External factors - Factors Controlling Metamorphosis in Insects
In few cases an external factor may be accountable for initiating moulting, as for instance in the blood sucking Rhodnius the intake of food (blood meal) is such type of a factor. Another example where external factor is essential to initiate a moult is the case of the pupa moth Platy samia cecropia. After pupation the insect falls into a quiescent state along with a reduced rate of metabolism - diapause - that continues throughout winter. It is essential that during this time the pupa be exposed to cold, or else the diapause is prolonged indefinitely. The diapause might be broken precociously if the pupa is exposed to cold (3° to 5° C) for at least two weeks.
The temporary cooling activates the very important processes in the pupa and on return to a warmer environment development is completed, the pupa moults and the imago emerges. In other insects factors like humidity, population density etc., appear to start metamorphosis. Though in the majority of insects no external cause of any moult has been detected and moults follow one other at intervals that appear to be ascertained entirely by the internal processes in the animal.