wind energy colony metabolism colony recruitment
Figure 11.12 Zones of gas exchange in mounds of Macrotermes michaelseni.
Figure 11.13 Schematic diagram of the construction and maintenance of the adaptive structure of a Macrotermes michaelseni mound.
the exchange of respiratory gases in the alveolus of the mammalian lung. Variation in wind speed and direction induces a tidal ventilation in the surface conduits. The admitted energy is damped by the lateral connectives, where it helps to mix chimney air with surface conduit air. Variation of gas exchange is driven partly by variation of wind speed and partly by variation of the colony's metabolism.
Together, these two energy sources form the basis of a feedback meta-loop that conveys information to the colony about the mound's performance as a gas exchanger (Fig. 11.13). Coupling this information to the stigmergic reflex described above probably completes the feedback, and in this way the mound can act as a truly adaptive structure. It probably works like this. The gas composition of the air inside the nest is determined by the balance between the colony's rate of production of carbon dioxide and its consumption of oxygen and the rate at which these gases are ex-
changed between the nest air and the environment. As always, homeostasis of the nest atmosphere requires that these fluxes be matched (equation 11.5).
Any departure of the nest atmosphere from that preferred by the termites indicates a mismatch between the colony's PF and the TFF through the mound. If, for example, the colony pO2 is low, the TFF is comparatively low. This indicates that insufficient wind energy is available to power the mixing of colony air and surface conduit air. Conversely, if the colony pO2 is too high (or the pCO2 is too low), the TFF is relatively high, which means that too much wind energy is driving the mixing. Indeed, the rate of exchange of gases inside the mound is strongly influenced by wind (Fig. 11.14): high wind speeds increase flux, and the increase in flux alters the gas composition inside the colony. If these colony-level variations of atmospheric composition are coupled to recruitment of workers and the activation of the stigmergic reflexes described above, then the mound can be considered an adaptive structure.
For example, consider what must happen as the colony grows. From its inception (the queen and a few hundred workers in her nuptial brood) to its maturity, the colony's collective metabolic rate increases by about six orders of magnitude. Despite this large increase in demand for oxygen, the composition of the nest atmosphere stays pretty constant, with CO2 concentrations hovering between 2-5 percent at all stages of colony growth. Such large increases of respiratory flux are supported mostly by the upward extension of the mound into sufficiently energetic winds to power
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