HORMONES 67 



at the end of five days. The pairs were then separated and B and 

 B' were joined to further groups of fourth-stage nymphs (C and C) 

 transected through the prothorax twenty-four hours after feeding. 

 At the end of eight days the group C, i.e., insects whose partners 

 retained the corpus allatum, were obviously beginning to moult ; 

 the group C was probably just starting to moult. The pairs 

 were then separated once more and joined to groups D and D', 

 cut through the prothorax twenty-four hours after feeding. At 

 the end of seven days D' were clearly beginning to moult ; but 

 D died several weeks later without showing any signs of moulting 

 beginning. 



" Thus in the series A, B, C, D, in which no corpus allatum was 

 present, the moulting hormone appeared to diminish in con- 

 centration until it ceased to induce moulting at all. Whereas in 

 the series A', B', C, D', in which B' retained the corpus allatum 

 (and was presumably able, therefore, to supply an adequate 

 amount of the hormone to C), there was no sign of diminished 

 activity in the moulting hormone — ^the group D' being caused to 

 show signs of moulting as soon after joining to C as B' did after 

 joining to A'." 



Wigglesworth has also brought forward evidence which appears 

 to indicate that metamorphosis is controlled by an " inhibiting 

 hormone " which is also secreted by the corpora allata. Thus it 

 was found that if a fourth instar nymph had only the apex of the 

 head removed six days after feeding and was then joined to a 

 fifth instar nymph decapitated twenty-four hours after feeding, 

 the latter moulted into an extra nymphal stage instead of becoming 

 an adult. The head is, therefore, necessary for the production 

 of the inhibiting hormone. Also, fifth instar nymphs, twenty- 

 four hours after feeding, had the corpora allata from third or 

 fourth instar nymphs (removed a ^day or two after feeding) 

 implanted within them. These fifth instar nymphs moulted to 

 " sixth instar " nymphs. As the result of similar experiments, 

 " sixth instar " nymphs may give rise to " seventh instar " 

 nymphs when they moult again. In some cases the resulting 

 characters were intermediate between those of adults and nymphs : 

 in others they were wholly nymphal in type. From these and 



3—2 



