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Strange Ants: Zombie, Dracula, Herders, and Farmers
Ants are impressive insects that have intricate colonies. Some have developed very interesting lifestyles. In tropical rainforests, carpenter ants are invaded by a fungus that enters their brain. The fungus controls the behaviour of the ants, turning them into zombies. The queen of another type of ant feeds on the blood of her larvae, reminding scientists of the Dracula legend.
Ants may also be farmers. Some species herd caterpillars. They “milk” their charges by stimulating them to release a sweet secretion. Other ant species take care of aphid colonies, stimulating the aphids to release honeydew for food. Leafcutter ants bite pieces off leaves and take them to underground nests. Here the leaves serve as food for a fungal colony, which the ants harvest and eat.
Ants live on all continents except Antarctica. The greatest variety of the insects is found in tropical regions. All known species live in colonies in a home known as a nest. An ant nest is often a complex underground construction with many tunnels and chambers. There may be a raised anthill above the nest, which also has tunnels.
Ant colonies contain a queen who lays eggs, males to fertilize the queen, and non-breeding females known as workers. The workers collect food, maintain and protect the nest, rear the young, and take care of the queen. The males have one job—to fertilize the queen—and don’t live for very long.
Some ants form supercolonies. When ants from different nests meet, they are normally aggressive towards each other. Scientists have noticed that in a few species, ants from different nests are not only unaggressive when they meet but actually cooperate with each other as though they belong to the same colony. These collections of ants are sometimes huge and have been dubbed "supercolonies". Evidence suggests that the ants in a supercolony are closely related genetically.
A Fungus That Creates Ant Zombies
The creation of a zombie ant begins when spores of a fungus named Ophiocordyceps unilateralis enter certain species of tropical carpenter ants. The spores geminate inside the ants, producing the thread-like mycelium which makes up the body of the fungus. The mycelium spreads through the body of an infected ant, eventually reaching the insect's brain and releasing chemicals that affect its behaviour.
Infected ants leave their nest in the trees and fall to the forest floor. The temperature and humidity in this area are ideal for fungal survival and reproduction. Under the influence of the fungus, the ant moves to the underside of a leaf which is about ten inches above the ground. The ant then bites down hard on a leaf vein, attaching itself to the underside of the leaf.
The ant eventually dies but maintains its grip on the leaf. The fungal mycelium produces a reproductive structure that sprouts from the ant's head and releases spores from a spore case. The spores then infect more carpenter ants. Researchers have found that many fungus-controlled ants attach themselves to leaves at the same time, forming an ant graveyard.
Zombie ants are carpenter ants whose behaviour has been affected by a fungus. Carpenter ants belong to the genus Camponotus. They build their nests in the wood of trees or buildings, but they don't eat wood. They feed mainly on dead insects and honeydew.
Zombie Ants in North America
New fungi that can produce zombie ants have recently been discovered. These discoveries should enable scientists to learn more about the intriguing relationship between the two organisms.
One species of zombie fungus was found in the United States in 2014. The fungus has been studied by scientists at Pennsylvania State University. They've discovered that it releases a complex mixture of behaviour-altering chemicals when it enters an ant belonging to its host species. When the fungus infects an ant of a different species, however, it doesn't release the chemicals and doesn't alter the ant's behaviour, even though it may kill the the insect.
Somehow the zombie fungus "knows" when it's inside an ant that it can control. As David Hughes, an assistant professor of entomology at Penn State says, "the one without the brain controls the one with the brain".
The scientific name of the North American zombie fungus hasn't been finalized. It's believed to be a type of Ophiocordyceps unilateralis. This species appears to be a complex of similar but not identical insects rather than a species in the usual sense.
Dracula ants are classified in the genus Adetomyrma and are found in Madagascar. They are believed to be related to the first ants, which evolved from wasps. Their name was derived from their feeding method, which resembles that of Count Dracula, the vampire in Bram Stoker's famous novel.
Dracula ants live underground. The worker ants are blind. Both the queen and the workers bite holes in the ant larvae and then feed on their blood. Although this process sounds dramatic, the wounds and blood loss don't kill the larvae. Researchers refer to this feeding method as "non-destructive cannibalism". An ant’s blood is technically known as hemolymph and is colourless.
The feeding behaviour of Dracula ants may have been the precursor to the trophallaxis seen in some other ants. In trophallaxis, food or fluid is passed from one member of a community to another by mouth-to-mouth feeding or anus-to-mouth feeding.
Herding and Milking Caterpillars
The caterpillars (or larvae) of many butterflies in the Lycaenid family have a special relationship with ants. Lycaenid butterflies are sometimes known as blues, coppers, or hairstreaks. The relationship between the caterpillars and ants takes several forms, but it’s often beneficial for each insect. The ants crawl over the caterpillars and trigger them to release a sugary solution. They do this by touching glands on various parts of the caterpillars’ bodies with their antennae. The ants then drink the secreted solution. The insects are often said to be “milking” the caterpillars.
Some species of ants build shelters for their caterpillars. At night the ants protect the caterpillars in the shelters. At the start of the day the ants herd the larvae up a tree so that the caterpillars can feed on leaves. The ants guard their herd during the day.
Taking Care of Aphids
Aphids are small insects found around the world. They are most common in temperate areas, where they are major plant pests. Aphids have a wide range of colours. They may be colourless, pink, red, yellow, green, brown, or black. The green forms are sometimes known as greenflies.
Aphids have mouth parts than pierce plant veins and suck up the sugary sap that is being transported in the phloem vessels of the veins. Once they have digested the sap, the insects release a sweet liquid called honeydew from the ends of their abdomens. Some ant species drink this liquid.
An ant colony "farms" its aphids, carrying them around to new plants when necessary. The ants stimulate the aphids to release a drop of honeydew by tapping or stroking them with either an antenna or a foreleg. They also protect their herd from potential predators. Some ant species collect the eggs produced by the aphids and store them in their nest over winter. They take the eggs back to the plant leaves in the spring.
The relationship between ants and aphids that produce honeydew is an example of mutualism. In mutualism, both animals benefit from their association. In this case, the ants receive a regular source of honeydew and the aphids receive protection from predators.
Leafcutter ants are found in the tropics and in the southern United States. They cut pieces from leaves or petals with their sharp jaws and then carry the plant pieces to their nest. Leafcutter ants are sometimes known as “parasol” ants, since as they travel they hold their piece of leaf or petal above their head. They may roam as far as two hundred and fifty metres to collect plant material for their nest. The ants find their way home by following a trail of chemical pheromones which they deposited as they moved away from the nest.
The leafcutter ant colony contains several types of ants of different sizes, each with specialized functions. The largest ant in the colony is the queen. Next in size are the soldiers, who protect the nest. Smaller worker ants collect the plant pieces. In the nest, the leaf carriers pass the leaves to even smaller ants, who chew the leaves and turn it into a mulch. The mulch supports the growth of a specific type of fungus, which the ants—especially the larvae—eat. The smallest ants of all, called minima ants, take care of the fungal garden.
The insects tend their fungus crop carefully, removing debris and even destroying invading fungi of a different kind. Bacteria live on the surface of the ants. These bacteria produce antibiotics, which rub off the insects’ bodies and on to the fungus crop, killing the invading fungi.
Leafcutter Ants at Work
About 10,000 species of ants have been identified, but scientists think that around twice as many species actually exist. According to the AntWeb website run by the California Academy of Sciences, more than one thousand trillion individual ants are thought to exist on Earth.
It's true that some ants can be annoying and that some are harmful for plants, other animals, or humans. I think that they are interesting insects, though. There may be many more strange ant behaviours waiting to be discovered.
Information about zombie ants from Pennsylvania State University
Pensoft Publishers. (2015, June 23). Tiny Dracula ants hunting underground in Madagascar and the Seychelles. ScienceDaily. Retrieved July 30, 2017 from www.sciencedaily.com/releases/2015/06/150623095511.htm
Relationships between lycaenid butterflies and ants from the Australian Museum
Imperial College London. (2007, October 11). Herding Aphids: How 'Farmer' Ants Keep Control Of Their Food. ScienceDaily. Retrieved July 31, 2017 from www.sciencedaily.com/releases/2007/10/071009212548.htm
Information about leafcutter ants from the American Academy for the Advancement of Science (AAAS)
© 2011 Linda Crampton