I am assuming ...

... that 'decay' was used to make some of the music effects. Here.

And, in case you are wondering, the fungus is a real world creepy thing that attacks ants and a few other insects. It is called "cordyceps campanotus" and there are several varieties of it.

I was recently watching an episode of Life On Earth where it was featured. It is a fungus found in the forest canopy that, at a certain point in its life, drops to the forest/jungle floor. Apparently, an ant then eats it or it gets on their body. It then invades their brain and causes the ant to go crazy. The specific ant is normally a ground dweller but, having been attacked by the fungus, it climbs to the roof canopy where it dies. And the whole cycle begins again with that weird stem coming out of its head. Yummy!

Here is the Wikipedia article.

Here is a photo.