Formica rufa

Formica rufa, also known as the red wood ant, southern wood ant, or horse ant, is a boreal member of the Formica rufa group of ants, and is the type species for that group, being described already by Linnaeus. It is native to Eurasia, with a recorded distribution stretching from the middle of Scandinavia to the northern Iberia and Anatolia, and from Great Britain to Lake Baikal, with unconfirmed reportings of it also to the Russian Far East. There are claims that it can be found in North America, but this is not confirmed in specialised literature, and no recent publication where North American wood ants are listed mentions it as present, while records from North America are all listed as dubious or unconfirmed in a record compilation. The workers' heads and thoraces are colored red and the abdomen brownish-black, usually with dark patches on the head and promensonotum, although some individuals may be more uniform reddish and even have some red on the part of the gaster facing the body. In order to separate them from closely related species, specimens needs to be inspected under magnification, where difference in hairiness is among the telling characteristics, with Formica rufa being hairier than for example Formica polyctena but less hairy than Formica lugubris. Workers are polymorphic, measuring 4.5–9 mm in length. They have large mandibles, and like many other ant species, they are able to spray formic acid from their abdomens as a defence. Formic acid was first extracted in 1671 by the English naturalist John Ray by distilling a large number of crushed ants of this species. Adult wood ants primarily feed on honeydew from aphids. Some F. rufa colonies are large networks of connected nests with multiple queens, while others have a single queen.

Nests of these ants are large, conspicuous, dome-shaped mounds of grass, twigs, or conifer needles, often built against a rotting stump, usually situated in woodland clearings where the sun's rays can reach them. Large colonies may have 100,000 to 400,000 workers and 100 queens. F. rufa is highly polygynous and often readopts postnuptial queens from its own mother colony, leading to old, multigallery nests that may contain well over 100 egg-producing females. These colonies often may measure several metres in height and diameter. F. rufa is aggressively territorial, and often attacks and removes other ant species from the area. Nuptial flights take place during the springtime and are often marked by savage battles between neighbouring colonies as territorial boundaries are re-established. New nests are established by budding from existing nests in the spring, or by the mechanism of temporary social parasitism, the hosts being species of the F. fusca group, notably F. fusca and F. lemani, although incipient F. rufa colonies have also been recorded from nests of F. glebaria and F. cunicularia. An F. rufa queen ousts the nest's existing queen and lays eggs of her own, and the existing workers care for her offspring until those offspring take over the nest.

These ants' primary diet is aphid honeydew, but they also prey on invertebrates such as insects and arachnids; they are voracious scavengers. Foraging trails may extend 100 m. Larger workers have been observed to forage farther away from the nest. F. rufa commonly is used in forestry and often is introduced into an area as a form of pest management.

Worker ants in F. rufa have been observed to practice parental care or perform cocoon nursing. A worker ant goes through a sensitive phase, where it becomes accustomed to a chemical stimulus emitted by the cocoon. The sensitive phase occurs at an early and specific period. An experiment was conducted by Moli et al. to test how worker ants react to different types of cocoon: homospecific and heterospecific cocoons. If the worker ant is brought up in the absence of cocoons, it will show neither recognition nor nursing behaviour. Both types of cocoons are opened up by the workers and devoured for nutrients. When accustomed to only the homospecific cocoons, the workers collect both types of cocoons, but only place and protect the homospecific cocoons. The heterospecific cocoons are neglected and abandoned in the nest and eaten. Lastly, if heterospecific cocoons were injected with extract from the homospecific cocoons, the workers tend to both types of cocoons equally. This demonstrates that a chemical stimulus from the cocoons seems to be of paramount importance in prompting adoption behaviour in worker ants. However, the specific chemical / stimulus has not been identified.

The foraging behaviour of wood ants varies depending on the environment. They have been shown to tend and harvest aphids as well as prey on and compete with other predators for food resources. They tend to prey on the most abundant members of the community, whether in tree canopies or forest foliage. Wood ants seem to favour prey living in local canopies near their nest. However, when food resources dwindle, they seek other trees further from the nest and explore more trees instead of exploring the forest floor more thoroughly. This makes foraging for food significantly less efficient, but the rest of the nest does not help the foraging ants.

Wood ants have shown aggressive behaviour toward their own species in certain situations. Intraspecific competition usually occurs early in the spring between workers of competing nests. This aggression may be linked to the protection of maintaining territory and trail. By observing skirmishes and trail formation of wood ants, the territory surrounding each nest differs between seasons. Permanent foraging trails are reinforced each season, and if an ant from an alien species crossed it, hostile activity occurs. Most likely, the territory changes based on foraging patterns are influenced by seasonal changes.

Ants recognise their nestmates through chemical signals. Failure to recognise each other causes the integrity of the colony to decay. Accumulation of heavy metals in the environment also alters aggression levels. This could be due to a variety of factors such as changes in physiological effect or changes in resource levels. The ants in these territories tend to be less productive and efficient. Increased resource competition would be expected to increase level of aggression, but this is not the case.

Wood ants, particularly those in the Formica species, perform organised and planned attacks on other ant colonies or insects. These planned attacks are motivated by territory expansion, resource acquisition, and brood capture. Raids are performed at certain times of the year, when resources may need restocking, and during the day when ants are most active. Organised and cooperative strategies for raiding are more specific tactics used by the F. polyctena species. However, raiding is still an integral behaviour of the F. rufa group. Scouts will investigate neighbouring nests to raid, marking their targets using pheromones. Wood ants are also capable of counterattack/defending retaliation. Strong defensive measures include guarding entrances to tunnels and having routine patrols of the areas to watch neighbouring nests. Some wood ant species, such as F. sanguinea, will raid brood, which is then integrated into their colony as workers. This behaviour enables the colony to bolster its workforce without expending energy on raising its brood. The captured brood matures and functions within the raiding colony, helping with foraging and nest maintenance tasks.