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Nov 29

Bizarre Animal of the Week: Ant-Decapitating Fly

Ants are the ultimate soldiers. They never question orders from their superiors, they complete their missions to the full or die trying, and they never take a day off sick. They are so focused on the task at hand, in fact, that they will continue marching even if they lose their heads… literally.

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Photo: Core A, Runckel C, Ivers J, Quock C, Siapno T, et al

The picture above shows a tiny fly called Apocephalus from South America. Although the female fly is many times smaller than your average ant, this doesn’t stop her from landing on one and injecting an egg into its body. When the larva hatches, it immediately makes its way to the ant’s head cavity and starts gorging on the muscles and other tissue. Eventually, when the inside of the head has been emptied of all edible matter, the head capsule falls off and the fly maggot completes the rest of its development within the safety of this shell. This grisly behaviour has given this species the common name of ant-decapitating fly.

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An ant-decapitating fly emerging from the head capsule of an ant
Photo: S.D. Porter, USDA-ARS

Unlike higher organisms, such as vertebrates, which have a massively important centralised brain located in their heads, invertebrates have a nervous system based on ganglia – small nerve nodules – that are repeated in each body segment. The ‘brain’ ganglion is the most important but it is not the vital, ‘all-controlling’ organ that we see in vertebrates. Therefore, a decapitated ant doesn’t instantly die. In fact, if we assume that it doesn’t lose a lethal amount of haemolymph (insect blood) when the head is removed, an ant can carry on with its life for hours or even days with little to no change to its body function, breathing or behaviour. It will only finally keel over when it dies of starvation.

Having your head cut off and then blindly trying to carry on with your life until your food reserves run out may seem like a fairly horrific demise, even for an ant, but they have started to defend themselves at least. They are most at risk when out on foraging trips so the workers of leafcutter ants take with them very small members of the colony called minims.  The minims ride about on the backs of the larger ants, ready to ward off nearby ant-decapitating flies and stopping them from landing on their sisters. Being so small, the minims are unlikely to be targeted by the flies themselves, and they offer a good defence to the workers below as they slave away.

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