66 METAMORPHOSIS 



by Nabert in 1913. Wigglesworth (1934) showed that these 

 organs exhibit cychc changes in that they pass through a secretory 

 stage associated with moulting, which is followed by a resting 

 condition. In the blood-sucking Reduviid bug Rhodnius, 

 Wigglesworth states that ecdysis takes place at a definite interval 

 after feeding, only one meal being necessary during each instar. 

 There is a " critical period " in the moulting cycle and decapitation 

 before the advent of this period prevents moulting. The critical 

 period synchronises with the beginning of mitosis in the cells of 

 the hypodermis. He further states that insects which have 

 passed the critical period contain a hormone in their blood. If 

 blood from an insect in this stage be transferred into an insect 

 which has been decapitated before the critical period, moulting, 

 which otherwise does not take place, is induced. (The critical 

 period referred to is about seven days after feeding in the fifth 

 instar and about four days after in the earlier nymphs.) That the 

 corpora allata are the organs involved is suggested by the fact 

 that the cells of these bodies show their greatest secretory activity 

 during the critical period. In a later communication, 

 Wigglesworth (1936) showed that the moulting hormone from 

 Rhodnius will induce moulting in the allied genus Triatoma and 

 in the bed-bug Civiex. While the available evidence points to 

 the corpora allata as the seat of the hormone involved, comjjlete 

 proof is, as yet, not available : for one thing, their extirpation 

 is extremely difficult to carry out. Rather indirect evidence, 

 however, is afforded by some other experiments described by 

 Wigglesworth as follows : " fourth-stage nymphs of Rhodnius 

 in two groups (A and A') were decapitated seven days after 

 feeding and joined to groups of fourth-stage nymphs (B and B') 

 twenty-four hours after these had fed. Of these, B were transected 

 through the prothorax, the corpus allatum being removed, while 

 B' were transected through the posterior part of the head behind 

 the brain but in front of the corpus allatum. 



" Groups B and B' both showed that moulting was beginning ^ 



^ At the onset of moulting, when the epidermal cells separate from the 

 cuticle and divide, there are visible changes in the abdominal tergites of the 

 living insect (see Wigglesworth, 1934). 



