One of the restrictions on the size of large buildings is the space required to deliver utilities to the top floors. As the height of a building increases, the amount of floor space that must be devoted to elevators, water pipers, sewer pipes, heating and cooling ducts and electrical conduit must expand. This space requirement imposes an effective limit on the size of our tallest skyscrapers.
The insect respiratory system is a system of tubes that delivers oxygen directly to every cell in the insect. Like our skyscrapers, the size of an insect is limited by the amount of space required by the piping, in this case the trachea of the respiratory system. The size of the trachea is fixed at each larval instar. A newly molted larva has excess tracheal capacity. As it grows, the oxygen consumption of the tissues rises to the limit of the ability of the tracheal system to deliver oxygen.
In an elegant series of experiments published in PNAS (Viviane Callier and H. Frederik Nijhout, Control of body size by oxygen supply reveals size-dependent and size-independent mechanisms of molting and metamorphosis. PNAS 2011; published ahead of print August 22, 2011, doi:10.1073/pnas.1106556108) Callier and Nijhout demonstrated that the critical mass (size necessary to trigger molting), is correlated with a tissue demand for more oxygen than the trachea can deliver.
They predicted that if oxygen demand was a trigger for molting, then:
1. Caterpillars placed in oxygen deficient conditions would molt at a smaller size than normal caterpillars and
2. Caterpillars given air enriched in oxygen would grow to a larger size before molting.
The authors concluded that the tracheal system can deliver a maximum “fixed” amount of oxygen to the tissues at normal atmospheric oxygen concentration. An oxygen deficit may be an important signal to the caterpillar to induce molting. Reducing atmospheric oxygen concentration causes the caterpillar to have an oxygen deficit at a smaller size and molt at a lower mass. Increasing the oxygen concentration will allow the caterpillar to reach a larger size before it experiences oxygen deficit.
This research offers important new insights into the control of insect molting.