Nematodes & Fungi
Nematodes are one of the most abundant animals in the world. Roundworms communicate with each other via ascarosides, which are small signaling pheromones. This is an extension of nematode intercellular communication, which also employs ascarosides.
Nematodes are preyed upon by fungi, which build sticky webs to trap the worms. Building a web takes considerable effort. Fungi only rise to the occasion after eavesdropping on nearby worm talk, which signals that a catch may be made.