The mound as a gas exchange device. 1

Section through the respiratory passages of a mound

The termites and their fungal symbionts respire in the same way we do: obtaining chemical energy for work by oxidizing food with oxygen, producing carbon dioxide waste in the process. As in ourselves, the higher the colony's rate of energy consumption (or its metabolic rate), the faster it must exchange of oxygen and carbon dioxide with the atmosphere.

The Macrotermes colony stands out in the termite world as having a very high rate of metabolism: various estimates put the colony's metabolic rate at 55 watts up to more than 200 watts. To put this number in perspective, the resting metabolic rate of a typical human is in the range of 80-100 watts.

To sustain these high metabolic rates, oxygen must flow into the colony at a rate that can match the demand for oxygen. Similarly, carbon dioxide waste must leave the nest as rapidly as it is produced, just as we must do. We sustain our high rates of gas exchange through muscle powered ventilation of the lungs. Without this, our bodies would suffocate. The same is true for the termite colony.

For many years, entomologists have assumed that the mound mediates a bulk flow of air through the mound, driven either by waste heat from metabolism lofting powering a thermosiphon flow, or by vertical gradients in wind speed drawing air through the nest, as a chimney works. There's a little bit right in both of these, but these schemes are mostly incorrect.

Flow through models of termite colony ventilation

Since Martin Luscher's original paper on the "air conditioned" Macrotermes nest, two principal models have emerged that supposedly explain gas exchange in the termite colony. Both suppose a bulk flow of air through the nest.

Martin Luscher's original model has come to be known as thermosiphon ventilation (left). Here, nest air warmed by waste heat from metabolism, is lofted up into the mound by buoyancy. This forces air down supoerficial channels just beneath the mound surface, where the excess heat and carbon dioxide is lost, and oxygen refreshed. This makes this air more dense, so that it sinks down to the nest, ready to be drawn in another circuit. Thermosiphon flow supposedly operates in so-called "closed chimney mounds, that lack an obvious vent hole to the outside. Macrotermes michaelseni and M natalensis are species that commonly build capped-chimney mounds.

two models of termite nest ventilation

The second model, induced flow, supposedly operates in open chimney mounds (right), that have a large-calibre vent hole at the top. Macrotermes subhyalinus, and Odontotermes transvaalensis build open chimney mounds. Induced flow taps boundary layer gradients in wind speed. Air moving rapidly over the vent hole draws spent nest air out through a Venturi effect. Fresh air is then drawn in through smaller entrance holes around the base of the mound.

Neither model is based upon actual measurements of flow through the nest. Both are inferences based upon mound morphology or distributions of temperature, oxygen and humidity.

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Multi-phase gas exchange in the lung

Actual measurements of air flows in the mounds and nests of Macrotermes michaelseni show that there is no bulk flow of air between the mound and nest. This seems impossible, until we realize that there is also no bulk flow of air through lungs, and yet they work perfectly well as gas exchangers. How do they do it?

Lungs are a so-called multiphase gas exchange system. This simply means there are at least three mechanisms of gas exchange at work in the lung.

Multiphase gas exchange in the lung

  • There is bulk flow of air only in the upper respiratory passages if the lung. This is the in-and-out breathing powered by the respiratory muscles of the chest. This is the convection phase.
  • At the lung's ultimate air spaces, the alveoli, there is virtually no bulk flow of air. Here, gas exchange is principally by diffusion. This is the lung's diffusion phase.
  • Bridging the two is a region, involving the lung's penultimate passageways of the bronchioles and alveolar ducts, diffusion and convection are of roughly similar importance. This is the lung's mixed regime phase.

What governs the lung's ultimate rate of gas exchange, say how rapidly oxygen moves from the air to the pulmonary blood, is determined mostly by how effectively air in the diffusion phase can be mixed with air in the convection phase.

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Multi-phase gas exchange in the termite colony

Something similar operates in the Macrotermes colony.

Like the lung, the termite mound and nest complex contains an elaborate network of convoluted and differentiated passageways. This belies multiple regimes of gas exchange regimes are analogous to the multiphase exchanger of the lung.

Multiphase gas exchange in the termite colony

  • The nest itself is a network of galleries connected to one another through tiny passageways that allow only limited flow between them. This is analogous to the diffusion regime of the lung. Although air is very still in the nest, there may be weak flows due to natural convection.
  • In the mound, there is a complicated regime of wind-driven flows that are analogous to the muscle-driven flows in the upper respiratory passages.
  • In between, situated roughly at the boundary between the nest and base of the mound is a mixed-regime region. Gas exchange in the mound-colony system is mediated by the extent of mixing across this mixed-regime phase.

Contrary to both the bulk flow models that have been proposed, the nest air and mound air constitute two poorly-mixed air spaces.

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Termite pages

Termite home

Structure

Endocasting

Social homeostasis

Nest temperature

Water homeostasis 1

Water homeostasis 2

Water homeostasis 3

Fungal symbiosis

Fungal symbiosis and water 1

Fungi and water homeostasis 2

Gas exchange 1

DC vs AC Gas Exchange

Gas exchange 2

Gas exchange 3

Gas exchange 4

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