86 INSECT PHYSIOLOGY 



has proved to be a hormone circulating in the blood. For example, 

 the blood-sucking bug Rhodnius takes only a single feed in each of its 

 five larval stages; moulting takes place at a definite interval (varying 

 from 10 to 20 days in the different instars) after each meal. Now if 

 the recently fed insects are decapitated before a certain 'critical 

 period' after feeding they fail to moult, although some of them may 

 survive for more than a year. But if an insect decapitated soon after 

 the critical period is connected by means of a capillary tube to an 

 insect decapitated one day after feeding, so that the blood passes 

 from the one to the other, the second insect is caused to moult. 



A similar effect, of decapitation preventing pupation, was ob- 

 served by Kopec (1917) in the caterpillar of the Gipsy moth Lyman- 

 tria; and since he found that pupation could be prevented by removal 

 of the brain, though not by section of the nerve cord, Kopec sug- 

 gested that the brain was secreting the hormone in question. In 

 Rhodnius, and in all other insects so far examined, there are groups of 

 'neurosecretory cells' in the dorsum of the brain, and the implanta- 

 tion of these cells into the decapitated insect will induce moulting. 



But this factor from the brain is not the 'growth and moulting 

 hormone' which acts upon the tissues, for implantation of the neuro- 

 secretory cells will not induce moulting in the isolated abdomen. The 

 brain factor seems merely to activate another gland of internal 

 secretion, the 'ventral gland' in the head of some insects, the 'thora- 

 cic' or 'prothoracic gland' in others, and it is this structure (which 

 had been observed by Lyonet in the larva of Cossus as long ago as 

 1762) that is the source of the hormone that induces moulting. That 

 has been proved experimentally in Lepidoptera, in Diptera (where 

 the gland in question forms part of the 'ring gland' of Weismann), in 

 Rhodnius, in the cockroach and in other insects. When the adult 

 stage is reached the thoracic gland degenerates and the insect does 

 not moult again - except in the primitive Thysanura in which moult- 

 ing continues in the adult and the ventral gland persists. 



As to the stimulus which causes secretion of this hormone : there 

 is evidence that the stimulus to moulting in Rhodnius is the stretching 

 of the abdomen by the meal ; and section of the nerve cord in the 

 prothorax inhibits moulting like decapitation. This suggests that 

 moulting is started in Rhodnius by a nervous stimulus to the brain. 



