Factors Controlling Moulting and ‘Metamorphosis’ in an Insect

Abstract

IN the blood-sucking bug, Rhodnius prolixus, moulting occurs at a definite interval after feeding ; only one meal being necessary in each stage. The morphological changes at moulting are relatively slight ; save at the fifth moult, when the insect becomes adult¹. It is therefore convenient (without prejudice to questions of homology) to refer to this final moult as ‘metamorphosis’. In this last stage, the interval between feeding and moulting averages twenty-eight days. If its head is removed soon after feeding, the insect will not moult—although some of these headless individuals have remained alive more than eleven months. But there is a critical period, about seven days after feeding, after which moulting is no longer prevented by decapitation. If the blood from an insect decapitated after this critical period is allowed to circulate in an insect decapitated before this period, the latter is caused to moult. Clearly, a moulting hormone is present ; and it is probably secreted in the head. Of the organs in the head, the only one which shows distinct changes during this period is the corpus allatum, the cells of which swell up to a maximum at about the seventh day after feeding, and then diminish. Perhaps this is the source of the moulting hormone—though the evidence on this point is still incomplete.