Four or five times along its road to moth- or butterfly-hood, a caterpillar molts to make room for a larger exoskeleton. How does the bug know when the moment has arrived? A stressed respiratory system seems to be the cue, according to a study published this week in the Proceedings of the National Academy of Sciences. Researchers measured tracheal tube sizes in tobacco hornworm caterpillars (Manduca sexta ) throughout their development and found that, while the rest of the organisms' bodies grow between molts, the tubes do not. So, at some point in each stage the caterpillar's body becomes too big for its respiratory system, the bug starts suffocating, and the resultant low oxygen levels spur exoskeleton shedding. After repeating the process a few times, the larvae eventually reach metamorphosis.
See more ScienceShots.