The world is an icy wasteland, populated only with dark fungi burrowing through the irradiated earth. The last plant life in this desert is cultivated in great cities of ants and honeybees. A few other, less sapient insect species live alongside them, mostly termites and other eusocials. While they can be physically protected by making alliance with the larger sophonts like humans, there is still the issue of defending against the elements
Insects are small, and most species are cold-blooded, which is not very good in an apocalyptic ice age. While trade and agriculture can assuage the need to leave the city in person, this still requires that the city itself must be warm (at least 15°C), rather than the 0°C that is usual outside. As this is a constant cold, the heating must work continuously. The city should also have warmer areas with a temperature around 35°C for the sake of productivity and comfort. The insects should be able to set up this heating themselves, and ideally should be able to synthesise the materials without the need for outside help
Given these requirements, is there any way for these insects to defend against the cold?