pupating larvae in- 

 duces pupation in non-pupating posterior 

 halves. That this blood-borne factor is not 

 itself a sufficient stimulus for pupation is 

 shown by the fact that it is effective in the 

 abdomen only if it is ligatured within 24 

 hours of normal pupation. It therefore seems 

 possible that the action of the injected blood 

 is nutritive rather than being hormonal as 

 Fraenkel inferred. Caspari and Plagge ('35), 

 working on two lepidopterans, obtained evi- 

 dence that removal of the brain prevents 

 pupation. Brain implants seemed to restore 

 the pupation capacity though the evidence 

 presented on this point is not adequate. Kiihn 

 and Piepho ('38) on Ephestia (Lepidoptera) 

 and Bodenstein ('38a) on Drosophila (Dip- 

 tera) by ligature experiments showed the 

 importance of some factor in the anterior 

 end of the larva for pupation. Piepho ('38) 

 showed that pieces of larval epidermis im- 

 planted into the fat body of host larvae pu- 

 pate synchronously with the host. Plagge 

 ('38) found brain removal before a critical 

 period in the caterpillar prevented pupation. 

 Reimplantation of the brain or injection of 

 blood from post-critical-period larvae seemed 

 effective in inducing pupation, though the 

 evidence here is not clear-cut. Similar results 

 on the hymenopteran Trypoxylon were re- 

 ported by Schmieder ('42). In silkworms, 

 Fukuda ('40a,b) found that decapitation pre- 

 vented pupation when done early enough. 

 Later, i.e., after the critical period for decapi- 

 tation, the prothorax is still necessary for 

 pupation. At this time implants of protho- 

 racic glands are capable of bringing on pupa- 



