  Many species of millimetric fungus-harvesting termites collectively build
uninhabited, massive mound structures enclosing a network of broad tunnels
which protrude from the ground meters above their subterranean nests. It is
widely accepted that the purpose of these mounds is to give the colony a
controlled micro-climate in which to raise fungus and brood by managing heat,
humidity, and respiratory gas exchange. While different hypotheses such as
steady and fluctuating external wind and internal metabolic heating have been
proposed for ventilating the mound, the absence of direct in-situ measurement
of internal air flows has precluded a definitive mechanism for this critical
physiological function. By measuring diurnal variations in flow through the
surface conduits of the mounds of the species Odontotermes obesus, we show that
a simple combination of geometry, heterogeneous thermal mass and porosity
allows the mounds to use diurnal ambient temperature oscillations for
ventilation. In particular, the thin outer flute-like conduits heat up rapidly
during the day relative to the deeper chimneys, pushing air up the flutes and
down the chimney in a closed convection cell, with the converse situation at
night. These cyclic flows in the mound flush out $\text{CO}_2$ from the nest
and ventilate the colony, in a novel example of deriving useful work from
thermal oscillations.
