Trap-jaws, silk and agriculture – just a few examples of convergence in the arguably most successful group of insects, the ants…

Although they may be far outnumbered by the beetles in terms of species richness, the ants are arguably the most successful of insects and provide a rich source of insights into convergence. Probably of greatest interest is the independent evolution of eusociality. Fascinatingly, this highly complex social system has opened the door to armies, warfare and slavery as well as facilitating the evolution of agriculture.

Eusociality

The social complexity of the ants has long been appreciated. It is, of course, based on eusociality, in itself a highly convergent social system that involves co-operative brood care, the overlap of generations and, most importantly, reproductive castes, with sterile workers caring for the offspring of one or a few queens. Eusociality has evolved probably only once within the ants, but several times in other insect groups (including bees, wasps, termites and aphids) as well as in other arthropods (the alpheid shrimp genus Synalpheus) and more remarkably in the mole rats.

Eusociality and cognition

Whilst ants and other eusocial insects are often dismissed as robotic, in point of fact they are capable of individual recognition. Their complex eusocial organisation unsurprisingly includes many associated aspects of activity, notably the convergent phenomenon of quorum sensing, a decision-making process used by social groups. Sound production by stridulation with a corresponding faculty for hearing is found in some ants and so analogous to other animals such as grasshoppers (and even birds!). In addition, tool-making, which is rampantly convergent, has evolved in a number of arthropods, including ants. Whilst the cognitive abilities of some insects, notably the bees, are quite well understood, less is known about the ants. It is appreciated, however, that the impressive navigational abilities of some species reflect quite complex cognitive powers.

Armies, wars and slavery

Amongst the most striking social arrangements are those found in the army ants, a name used for more than 200 species in various lineages. They are characterised by their high aggressiveness and predatory “raids”, during which they act as highly co-ordinated swarms that can form long files or fan out across the forest floor in search of prey. Although they might temporarily set up bivouacs, army ants are nomadic and have no permanent nest. The main genera, Dorylus and Eciton, had long been thought to have arrived at this syndrome independently, but recent molecular data suggest they are closely related. It remains unclear, however, how deep the divergence is, and they could still have acquired the main features of aggressive nomadism independently. In other words, this group may well have had a disposition to this type of sociality, but it does not preclude at least parallelism.

The fire ants in the genus Solenopsis are highly territorial, with neighbouring colonies being at war and their aggressive workers fighting each other to the death. They “far exceed human beings in organised nastiness” (Hölldobler and Wilson 1995, Journey to the ants: a story of scientific exploration, Harvard University Press, p. 59). Interestingly, the youngest workers in the species Solenopsis invicta feign death, a behaviour known as thanatosis that is widespread in animals, especially spiders.

Some ants have become slave-makers. These social parasites rely on other ant nests for their workforce, usurping and regularly raiding them. Slavery (or dulosis) is assumed to have arisen at least nine times within the two ant subfamilies Formicinae and Myrmicinae. Within the latter, the small tribe Formicoxenini seems to be a particular hotspot with six independent origins of slavery. The phenomenon of slavery involves important molecular convergences of pheromone-like chemicals.

Ants and mutualism

More benignly, many ants “farm” or tend other insects, such as aphids or mealybugs, for their sugary excretions (honeydew). This too is convergent, having evolved not only many times independently within the ants, but also in a Madagascan gecko lizard that feeds on planthopper honeydew. In some cases, the ant-hemipteran mutualism is obligate and can be effectively described as nomadic shepherding, because the ants move their symbiosis partners from one “pasture” to another, whilst protecting them from predators and parasites. Another feature of a number of ants is the construction of ant gardens, which is not specifically to do with agriculture but a mutualistic association between the ants and epiphytic plants. This too has arisen a number of times. Amongst the most remarkable evolutionary developments, however, is the independent emergence of true agriculture, mainly (but not only) in leafcutter ants. It shows many striking similarities to that of humans and involves the construction and maintenance of a fungal “farm”. Such fungal farms are also found in termites, ambrosia beetles and even a marine snail.

Ant dexterity: gliding and opposable spines

Ants are typically ground-dwellers and winged morphs emerge only temporarily, for example females in nuptial flight. Some tropical tree-dwelling species, however, regularly take to the air – they are capable of gliding, a highly convergent method of controlled descent. These species belong to three separate families (Formicinae, Myrmicinae and Pseudomyrmecinae) and possess morphological adaptations such as a dorso-ventrally flattened body with lateral flaps. 

The great majority of ants, of course, walk and manipulation of objects is typically achieved by using the mandibles. Some fire ants, however, have evolved the ability to handle objects by using their forelimbs, which bear opposable spines. This is convergent not only with other arthropods, such as the mantids and crustaceans, but also more remarkably with the opposable thumbs of some mammals, including racoons and primates.

Weaving ants

Some ants have the ability to produce silk, which is an excellent example of molecular convergence, having evolved at least three times in the arachnids and at least eight times in the insects. The functions of silk are manifold, including not only the well-known spider webs and other sorts of traps, but also escape lines, cocoons, egg coverings and nuptial gifts. Silk production in ants is most familiar in the weaver ants, where this weaver ecology has evidently evolved several times. As the name suggests, weaver ants use the silk to bring leaves together to form nests. The adult holds the silk-producing larva, shuttling it backwards and forwards. In other groups of ants, silk is also produced by the adult and used in nest construction.

Ant eaters and ant followers

The capacity to eat ants (and termites), which is known as myrmecophagy, has evolved in several groups of animals, such as placental mammals (e.g. pangolins, aardvarks and anteaters), marsupial mammals (e.g. numbats) and reptiles (e.g. the extinct theropod dinosaur Mononykus as well as the extant desert-inhabiting horned lizards and thorny devils). Convergent morphological adaptations to myrmecophagy include a long protrusible tongue, loss of teeth and sticky saliva to immobilise the aggressive prey.

Some animals are either opportunistic or obligate ant followers – they track down active swarms of army ants such as Eciton burchellii and follow them through the understory. Not to feed on the ants, but to catch other arthropods that try to flee from the approaching swarm. This behaviour has arisen independently in several bird families, but also in some mammals, lizards and other insects.

Ant mimicry

Not all animals that appear to be ants are ants – a number of insects and even some spiders closely mimic ants, often for predatory purposes. Many beetles show morphological resemblance, particularly species that spend time with the ants outside the nest. Myrmecophilous beetles that stay mainly underground primarily employ chemical mimicry. An example is Myrmecaphodius excavaticollis, a social parasite in fire ant colonies that lacks morphological mimicry but imitates the cuticular hydrocarbons of its host, thus manipulating the ant workers into feeding it, while also preying on the ant larvae.

Trap-jaws

Unsurprisingly, jaw-like structures have evolved many times, and for the most part the examples are so general as to be of relatively little use in the study of convergence. There are, however, some important exceptions, such as the remarkable trap-jaws, which have evolved independently in at least three ant subfamilies. The mandible strike of these species is extraordinarily fast and used to catch prey or propel the animal into the air to facilitate escape from predators. It is based on a catapult design that stores energy, as first shown in the well-studied trap-jaw ant Odontomachus. Other ants with trap-jaws employ very similar mechanisms, thus representing a remarkable example of convergence.