Leafcutter is the common name given to 39 species of Central and South American fungus growing ant from two genera. These two genera are Atta (15 described species) and Acromyrmex (24 described species).
All 39 species of these ants have evolved an advanced agricultural system and have a symbiotic relationship with the fungus they grow. The ants eat structures called gongylidia produced by the fungus and this fungus (different species of ant grow different species of fungus) grows only in the underground chambers of the ants' nest.
The mutualist relationship the ants have with the fungus is further augmented by another symbiotic partner, a bacterium that grows on the ants and secretes antimicrobials.
Whilst the two genera have much in common anatomically, they can still be identified by their external differences. Members of the Atta genus ants have 3 pairs of spines and a smooth exoskeleton on the upper surface of the thorax while species belonging to Acromyrmex have 4 pairs and a rough exoskeleton.
Mature colonies of either genus can contain more than 8 million ants. Most of these are sterile female workers. They are divided into castes, based primarily on size, that perform disparate functions. Both Atta and Acromyrmex exhibit a high degree of polymorphism, though this is somewhat more pronounced in Atta than in Acromyrmex.
The castes are often grouped as follows:
- Minims are the smallest workers, and tend to the growing brood or care for the fungus gardens. Minims have a head width less than 1 mm.
- Minors are slightly larger minima workers and are present in large numbers in and around foraging columns. These ants are the first line of defense and continuously patrol the surrounding terrain and vigorously attack any enemies that threaten the foraging lines. Headwidth being 1.8-2.2 mm
- Mediae are the generalised foragers, who cut leaves and bring the leaf fragments back to the nest.
- Majors are the largest worker ants and act as soldiers, defending the nest from intruders, although there is recent evidence that majors participate in other activities, such as clearing the main foraging trails of large debris and carrying bulky items back to the nest. The largest soldiers (belonging to Atta laevigata) may have total body lengths up to 16 mm and head widths of 7 mm.
When the ants are out collecting leaves, they are at risk of being attacked by the phorid fly, a parasitic pest which lay eggs into the crevices of the worker ants head. Often a minim will sit on a larger caste and ward off any attack.
Image Copyright© pbertner
Tags: Atta | Acromyrmex | Morphology & Physiology | Evolution
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