The ruins of a large city fan out over a 10 mile square, visible mostly as the rotted stumps of walls amid the overgrowth. Except for an unexplained clicking sound which can barely be heard over the moan of the wind, the city is silent.

When night falls, from the sewers and storm drains flow cockroaches. Billions of cockroaches. Carnivorous cockroaches.

These swarms are controlled by 'brains' which are formed by a million roaches connected by linking antennae to become a single neuron in the collective mind which controls swarms of resource gatherers through pheremones.

There are a dozen such brains in the city at various levels of intelligence, with another thirty or more less intelligent brains either newly formed and growing, or aged and declining.

Keep in mind that specialization is an insect trait. So there may be pheremone spreaders, warriors, food gatherers, etc.