Castes

Within a colony, there is a lot of work to be done. So many different tasks need completing. From foraging to feeding, from defence to digging, all of these roles require different actions and abilities. An average ant could probably complete these tasks, although not very well. Some tasks they couldn’t complete at all such as cutting up large food items and, in some cases, even feed themselves. Some of the soldiers in different species struggle to feed themselves. Why aren’t there different ants in a colony that are built for different roles? Well that’s because there are! This mislead was terrible and I will carry on with the facts now. Division of labour is one of the defining characteristics of eusocial insects. This division goes beyond merely their roles and extends to their very physiology. Queens, males and workers are castes as we have discussed before but many species go further than this. Much further.

A Lasius aphidicola worker. Jack of all trades, master of none (Alex Wild).

In some species, such as leaf cutter ants, Atta sp., there can be 7 worker castes. Each caste has a very specific job to do. This is known as Polymorphism. The smallest workers are called minims and their job is to care for the brood, queen and fungus gardens. Leafcutter ants have developed agriculture and this deserves its own post so stay tuned. These minims are small because they need to be able to access all areas of the fungus. A greater size would not benefit them at all. The medium sized workers are called ‘Medians’ do pretty much everything else. They forage for leaves and excavate the nest. Their jaws are shaped to allow for digging and cutting of leaves. These are the workers you are most likely to see. The largest are the majors or soldiers. Their job, as their name suggests, is to defend the nest and the colony. They have massive heads packed with huge muscles. These drive their jaws into whatever is unfortunate enough to think messing with leaf-cutter ants is a good idea. It is not.

A soldier and a minim of leaf cutter ants, Atta sp. (Alex Wild)

Some of the largest size differences between castes are found in Carebara diversa. These are more commonly known as Asian army ants. They aren’t true army ants but they hunt in similar swarms. The queens and soldiers are over 20mm long while the smallest workers are around 3mm long. The soldiers are over 500 times heavier than the minims. That would be like the average American having as sister that weighed 40,000Kg! Huge! Their soldiers defend the colony and cut any larger prey items into more manageable chunks for the smaller workers. Soldiers are often equipped with stings and/or powerful mandibles that the other castes may not have.

A selection of Carebara diversa castes. (Antstore)

Many ant species or genera have specific castes based on their lifestyles. I will be covering these all in their own posts but we will talk about one example now. One of my favourite ants are honeypot ants in the genera Myrmecocystus. They, along with the other genera of honeypot ants, have a special caste called repletes.

These species live in deserts which have dry seasons that give them almost no food or water. This means they need to store any and all food they gather in the wet seasons. Food has an annoying tendency to rot and go off so these ants needed a special way to store food for months on end. All ants have 2 stomachs. A normal one which digests food and a social stomach which stores food and water to be shared with the rest of the colony. When some of the ants eclose from their pupal stage, they are instantly overly fed. Because their exoskeletons and social stomachs are still soft and malleable they grow to enormous sizes. Their exoskeleton plates part making their abdomens transparent.

Myrmecocystus mexicanus repletes (Alex Wild).

They hang from the tops of their nests to avoid being damaged. These repletes sustain the colonies in the dry months of no food or water. They have the added bonus of looking cool. If you want to learn more about specialsist adaptations we have the first post of a series covering specialist adaptations. This one covers Saharan Silver ants.

I hope you enjoyed this post and learned something! Please check out the series ‘Weird and Wonderful Adaptations’ in the blog posts. The first one is already up and is defiantly worth a read. You can find it here. You may also have noticed we have a store page. stay tuned for some exciting products! If you have any questions please leave them below or join and comment on our Facebook page. Thank you,

Alex

Glossary

Caste- A specialised level in a colony of social insects, such as ants, in which the members, such as workers or soldiers, carry out a specific function.

Polymorphism- Different ants having different appearances and functions within a colony.

Physiology- All the functions of a living organism or any of its parts.

Replete- A specialised worker in a honey ant colony that stores food in its distensible abdomen for later use by other members of the colony.

References

Burgett, D.M. and Young, R.G., 1974. Lipid storage by honey ant repletes. Annals of the Entomological Society of America67(5), pp.743-744.

Conway, J.R., 1986. The biology of honey ants. The American Biology Teacher48(6), pp.335-343.

Hölldobler, B., 1985. Liquid food transmission and antennation signals in ponerine ants. Isr J Entomol19, pp.89-99.

Hölldobler, B. and Wilson, E.O., 1990. The ants. Harvard University Press.

Moffett, M.W., 1987. Division of labor and diet in the extremely polymorphic ant Pheidologeton diversus. National Geographic research (USA).

Moffett, M.W. and Tobin, J.E., 1991. Physical castes in ant workers: a problem for Daceton armigerum and other ants. Psyche98(4), pp.283-292.

O’Donnell, S., Bulova, S., Barrett, M. and von Beeren, C., 2018. Brain investment under colony-level selection: soldier specialization in Eciton army ants (Formicidae: Dorylinae). BMC Zoology3(1), p.3.

Wilson, E.O., 1980. Caste and division of labor in leaf-cutter ants (Hymenoptera: Formicidae: Atta). Behavioral ecology and sociobiology7(2), pp.157-165.

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