Species of the Week: Leafcutter Ants

If you're a naturalist or have an interest in wildlife, then you'll most likely have heard of Leafcutter Ants. These industrious little insects are a wonder of the natural world, and well worth a Species of the Week dedicated to them.
The common name Leafcutter Ants is pretty vague, since it can mean any of the species belonging to either genus Atta or Acromyrmex. Distinguishing the genus is easy: Atta species have a smooth exoskeleton with 3 pairs of spines, whereas Acromyrmex species have a rough exoskeleton and 4 pairs of spines. There are 47 known species in total, and they can be found in Mexico, South and Central America, and also parts of southern United States.

Within a species there are several variations, known as castes, which specialise in doing a certain job. In most Leafcutter species there are the following castes:
Minims - these look after the young and fungus gardens (more on them later). They are the smallest caste.
Mediae - the actual leaf cutters. They cut up vegetation using their mandibles and transport it back to the nest.
Minors - these patrol the borders of the nest and attack intruders.
Majors - also known as soldiers, this caste deals with enemies that get past the minors. They have enormous mandibles to bite at intruders.
All these, as well as a Queen, larva, pupa, and eggs, live in an underground nest that can span over 30m across! The nest may contain up to 8,000,000 individual ants!

 An Acromyrmex octospinosus mediae cutting a leaf.

This Acromyrmex octospinosus mediae is making good progress cutting the leaf.

Some Atta columbica mediae take leaves back to their nest. Often the minors will ride on the leaves being carried, possibly keeping an eye out for parasitic flies that may endanger the mediae.

You might think that the only creature on Earth that can farm plants and animals are humans. But ants also do it - several species 'farm' aphids for honeydew, and Leafcutters farm fungus. It is a common misconception that Leafcutters are taking the cut foliage back to their nest to eat - but they're actually growing fungi on it. They have a chamber in their nest called the fungus gardens, in which all the foliage is placed. The fungi grows on the cut leaves and is tended to by the minims. The fungi also releases a chemical that the ants can detect, telling them that a particular type of leaf is toxic to them. The ants will remove the offending foliage and never collect it again. The farmed fungus is fed to the larva - the adults never eat it, instead they have a diet of tree sap.

Some Leafcutter Ants in their fungus garden.

A group of hard-working mediae can strip a whole citrus tree in just 24 hours. Because of this, they aren't too popular amongst farmers and are seen as an agricultural pest.

The Queen is always hard at work in the nest laying eggs. She may lay up to 1000 in a single day! When the larva hatch out, they are immediately fed pieces of fungus by the minims.

A Leafcutter Ant Queen amongst several different castes

Every so often, young Queens and males will leave the nest and fly up into the air. This is known as the revoada. A female will mate with several males while she's flying, then land on the ground and shed her wings. She begins to make a burrow: the beginning of her own colony. She takes with her a little piece of fungus from her previous home to kick-start a fungus garden. She will begin to lay eggs in her new nest, and when they hatch and the larva turn into adults, she will have her first workers.

 Some captive Atta cephalotes take leaves back to their nest

I hope that you have enjoyed this post as much as I have enjoyed writing it.

These ants really are incredible creatures. Next to humans, they have the most complex societies of any animal. If that's not amazing, then I don't know what is.

Until next time, keep on the wild side!