From “Low Oxygen Triggers Moth Molt: Caterpillars Have a Respiratory System That Is Fixed in Size” (ScienceDaily, Aug. 23, 2011), we learn a valuable clue about when caterpillars decide to molt (split out of their old skin – with a new one growing underneath):
The research shows that a baby moth’s respiratory system is fixed in size at each stage of development, which limits its oxygen intake. Sensing it is low on oxygen apparently signals to the insect that it cannot continue to grow without proceeding to the next stage of its development, by molting.
“This is a neat paper,” said Arizona State University biologist, Jon Harrison, who was not involved in the research. It shows how oxygen is one important factor in controlling the size, growth and overall development of insects, he said.
Lack of oxygen seems, on reflection, to be an obvious trigger. What else would distract the larva’s attention away from eating? See also: Researchers discover supergene that controls a butterfly’s mimicry patterns
Metamorphosis trailer here: