'The parasite was in the driver's seat'

'The parasite was in the driver's seat': The zombie ants that die gruesome deaths fit for a horror movie." Once the egg hatches, the ant has only a few weeks of life before it succumbs to the manipulations of its attacker, stumbling away from its home and family and then undergoing decapitation from the inside out."


A phorid fly (left), Pseudacteon cultellatus, about 1 mm long, and a red imported fire ant (right), Solenopsis invicta, about 3 mm long. The fly can lay eggs inside the ant’s head. Fly maggots that hatch will then slowly decapitate the ant. (Image credit: Sanford Porter)

Within days after hatching, the phorid larva migrates from the thorax into the ant's head; little is known about how the parasitoid avoids being destroyed by the ant's immune system, but one possibility is that moving quickly into the host's head may help the larva evade an immune response. For the duration of the larva's second instar — about two to three weeks — it makes itself comfortable in the ant's head cavity, sipping on hemolymph.

This liquid nutrition is all the larva needs until it reaches its third instar. For an infected ant during those initial honeymoon weeks, despite carrying and nourishing a growing parasite inside its head, life is pretty much business as usual; the ant looks and behaves normally, according to scientists with the Louisiana State University (LSU) College of Agriculture.

Through "intensive observation" of ant parasitism by the phorid fly Pseudacteon tricuspis, LSU researchers found that a parasitized ant stayed with its nest mates until about 8 to 10 hours before the larva in its head was ready to pupate. It would then depart the nest on what appeared to be a normal foraging expedition, alongside its non-parasitized sisters. But for the zombified ant, this final excursion was a one-way trip. Once the ant turned its back on the colony and walked away, it was on a death march. And the parasite was in the driver's seat. "Parasitized ants were highly mobile after they left the nest and ultimately entered the thatch layer at the soil surface," the scientists reported. "The term 'zombie' fire ant workers was coined to characterize the behavior while under parasitoid control."

Finally, the phorid larva is ready for its metamorphosis. It releases an enzyme that degrades membranes in the wandering ant's exoskeleton, causing the ant to stop walking and eventually collapse. The ant's head loosens from the body, as does the first pair of legs; other legs may be affected, too. Its mandibles weaken, rendering it unable to bite or burrow. As for the larva, it indulges a new appetite for solid food; namely, ant head tissue. You can probably guess where this is headed; the ant's hollowed-out, larvae-stuffed head falls off (the ant, unsurprisingly, is already dead by now, even though its legs are often still twitching as its head rolls away).

https://www.livescience.com/animals/insects/the-parasite-was-in-the-drivers-seat-the-zombie-ants-that-die-gruesome-deaths-fit-for-a-horror-movie


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