An expansion based off of two features: miasma and water.
Fungus grows where its damp and dark. Water spreads down my underground fortress channels to my well, as the water recedes and drys up a fungus is left in it's place. The fungus spreads slowly beyond the channel, under door cracks, up stairs and down my twisting hallways. The fungus itself is simply an eye sore, making my dwarfs unhappy about the lack of tidiness unless they clean it up.
However, if miasma comes in contact with this fungal field then a Black Maggot starts to grow in its dank dark folds. It's an uncommon event, even with fungus and miasma. But these maggots aren't as friendly as the purring variety.
The maggot squirms about the floor and eats everything it can get its mouth on, starting with the fungus. Although this removes your fungal problem it creates a new one: As the maggot eats it multiplies into a larger swarm of maggots.
3 units of food eaten: The Black Maggot Upgrades to a Black Maggot Pile.
10 more units of food eaten: Black Maggot Pile Upgrades to Black Maggot Cluster and produces a Black Maggot
30 more units of food eaten: Black Maggot Cluster Upgrades to Black Maggot Swarm and produces a Black Maggot Pile.
90 more units of food eaten: Black Maggot Swarm Upgrades to Black Maggot Flood and produces a Black Maggot Cluster.
The Maggots continue to swarm around the original Black Maggot group and eat everything biological, regardless if it was living or dead, goblin or dwarf, cooked or raw. They don't eat trees or plantlife.