Ants are members of the insect order Hymenoptera, which also includes the bees and wasps. All ants are in the family Formicidae, which is divided into a dozen or so subfamilies. Worker ants are easily recognized by their combination of a constricted waste at the front of their abdomen combined with their lack of wings. The constricted part of the abdomen, or petiole, has one or two nodes. While we usually think of ants as having a sting, many common ants lack a sting; these are in subfamily Formicinae; if you pick up an ant in this subfamily and sniff it, you'll detect the sharp odor of formic acid, which is used as a defensive chemical by these ants.
All ants are either highly eusocial or are parasites on other ants. Because no intermediate forms between solitary wasps and the present-day ants are known, little is understood of the initial phases of social evolution in ants. From a morphological standpoint, the most "primitive" ants are found in Australia. In fact, the Australian Nothomyrmecia is a true living fossil--this genus was first found as a fossil and later living representatives of the same genus were found. Even in Nothomyrmecia, though, queens and workers are well differentiated, with the wingless workers being well-specialized for their role. The few vestiges of wasp-like behavior in the "primitive" Australian ants--Nothomyrmecia and the bulldog ants, Myrmecia--include the use of their sting when capturing insect prey, relying on vision to find and track prey, and moving on the ground with rapid, jerky movements, much like the movements of spider wasps.
As mentioned above, systematists have divided ants into a dozen or so subfamilies. These are listed on the ant diversity page (follow the link at the end of this paragraph). Another way of looking at ant diversity is to consider their ecological roles. Ants occupy many ecological niches; in contrast with other social insects, ants, as a group, have substantially more ecological roles than termites, bees, or wasps. While the most "primitive" ants are predators, usually specializing on other insects, ohter ants feed on such diverse items as leaves (which are digested by symbiotic fungi maintained by the ants), seeds (the harvester ants), and nectar (many different ant species). This evolutionary switch from carnivorous to omnivorous or herbivorous diets has been an important factor in the development of ant diversity and abundance. More on ant diversity.
Within a species, ant workers may be monompophic--all about the same size and all more or less equally capable of performing all worker tasks, or they may be polymporphic. Polymporphic ant species may feature workers which range 10 or even 100-fold in size. Perhaps most the most well-known polymporphic ants are the leafcutters, which appear in many New World tropical and subtropical habitats. The army ants are another well-known polymorphic group. More on caste and division of labor in ants.
Pheromones are the best-known form of communication in ants. Ants use these chemical signals to communicate how to find food (trail pheromones), the presence of threats to the colony (alarm pheromones), and More on communication in ants.
The typical ant life cycle begins with the release of winged gynes and males from a colony. After mating the gyne breaks off her wings and starts a nest, rearing workers which will take over most of the colonys tasks. Some ants found new colonies by budding, in which a portion of the colony starts a new nest by splitting from the parental colony.
In many ant species there is more than one queen in a colony, a condition called polygyny. More on queen number, life cycles, and reproduction in ants.
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