Zombie flies: Fungal spore dispersion using a soft water cannon*

When the pathogenic fungus Entomopthora muscae infects a house fly, it strongly impacts motor control of the insect. The fungus forces the infected fly into a “zombie-like” state: it instinctively moves to an elevated position allowing the fungus to shower spores over nearby flies. Fungal dispersion efficiency critically depends on the range of these spores. We study the mechanism of forcible ejection of the fungal spores from the tip of the small stalks which penetrate outward from the fly cadaver. Each stalk act as a soft water cannon, using a liquid pressure buildup to eject a jet of cell contents and the spore initially attached to the end of the stalk.

 

We study the ejection using both a force-balance model and a millimetric experimental model system - an elastomeric fluid-filled barrel plugged with a projectile - that allows variation of geometrical and elastic properties of the cannon. We identify critical liquid pressure for break-through, the projectile’s exit velocity and the energy conversion efficiency. We compare these results to high-speed visualization of micrometric fungal spores, and predict an optimum projectile range versus system size based on our model for the projectile’s exit velocity and aerodynamic drag.