Structure of Macrotermes mounds
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The mounds of macrotermitine termites have stereotypical architectures, yet are highly variable in structure.

All species locate their colonies underground, where they cultivate fungi that aid in cellulose digestion. The mounds enclose a ramifying network of tunnels that forms a ventilation system for the nest. Some species may build open chimneys or vent holes into their mounds, (like the Odontotermes chimney below), while others build completely enclosed mounds that exchange gas through porous thin-walled tunnels (like the Macrotermes mound to the right). Even within populations, the variation of mound structure is prodigious.

The basic problem for understanding these structures is this: how can the millions of individuals in a colony work together to build a structure many times larger than themselves that has a characteristic architecture? Is there a "collective plan" for the mound that is encoded into the assemblages of termite brains that make up a colony? Or is there a more prosaic explanation?

The mound's external structure
The mound's internal tunnel network
The mound's surface structure

External stucture of the Macrotermes michaelseni mound

Outwardly, the mound consists of three parts:

A distinctive feature of these mounds is the northward tilt of the spire. At our study site in northern Namibia, this tilt is about 19 degrees on average, which corresponds to the average zenith angle of the sun (and to the latitude). The spire's northward tilt is probably due to termites building more avidly on the warmer north-facing surface of the mound (remember that Namibia is in the southern hemisphere). You can read more about this, including a video of how it happens, here.

Mounds are also often built around trees, as shown to the right. This may be due to a shelter effect: a foundling colony's chance of survival to maturity may be enhanced when the founding king and queen land in the shelter of a tree, rather than in the harsher environments between trees. The tree is not harmed by this - indeed it thrives. You can read more about this here. TOP

Internal structure of the mound and nest

Inside the mound is an extensive system of tunnels and conduits that serves as a ventilation system for the underground nest. This network of tunnels is made visible by casting them: filling the air spaces with concrete or plaster and then washing away the soil. Perhaps the first to do this was the Belgian entomologist Jean Ruelle (below left), who cast the tunnel network of a Macrotermes natalensis mound. Clearly visible is a vertical system of surface conduits, festooned with numerous fine egress tunnels that connect to the surface.

Rupert Soar (of Loughborough University) and I have cast the internal tunnel network of Macrotermes michaelseni (above, middle and right). As in Ruelle's cast, the tunnel network of the M michaelseni mound consists of an elaborate reticulum of surface conduits (middle) overlying a more extensive reticulum of large-calibre deep ventilatory tunnels (right).

More detail is revealed by sectioning the mound in various ways. The regular array of surface conduits is revealed, for example, by cutting out a slice from the mound, as seen to the right. A more systematic sectioning of the mound (see here for a detailed series of 10 cm vertical sections through a mound) reveals that the tunnel network is roughly differentiable into three types of tunnels (below):

The mound is built above the subterranean nest. The nest itself (above, right) is a spheroidal structure consisting of numerous gallery chambers. Each gallery contains a fungus comb, where raw forage, such as grass and wood, is digested by the symbiotic fungi (Termitomyces) that the termites cultivate within their nest. The chimney clearly arises from deep within the nest.

The queen herself resides in a queen cell (above, right), a specialized structure built deep within the nest. She is tended by the workers, who bring food to her and remove eggs through several access ports, shown in the photograph below.

As in all social insects, the queen (above, left) is essentially the reproductive slave of her offspring. When she matures, her abdomen swells into a grotesque egg-laying machine, which produces eggs at a prodigious rate: roughly one egg every three seconds. Unlike the bees, wasps and ants, in which the workers are all female, the termite colony consists of roughly equal proportions of males and females. Top

Surface architecture of the mound

The mound surface appears to be solid and impermeable, but it is actually quite porous. The porosity arises from the method that termites employ to grow the mound.

The mound grows by termites transporting soil onto the mound surface and depositing it there (below left). To get to the surface, termites dig numerous egress tunnels from the surface conduits to the mound surface. These are shown as the finely-divided filigree on the cast surface conduits (below right).

Deposition of new soil onto the surface is evident as patches of moist or rough deposits of soil (below left and middle) These new deposits are quite friable and air moves through them with relative ease. These patches of newly deposited soil open directly into the egress tunnels, which is revealed when a patch is removed (below middle).