Animals come in all sizes, but how does an animal know when it’s grown to the right size?  That’s been puzzling Fred Nijhout for a long time, but he thinks he has part of the answer—at least for tobacco hornworms.  It’s because of oxygen deprivation.

Instead of lungs, insects have branching tubes which carry air from holes in their bodies down to the cellular level.  Their lining is the same stuff as the exoskeleton, folded into cylinders and diving inside the animal.  And like the exoskeleton, it does not grow.  As the caterpillar gets bigger, it reaches a point when the tracheal tubes can’t supply enough oxygen:  time to molt, big fella.  The old exoskeleton cracks and falls off, taking the tracheal lining with it, making way for a new, slightly larger system.  Fred was able to control the hornworms’ growth by controlling the amount of oxygen available.

But how does the not-so-hungry caterpillar know that it’s time not just to molt but to metamorphose?  Its juvenile hormones switch off.  Fred originally planned to explain this in his Ph.D. thesis but it’s still a great mystery.  “Hope springs eternal,” he says.