What is an ‘ant’?

Ant closeup 2 bamboo bottom-up view
Ant closeup 4

What is an ‘ant’?

A brief introduction to the biology and ecology of ants

Ants belong to an order of insects called Hymenoptera, which also includes bees, bumblebees, and wasps. Scientists estimate that ants evolved about 120 to 140 million years ago, most likely from a wasp-like ancestor. They remained relatively uncommon for several million years, and only at the start of the Eocene, about 55 million years ago, ants started becoming abundant and diversified to the level that we observe today.

All ants belong to a single family, the Formicidae, which is, without a doubt, one of the most successful group of organisms on this planet, with nearly 16,000 species described. This represents as many species as all bird and mammal combined, and yet scientists estimate that just half of the ant species have been discovered! Contrary to what is sometimes thought (and how the Chinese common name may have suggested), termites are not closely related to ants, nor to other Hymenoptera, but instead belong to a different order Blattodea and are related to cockroaches. Yes, termites are social roaches!

Ant closeup 2
A Camponotus sp. worker on a bamboo, taken at Saiyuen.

The diverse world of ants

But let’s come back to ants. Ants have conquered all continents (except Antarctica) and are found in all types of habitats, from the most arid deserts to the humid tropical rainforest, from savannah to inside our home, from caves hundreds of meters deep to the peak of mountains above 4000 m. In terrestrial habitats, they have evolved and colonized all kinds of places (what ecologists call niches). Some build nests deep underground (as deep as 7m!), while other live within decaying wood on the ground, under rocks, or at the top of the tree canopy inside hollow branches or within complex ant-made nest structures constituted of sewed leaves or carton pulp.

All these ant species have evolved a wide range of lifestyles and diets. If in general ants have a preference for animal proteins, and in particular for other insect or arthropod species, they do not all eat the same food. Some species are extremely specialized on hunting a single group of arthropods, such as the miniature trap-jaw ants (the genus Strumigenys) which feed exclusively on springtails (Collembolan), while other will have a more generalized diet, which includes proteins regardless of their origin, as well as lipids from seeds, or sugar from plant secretion, fruits and honeydew produced by sap-sucking insects. Let’s not forget to mention that some species feed on fungi either directly, or more surprisingly, by cultivating fungi for consumption inside their nests. Yes, that’s right, some ant species (although they can only be found in Americas) have developed agriculture at around 25 million years ago, well before human (12,000 years ago).

The social ants

Ants’ astonishing success can be attributed to several reasons, but one in particular: sociality. Ants are social insects, or rather eusocial insects to be exact, like termites, some bees and wasps (but note that the majority of bees and wasps are non-social). To date, none of the 16,000 ant species is known to be solitary. Eusocial insects are species for which individuals, usually related to each other’s: 1) cooperate to perform a number of tasks, including taking care of their offspring; 2) with a division of labor such that there are reproductive and non-reproductive individuals; and 3) characterized by an overlapping of two or more generations within a colony. In another word, only certain individuals in a colony are responsible for reproduction, and this is, mostly, the only thing they do throughout their lives! We would say that they are specialized for reproduction. At the same time, the other, non-reproductive adults in the colony are responsible for all the other duties, such as taking care of brood (eggs, larvae and pupae), finding food, building and defending the nest, etc. Being a social insect means that ants are able to, often, utilize numerical advantage and collectively build their nest, defend it and forage for food, and this is why ants are so successful!

One thing that often comes into our minds when we talk about ants is that they form colonies. But what is a colony? What is its difference with the term ‘nest’? In short, nest is the physical structure where the ants live, while colony is the social structure for which members cooperate with one another. Under most circumstances, ants will be aggressive towards ants that are from a different colony. For most ant species, one colony will only have a single nest. However, it is possible, for some species, for a colony to have multiple nests! We describe these ants as polydomous (in contrast with monodomous for ants having a single nest). Members from different nests of the same colony are still able to recognized one another, despite their physical separation, using chemical signature that is unique to each colony. Why having multiple nests then? Simply to be more efficient! They can control a territory better and exploit resources like food more efficiently by reducing distance between potential food source and the nearest nest. Clever ants!

Queens, workers and males

Introduction caste
Dealate queen (=wing-shedded queen; top) and worker (bottom) of the same species of ant (Strumigenys hirsuta) under the same magnification (specimens not collected from Saiyuen). Notice that queen has a slightly bigger overall size, and noticeably larger eyes and much more robust body (with extra flight muscles).

As mentioned above, a division of labor in found within a colony. The ants’ different roles (which are predetermined before they become adults) are known as ‘caste’. There are three common castes of ants: male, reproductive female (queen or gyne) and non-reproductive female (worker or ergate). Male ants are usually very short-lived and die soon after mating (many do not even have a developed mouth for feeding!). They hatch from unfertilized eggs and only appear during certain period of the year (known as the swarm or nuptial season). Females are usually much more long-lived (for worker it may be several years, for queen it may even be a few decades!) and they hatch from fertilized eggs. Queens are responsible for egg-laying, while workers are responsible for all the other duties: taking care of brood, foraging for food, building, maintaining and defending the nest. Queens and males are usually winged (alate) such that they can fly to disperse (but in some species there are wingless queens or males!), mate during swarm season and form new colonies (by queens only). The term of queen may be a bit misleading as in human societies we tend to associate it with some kind of decisional power. In ants, however, the queen does not govern, nor any other ants in the colony. The society is self-organized without any central power, and with each individual contributing to the growth of the colony in its own way. Of course, this does not mean that ant colonies are devoid of conflicts and errors (all the opposite).

Introduction subcaste
Major worker (top) and minor worker (bottom) of the same species of ant (Pheidole zoceana) under the same magnification. Notice major worker has a bigger overall size and much more robust head.

Be aware that size difference between major and minor workers depends on species, hence are not always this extreme.

For some ant species, the castes in a colony can be further subdivided into subcastes. They are defined based on the morphological differences of individuals and their roles within the colony. When those occur in the worker caste, we call these species polymorphic (in contrast with monomorphic ants, where caste cannot be further broken down into subcastes). For polymorphic ants, workers can be subdivided into major and minor workers (or minima, media, supermajor, if more than 2 sizes are recognized). Major workers (or soldier) are often bigger in size and have stronger mandibles. They are responsible for defending the nest and performing other heavy duties close to the nest (e.g., bringing large food items back to colony, cutting big pieces or even milling seeds). On the other hand, minor workers focus on nest construction, brood care and foraging relatively far away from the nest. Even though workers are usually non-reproductive, they are still able to lay non-fertilized eggs if they have complete ovaries. These non-fertilized eggs (known as trophic eggs) can serve as an alternative food source for their queens and larvae! When the colony has lost its queen, non-fertilized eggs will hatch into males instead. In rare cases, workers known as gamergates can mate with male and are responsible for reproduction as well (sometimes even entirely replaced queen ants!).

When there is only one egg-laying queen in the colony (i.e., monogynous colony), workers in the same colony are siblings with one another. But it is also possible that there are multiple egg-laying queens in the same colony (i.e., polygynous colony)! If the queens do not tolerate one another and live far apart in the nest, it is known as oligogyny, which is a special kind of polygyny. For both general polygyny and oligogyny, workers from the same colony take care of all the queens equally. While workers in a polygynous colony are less related with one another than in monogynous colony, it allows the formation of a much bigger colony and be more competitive within the habitat. Polygynous colonies are also much more resistant to disturbance as the death of one or a few queens does not mean the end of the colony. As a result, some polygynous colonies are virtually immortal as they continuously produce or adopt new queens!

This is only a simple introduction on the fascinating world of ants. As you can imagine, with so many living species, many more interesting strategies have evolved in ants, some likely remaining unknown to us! We will talk about other aspects of ants in the following sections. We hope you will find this interesting and come to appreciate these amazing tiny creatures a little more!

Next

Ant closeup 3

Ants and where to find them

Glossary

A･B･C･D･E･F･G･H･I･J･K･L･M･N･O･P･Q･R･S･T･U･V･W･X･Y･Z

A

Alate queens / alate males. Queens or males that are born with wings and are capable of flying. After mating, queens would shed their wings and are now known as dealate queens.

Antennal club. Apical segments of the antenna that are distinctly enlarged.

Antennal scape. The basal-most segment of the antenna that is elongated.

Arboreal. Nesting and/or active on trees.

Army ants. Ants that do not build permanent nest and are constantly on the move. They often forage collectively in mass.

B

Brood. Non-adults, including eggs, larvae and pupae.

Budding. New queens leaving with some workers and brood from the original colony and found a new colony nearby. In contrast with alate (winged) queens founding a new colony alone after mating.

C

Castes. Roles of ants in the colony, usually predetermined before they become adults. Defined by sex, morphology and reproduction capability of the ants. Further subdivisions based on minor differences in morphology and role are known as subcastes.

Clypeus / clypeal margin. The anterior part of the head, in-between the insertion site of antennae and the mandibles of the ant. Clypeal margin refers to the anterior margin of the clypeus.

Coccids. A family of insects (Coccidae) belonging to the infraorder Coccomorpha (scale insects) of order Hemiptera, also known as soft scales or wax scales. They may form mutualistic relationship with ants, offering honeydew in return for protection.

Colony. The social structure of ant, constituted by workers and reproductive individuals that cooperate with one another, as well as their brood. Members of the same colony are able to recognize and are not aggressive with each other. Number of individuals can range from a few to several millions.

Cosmopolitan distribution. Global or near global distribution.

Cryptic. Obscure lifestyle, often refers to subterranean species and species that are active within leaf litter.

D

Dimorphic. There are two distinct subcastes for workers. There is no intermediate form in-between the two subcastes.

Disturbance / disturbed environment. Human activities that change the environment and impact ecosystem, which can be momentary or prolong. Agriculture and environmental management (e.g., cutting or trimming trees, use of pesticides) are two typical examples of the disturbance that may affect ants.

Dorsal view. Top-down view on an ant.

E

Epigaeic. Nesting and/or active above the ground and on lower vegetations.

Ergatoids. Queens or males that are born wingless. Ergatoid queens may look very similar to workers.

Exotic species. Species that has been introduced to place outside of where it is originated from (=introduced species).

F

Forage. Hunting or gathering food.

Full-face view. Frontal view on the head of an ant.

G

Gamergate. Workers that have mated with males and are able to lay fertilized eggs for reproduction. Depending on the ant species, gamergates may coexist with queens, or entirely replace queens.

Gaster. The posterior part of an ant’s body, connected with mesosoma through waist segment(s). In some ant species a sting is present at the end of the gaster for hunting and/or self-defense.

Gastral tergites. The dorsal part of the gaster, can be subdivided into segments.

H

Habitat. Where the organism live, characterize by the landscape and dominant plants (e.g., tree, shrub, grass, mangrove tree) present.

Hemiptera. An order of insects that includes aphids, cicadas, leafhoppers, scale insects and shield bugs. Most of them feed on plant sap, and sometimes form mutualistic relationship with ants, offering honeydew in return for protection.

Hypogaeic. Nesting and/or active within or below leaf litter, inside rotten woods, beneath rocks and within the soil.

I

Intraspecific variation. Variation between individuals of the same species, in contrast with interspecific variation, which describe variation between species.

Introduced species. Species that has been introduced to place outside of where it is originated from (=exotic species).

Invasive species. Species that has been introduced to place outside of where it is originated from and has a significant impact on the local ecosystem.

L

Leaf litter. The layer of plant debris above the soil, which is made up of fallen leaves, branches, fruits, seeds and flowers.

M

Mandibles. The pair of hard mouthparts of insects for biting, cutting and holding food. It is a part of the exoskeleton.

Mesosoma. The anterior part of an ant’s body, connected with gaster through waist segment(s). It is also where the legs and wings (when present) of the ant attached.

Microhabitat. Where the organism live, at a finer scale than habitat. It is further characterized by the local temperature, humidity of the environment. For example, beneath a rock and on the surface of tree branches are two different microhabitats of the same habitat (forest).

Monogyny. Colony with only one egg-laying reproductive individual.

Monomorphic. There is only one size and form for workers, and cannot be further subdivided into subcastes.

Morphology. The form and structure of an organism, both internal and external. For external morphology, it includes the shape, size, color, pattern and external structure of the various parts of the organism.

Morphospecies. A species defined solely base on its morphological differences with respect to other species. It may not have been formally described and named.

Mutualism. Mutually-beneficial relationship between organisms.

Myrmecophily plants. Plants that are positively associated with ants.

Myrmecophyte plants. Plants that have formed mutualistic relationship with ants, and possess adaptations for such relationship, such as food bodies and extrafloral nectaries dedicated to ants and/or specialized structure for ants to nest (e.g., hollowed structures in tree branches).

N

Native species. Species that that naturally live in a particular place without human intervention. ‘Native fauna’ further specifies animal species, while ‘native flora’ specifies plant species.

Nest. The physical structure that ant colony lives in. It may be permanent and extensively built, or temporary and has only been minimally modified by its residents.

Niche. It can broadly refer to the role an organism plays within its habitat, or specifically refer to where an organism lives within its habitat. Here niche means the latter.

O

Oligogyny. A special case of polygyny, where there is more than one egg-laying reproductive individual in an ant colony, but these individuals do not tolerate one another and stay far apart from each other in the nest. Workers still cooperate with one another like in a normal polygynous colony.

Open habitats. Habitats such as open shrubland and grassland, in oppose to habitats with dense vegetation cover, such as forest.

P

Parthenogenesis. Asexual reproduction through unfertilized eggs. For ants, it means reproductive females are able to reproduce partial or complete clones of themselves without mating (i.e., theylotoky).

Pest. Species that has been a source of nuisance to people. Common categories include household pest and agricultural pest (which causes damage to crops).

Petiole. The first segment of waist segments (or the only segment, when there is only one waist segment) of an ant, connecting mesosoma and gaster. It may be reduced and difficult to observe.

Polygyny. Colony with, on average, more than one egg-laying reproductive individual. Facultative polygyny (=secondary polygyny) means that colonies were originally monogynous when founded, but would later adopt other queens (usually daughter queens of the colony) and become polygynous.

Polymorphic. There is more than one size and/or form for workers, hence, can be further subdivided into subcastes. When there are two distinct subcastes but no intermediate form, it is known as dimorphic.

Postpetiole. The second segment of waist segments, connecting mesosoma and gaster. Some species of ants do not have a postpetiole.

Profile view. Side view of the ant, usually with its head facing left.

S

Scale Insects. An infraorder of insects (Coccomorpha) belonging to the order Hemiptera, also includes coccids. They may form mutualistic relationship with ants, offering honeydew in return for protection.

Scavenger. Ants that predominantly gather food instead of through other means, e.g., hunting. Depends on the ants’ preference, it may include fruit, seed and other food bodies of plants, carcasses of invertebrates and small vertebrates, honeydew produced by other insects, etc.

Sculpture. Pit- or grove-like structure on the surface of the exoskeleton of the ant, in contrast with smooth and shining surface. Its pattern can be used to differentiate between species of ants.

Secondary forest. A forest that has been regrown after timber harvest or agricultural activities, in contrast with primary forest, which has not been deforested by human. There are only secondary forests in Hong Kong.

Soil arthropods. Small invertebrates from the phylum Arthropoda that are active within and above the soil, as well as within leaf litter.

Spine. Hard and pointed structure on the back of the ant. It may be long and sharp like a thorn, or low and inconspicuous. It most often appears at the posterior part of mesosoma in one pair (propodeal spine), with other possible locations include pronotal spine and petiole spine.

Subcastes. Roles of ants in the colony, a further subdivision based on minor differences in morphology and role under their respective castes.

Supercolony. The unusual case where multiple colonies unite and are not aggressive with one another, often occur in invasive species in introduced regions. Individuals range from millions up to billions.

T

Tramp species. Species that are associated with habitats frequently-disturbed by human activities. They may further be dispersed by human to places beyond its native range.

Trophic eggs. Unfertilized eggs that function as food. In ants, they are usually produced by workers and offered to queens and larvae.

W

Waist segment(s). The segment(s) connecting mesosoma and gaster of an ant. The first segment is called ‘petiole’ and the second segment (when present) is called ‘postpetiole’.

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