Myrmecophily
Myrmecophily consists of positive, mutualistic, interspecies associations between ants and a variety of other organisms, such as plants, other arthropods, and fungi. It may also include commensal or even parasitic interactions.
A "myrmecophile" is an animal that associates with ants. An estimated 10,000 species of ants are known, with a higher diversity in the tropics. In most terrestrial ecosystems, ants are ecologically and numerically dominant, being the main invertebrate predators. As a result, ants play a key role in controlling arthropod richness, abundance, and community structure. The evolution of myrmecophilous interactions has contributed to the abundance and ecological success of ants, by ensuring a dependable and energy-rich food supply, thus providing a competitive advantage for ants over other invertebrate predators. Most such associations are opportunistic, unspecialized, and facultative, though obligate mutualisms are common.
As ant nests grow and become more specialized, they are more likely to house larger numbers and a greater diversity of myrmecophiles.
Myrmecophile
A "myrmecophile" is an organism that lives in association with ants. Myrmecophiles have various roles in their host ant colony. Many consume waste materials in the nests, such as dead ants, dead larvae, or fungi growing in the nest. Some feed on the stored food supplies of ants, and a few are predatory on ant eggs, larvae, or pupae. Others benefit the ants by providing a food source for them. Most associations are facultative, benefiting one or both participants, but not being necessary to their survival, although many myrmecophilous relationships are obligate, meaning one or the other participant requires the relationship for survival. Many lycaenid caterpillars produce nectar in specialized organs, and communicate with ants through sound and vibrations. The association with ants reduces parasitism of the caterpillars. Myrmecophilous beetles occur in the families Coccinellidae, Aphodiidae, Scarabaeidae, Lucanidae, Cholevidae, Pselaphidae, Staphylinidae, Histeridae, and Ptiliidae. The myrmecophilous staphylinids are especially diverse. Myrmecophilous associations are also seen in aphids and treehoppers, in the hoverfly genus Microdon, and several other groups of flies. The mollusc Allopeas myrmekophilos is another non-insect myrmecophile.Ant nests provide environmentally stable environments that are well organized and protected by the host colony. The ant guests can have a positive, neutral, or negative effect on the colony. If the infiltrating species' impact is too negative, they risk discovery; this usually forces myrmecophile populations to be small. Some spiders use Ant mimicry and chemical mimicry to infiltrate ant nests, usually to prey on food supplies or on the ants themselves. Aribates javensis, a species of oribatid mite, is an obligate myrmecophile that lives in ant nests. These mites are cared for by their ant hosts in exchange for eating litter and bacteria in the nest.
Ant-plant interactions
Ant-plant interactions are geographically widespread, with hundreds of species of myrmecophytic plants in several families, including the Leguminosae, Euphorbiaceae, and Orchidaceae. In general, myrmecophytes usually provide some form of shelter and food in exchange for ant "tending", which may include protection, seed dispersal, reduced competition from other plants, hygienic services, and/or nutrient supplementation.Three structural adaptations of ant plants are extrafloral nectaries, domatia, and Beltian bodies. Domatia are nesting sites provided by the plant in the form of hollow stems, petioles, thorns, or curled leaves. Ant-specialized domatia exist in over 100 genera of tropical plants. Beltian bodies provide a high-energy food source to ants in the form of nutritive corpuscles produced on leaflet tips, and they have been described in at least 20 plant families. Extrafloral nectaries occur in at least 66 families of angiosperms, mainly in the tropics, with some in temperate species. They occur in some ferns, but are absent in gymnosperms. They are not used in pollination; their primary purpose is to attract and sustain tending ants. Many plants can control the flow of nectar from EFNs so that the availability of nectar varies according to daily and seasonal cycles. Because ants can respond quickly to changes in flow rate from EFNs, this may allow plants to induce greater ant activity during times of peak herbivory, and minimize overall costs of nectar production.
In exchange for nesting sites and food resources, ants protect plants from herbivores. For example, bullhorn acacias support and are protected by Pseudomyrmex ants in Central America. This system was studied by Daniel Janzen in the late 1960s. Since then many similar systems have been documented. In the bullhorn acacia system, in exchange for protection, the acacias provide domatia, Beltian bodies, and EFNs; the Pseudomyrmex ants can survive exclusively on these food resources. For many plants, ants significantly reduce herbivory from both phytophagous insects and organisms as large as grazing mammals. Obligately associated ant species are some of most aggressive ants in the world. They can defend a plant against herbivory by large mammals by repeatedly biting their attacker and spraying formic acid into the wounds.
Myrmecophily is an indirect form of plant defense against herbivory, though ants often provide other services as well. Some ants keep leaf surfaces clean, helping to deter disease, while others defend against fungal pathogens. Ants commonly prune epiphytes, vines, and parasitic plants from their host plant, and they sometimes thin the shoots of neighboring plants. This reduces plant-plant competition for space, light, nutrients, and water. In many ant-plant relationships, nutrient flow is bidirectional. While 80% of the carbon in the bodies of Azteca spp. workers is supplied by the host tree, 90% of the Cecropia tree's nitrogen is supplied by ant debris carried to the tree as a result of external foraging. In light of these services, myrmecophily assists a plant's survival and ecological success, sometimes at a high cost to the plant in providing for the ants.
Ant-arthropod interactions
Many species of arthropods are dependent on ant species and live amongst them in their nests. Mites are particularly adept at being myrmecophiles, being that they are small enough to enter nests easily and not to be evicted. Mites exhibit extreme myrmecophily in numbers far above other myrmecophiles.Ant-insect interactions
Ants tend a wide variety of insect species, most notably lycaenid butterfly caterpillars and hemipterans. About 41% of all ant genera include species that associate with insects. These types of ant-insect interactions involve the ant's providing some service in exchange for nutrients in the form of honeydew, a sugary fluid secreted by many phytophagous insects.Some insects are adapted to contend with ant aggression, resulting in either mutualistic or parasitic bonds with ant colonies. Some coccinellid beetles have behaviors, body shapes, and chemical mimicry that enable them to prey on ant-tended aphids.
Hemiptera
s are the most abundant myrmecophiles in northern temperate zones. Aphids feed on the phloem sap of plants, and as they feed, they excrete honeydew droplets. The tending ants ingest these honeydew droplets, then return to their nest to regurgitate the fluid for their nestmates. Between 90 and 95% of the dry weight of aphid honeydew is various sugars, while the remaining matter includes vitamins, minerals, and amino acids. Aphid honeydew can provide an abundant food source for ants and for some ants, aphids may be their only source of food. In these circumstances, ants may supplement their honeydew intake by preying on the aphids once the aphid populations have reached certain densities. In this way, ants can gain extra protein and ensure efficient resource extraction by maintaining honeydew flow rates that do not exceed the ants' collection capabilities. Even with some predation by ants, aphid colonies can reach larger densities with tending ants than colonies without. Ants have been observed to tend large "herds" of aphids, protecting them from predators and parasitoids. Aphid species associated with ants often have reduced structural and behavioral defenses.Ants engage in associations with other honeydew-producing hemipterans, such as scale insects, mealybugs, and treehoppers. These associations are usually facultative and opportunistic; some are obligate, as with inquilines that can only survive inside ant nests. Ants may provide other services in exchange for hemipteran honeydew. Some ants bring hemipteran larvae into the ant nests and rear them along with their own ant brood.
Lycaenid butterflies
Lycaenid caterpillars do not continually excrete honeydew, so they have evolved specialized organs that secrete chemicals to feed and appease ants. The secretions are a mixture of sugar and amino acids, a combination more attractive to the ants than either component in its own. The secretions of Narathura japonica caterpillars contain components that cause behavior alteration in the ants, with a reduction in the locomotory activity of caterpillar attendants, increased aggression and protectiveness by Pristomyrmex punctatus ants. This suggests that the association is parasitic rather than mutualistic. Because caterpillars do not automatically pass honeydew, they must be stimulated to secrete droplets, and do so in response to antennation, the drumming or stroking of the caterpillar's body by the ants' antennae. Some caterpillars possess specialized receptors that allow them to distinguish between ant antennation and contact from predators and parasites. Others produce acoustic signals that agitate ants, making them more active and likely better defenders of the larvae.Ants protect lycaenid larvae from predatory insects and parasitoid wasps. For example, larvae of Glaucopsyche lygdamus tended by Formica podzolica are much less likely to be infected by parasitoids. These interactions come at an energetic cost to the butterfly: ant-tended larvae reach smaller adult sizes than untended individuals due to the costs of appeasing ants. Ants are important partners for butterflies at all stages of their lifecycles. For example, adult females of lycaenid butterflies such as Jalmenus evagoras, preferentially oviposit on plants where ant partners are present, possibly by using ants' own chemical cues. While ant attendance has been widely documented in lycaenids and in riodinid butterflies such as Eurybia elvina, many other butterflies and moths associate with ants.