68 METAMORPHOSIS 



other experiments it is concluded that the corpora allata control 

 the characters of the various instars and also secrete an 

 " inhibitory hormone " which prevents metamorphosis taking 

 place until the insect attains its fifth instar. In this phenomenon 

 of " inhibition of metamorphosis " there seem to be two phases 

 involved : (i.) deposition of new cuticle following rapidly upon 

 the initiation of growth and hence the arrestation of adult char- 

 acters ; (ii.) so long as differentiation is arrested in this way the 

 cells are capable of new growth and will respond to the moulting 

 hormone. Whether the latter hormone is biochemically distinct 

 with that concerned with the inhibition of metamorphosis has 

 not been proved. 



In contrast with Wigglesworth's experiments it is noteworthy 

 that Weed (1936a), in an experimental study of moulting in the 

 grasshopper Melanoplus differentialis, obtained different results. 

 The account, however, of these experiments is very short and 

 they are stated to be preliminary only. It appears that in this 

 work the corpora allata themselves were removed from grasshopper 

 nymphs one to nine days after they had entered the last stadium. 

 In each case moulting resulted, and it was concluded that the 

 corpora allata do not influence this process. It is necessary, 

 how^ever, to await more extensive experiments before drawing 

 definite conclusions. 



With regard to the Holometabola, it has been known for some 

 time that removal of the brain prevents jDupation. The earlier 

 observers did not associate the corpora allata with any significant 

 role in the metamorphosis, and it is extremely probable that their 

 removal took place along with the brain in, at any rate, most of 

 the experiments recorded. Thus Kopec (1922) removed the brain 

 from caterpillars of Lymantria disjpar in their last instar. If the 

 brain be excised ten days before the last ecdysis, or later, the 

 caterpillars pupated, but if the experiment be performed seven 

 days after that moult, jjupation did not occur. Kopec concluded 

 from these, and other experiments, that a hormone is produced 

 in the brain and that this secretion induces pupation. Quite 

 recently Caspari and Plagge (1935) obtained very similar results 

 with larvae of Sphinx ligustri and Deilephila eupJiorbiw. Bounhoil 



