Grass-cutting ants build gigantic nests - underground cities where up to seven million insects live and tend a fungal garden that feeds their young. Scientists have now discovered how the ants build nests that stay at the right temperature for this precious fungus to grow. The ants build porous turrets, specifically to ventilate the nests. A study, reported in the Journal of Insect Behaviour, has revealed how they manage this feat. Highly magnified image of the grass cutting ant, Atta vollenweideri (Image: Antweb) The insects mould clay in their specialised jaws Marcela Cosarinsky from the Argentinian Museum of Natural Sciences, Bernardino Rivadavia, in Buenos Aires led the study. It was already known that the turrets, which jut out above the ants\' expansive nests allowed air into the underground labyrinths. But this study is the first to show that the ants carefully construct the turrets with highly porous walls that are perfect for this exact job. To show this was the case, the team carefully designed a construction task for the ants. Dr Cosarinsky and her colleagues brought a colony of grass-cutting ants into their laboratory and provided them with materials to built their giant nests. They gave the ants three building materials: clay, coarse sand and fine sand, and tested the insects\' building techniques by regularly changing the amount of each material that was available to them. They even simulated rain damage to the nest by pouring water on to the structure. \"When [the ants] finished a turret, we analysed the arrangement of the building materials [under] the microscope,\" Dr Cosarinsky told BBC Nature. This revealed that, whatever materials they used, the ants always made turret walls that were highly porous and allowed air to flow through. The ventilation turrets on a nest built by grass-cutting ants, Atta vollenweideri (Image: Marcela Cosarinsky) All of the ventilation turrets have highly porous walls that allow air to flow \"The ants construct the turrets by stacking sand grains and little balls of clay that they mould with their [jaws],\" Dr Cosarinsky explained. If the walls were watered, these pores would collapse, compacting the walls. \"Immediately workers removed the materials\", she said, and reworked the turret wall. The findings confirm that the ventilation turrets are \"built structures\". \"They don\'t simply result from a passive deposition of excavated soil, as is the case in many other ant species,\" the scientist explained. Ant architecture Prof Nigel Franks from Bristol University\'s Ant Lab described the study as \"beautiful research\". \"It shows that the ants change the way they build ventilation turrets depending on the materials that are available to them.\" CT scans of an ants\' nest showing its construction over a 24-hour period (Image: Nicholas J. Minter, Nigel R. Franks and Katharine A. Robson Brown) A medical CT scanner allowed scientists to capture 3-D snapshots of the nest being built over time Prof Franks recently published his own findings from a study of ant nest architecture. He and his team used medical CT scans to produce remarkable images of an ants\' nest being constructed over time. This generated accurate 3-D images of the nest, a feat that has previously been achieved only by pouring plaster or molten metal into the structure. Although this technique has created beautiful, accurate casts of the ants\' nests, \"it kills the poor ants and destroys the nest\", said Prof Franks. By taking CT snapshots over time, Prof Franks and the team revealed that yellow meadow ants, Lasius flavus, were able to sense the depth at which they were digging, and use this as a trigger to switch from vertical tunnelling to horizontal. The insects are able to sense minute differences in the density of the soil at different depths. This highly tuned sense allows them to build organised horizontal pathways that connect the nest chambers. \"Ants often operate at a minute scale,\" said Prof Franks. \"They seem well able to detect tiny differences in the substrate or even in minuscule air currents and then they can act on this information to improve the architecture of their buildings.\" The scientist said that humans had a great deal to learn from ants. \"They have been around for perhaps 100 million years,\" he said. \"Evolution by natural selection has equipped them with procedures that enable them to do certain things wonderfully well.\" The scale of grass-cutting ant-built structures are certainly impressive. The mounds they build are commonly 6-8m in diameter, almost 1m high and weigh approximately 40 tonnes.