Caste

All ants are eusocial (except for a few derived parasitic species): that is, there are gynes (queens) and males which reproduce and workers which have limited or no reproductive potential [workers of some species may produce male eggs, and in some rare queenless Ponerine ants, workers (called gamergates) actually mate and reproduce (Peeters 1993, 1997)].

The term "caste" is often used to describe groups of workers which perform a set of specific tasks. We will stick with the strict "reproductive caste" definition (worker, female reproductive, or male reproductive).

The Japanese Ant Database Group also has a description of ant castes

Below are images of the three castes, from left to right, female worker, female reproductive, and male reproductive. The first series shows the Myrmicine Solenopsis molesta.

The female reproductive is highly specialized compared to the female worker. Note the wings, large wing muscles, and fat and glycogen reserves in the gaster that make the abdomen swollen. The female reproductive also has enlarged eyes (note that the worker's are extremely reduced) and often ocelli on the top of the head (note the bubble-like structures on the top of the male's head). These specializations of the female reproductive caste allow the gynes to disperse, often through large-scale nuptial flights, and found colonies. After participating in this mating flight, the females shed their wings.

The male reproductive caste (there are no male workers) are sex machines. They have exremely reduced mandibles, enlarged eyes and ocelli, large wing muscles, and abdomens tipped with their genital armature. After mating, the males die.

This second series of images shows side views of the Ponerine Ponera pennsylvanica. Ponerines are generally less morphologically specialized, that is, there is not much difference in morphology between the female workers and reproductives. Notice that the Ponera queen does not have an enlarged thorax for large wing muscles like Solenopsis does. This is because Myrmicine queens (as well as Formicines and Dolichoderines) often found colonies through idependent claustral colony foundation. They hide themselves in a chamber and survive by metabolizing the large thoracic muscles used during the nuptial flight. Ponerine queens do not have large thoracic muscles and they must leave their chamber to hunt for food for themselves and their brood.

Notice the cerci at the tip of the Ponera male's abdomen that forms part of the male's genital armature. Also notice that the males of these two species do not have bent antennas with an enlarged first antennal segment (the scapes). Myrmicine and Ponerine males do not have long scapes and bent antennae, but Formicine and Dolichoderine males do (see long scapes and bent antennae on this male Formicine Formica obscuripes).

So how do you differentiate between the castes? The large majority of ants collected are female workers. The only time that winged reproductives can be captured is right before or during a nuptial flight. After this mating orgy the males die and the female reproductives shed their wings and start new colonies. Reproductive females (queens) in established colonies are often difficult to find but they are usually easily distinguished from workers because of their larger size. Scars on the thorax of new queens can also often be found. Males are also easy to distinguish from winged female reproductives because as shown in the above images, they have different, narrower, abdomens with genital armature. These differences are often very obvious and it is usually easy to determine the caste of an ant specimen in the field.

Please send any questions or comments regarding these pages to Tim Linksvayer or Andy McCall