The mound as a gas exchange device. 2

experimental mound

The principal problem for the colony's respiratory function is the mixing of the nest air (the focus of colony metabolism) and the mound air.

The well-known flow through models of nest ventilation assume the two air masses are well-mixed. In fact, they seem poorly mixed.

This means that any model of colony gas exchange that assumes good mixing of the nest and mound air will be erroneous.

nest versus chimney temperature

Stratification of nest air and mound air

The thermosiphin model in particular presumes that metabolic heat production from the nest will destabilize nest air, imparting a buoyant force to the spent air that lofts it up into the chimney.

In fact, the nest air actually is usually cooler than the mound air. We took detailed measurements of temperatures at various locations around the mound and nest, and logged these measurements over the course of a year. The graph at right summarizes the result.

The relevant measure here is the temperature difference between the nest (Tln) and the air above it, that is in the chimney, (Tmc). Cooler nest temperatures extend into the blue (to the left of 0), while warmer nest temperatures extend into the red (to the right of 0).

Contrary to the demand of the thermosiphon model, we see that the nest is most often cooler than the mound. Jn autumn and winter, it is about twice as likely that the nest will be cooler than the mound (about 67% vs 33%). This is due primarily to the very cold nighttime temperatures of the mound. In the spring and summer, the likelihood of a cooler nest is more than 90%.

This temperature distribution ensures the stable thermal stratification of the nest air from the mound air, and will impede, not promote, the mixing between them.


Air does not circulate between nest and mound

The thermosiphon model makes two predictions for the patterns of air flow between mound and nest.

First, air in the surface tunnels of the mound should at some point move into the nest. Second, air in the nest should at some point move into the chimney air spaces above the nest.

Both predictions can be tested by using tracer gases in a so-called pulse-chase experiment. This involves injecting a pulse of tracer at some location and measuring the time it takes to make its way to a detector at some other location.

We can test the predictions of the thermosiphon model with two experiments. In one, we inject tracer into a surface tunnel and try to detect its arrival in the nest. In the second, we inject tracer into the nest and try to detect its arrival in the chimney.

In both experiments, the thermosiphon model does not hold up. Tracer injected into a surface tunnel never appears in the nest. We know from other experiments this air is commonly drawn up and out through the mounds' porous surface.

mound to nest

Tracer injected into the nest does appear in the chimney, but not in the expected way. If there was flow through the nest, tracer injected there would appear as a pulse, or bolus. It does not. Rather, tracer appears to emerge slowly and in episodic "puffs."

nest to mound

Both of these are interesting patterns of exchange and flow. Suffice it to say they do not support the thermosiphon model's claim of bulk flow through the nest.

Structural impediments to mixing

fungus gallery passageway

Finally, there are structural impediments to the free flow of air between the mound and nest.

The nest is subdivided into many galleries that house the fungus combs. These connect to one another only through very small passageways, a few millimeters in diameter (wdie enough to allow passage by worker termites), and usually only one or two between adjacent galleries. The air space within a gallery is usually occupied by the convoluted structures of the fungus comb.

The air passageways connecting the chimney and nest are also small. Although the chimney extends into the nest, the chimney is formed from abandoned and partly excavated galleries. This means the passageways connecting the nest center to the chimney are the same small passageways that connect galleries to one another.

Peripherally, the extensive tunnels of the paraoecies connect to the air spaces of the peripheral galleries through the same small number of narrow passageways. The overall effect is a nest structure that impedes air flow through it.

This, combined with the termites' tendencies to block flows of air, and the thermal stratification of nest and mound air, all point to the nest and mound containing two air spaces that are very poorly mixed with one another.


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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