Wood ants (Formica aquilonia, Formica lugubris) play a very important role in the Caledonian Forest ecosystem.

Global distribution

Wood ants of the genus Formica are widely distributed in Europe and Asia, with both Formica aquilonia and F.lugubris occurring from Scandinavia in the north to Bulgaria and Italy in the south, and from the UK eastwards through France and Germany to Russia, while they are also found in the coastal areas of the Sea of Okhotsk in eastern Siberia. They live mainly in coniferous forests but they do also occur in some deciduous woodlands and have been found in open land. 

Distribution in Scotland

Formica aquilonia occurs in the pinewoods of the Caledonian Forest throughout the Highlands. Several recording have also been made in birchwoods including those of Inverpolly National Nature Reserve and on the Isle of Skye off the west coast. F.lugubris has a similar distribution in central Scotland but is less often found in the west than F.aquilonia and extends further to the east. 

Current distribution maps for the Highlands are best obtained from the HBRG set on NBN Atlas which is accessible here

Both Formica aquilonia and F. lugubris are included on the International Union for the Conservation of Nature's (IUCN) Red List of threatened species as near threatened, where they are classified as Lower Risk. 

We do not fully understand the status and distribution of F.aquilonia and F.lugubris as many colonies and nests are not recorded. 

All the wood ants have a distinctive red and black covering. With practice the two species can be distinguished by the distribution of hairs on the head and thorax, the two species have been widely confused, more information on identification can be found here. The queens may live for fifteen years or more, the workers about one year and the males are very short lived, dying after their mating flight in the spring.

Wood ants are social insects and live in colonies which can number up to half a million individuals, though in Scotland colonies rarely number more than 100, 000. Most of the individuals are worker ants. The workers are all female and as they are not fully developed, do not reproduce, though they do occasionally lay eggs which are used as food. As the name implies, they do all the work in the colony, maintaining the nest and tending the queen and her brood. The queen is the only ant that lays eggs that are reared to adulthood. The males do not work, and their sole purpose is to mate with the queen.

Wood ant nests are a common sight in the forest. They consist of a characteristic dome-shaped pile of pine needles that can be over one metre high and two metres in diameter. Formica lugubris and F. aquilonia build large mounds, though the nest of the former tends to be larger, flatter and more in the open.The nest provides protection from predators and the elements, as well as a steady environment in which to rear eggs and pupae. The nest temperature and humidity varies very little in comparison to the surrounding soil and the ants maintain this in several clever ways. The needles are not placed randomly but are very carefully maintained by the workers to make the dome like a thatched roof, very efficient at shedding water. If examined closely it can be seen that the dome shape is usually not symmetrical; the southern side of the nest is flatter to present a greater surface area to the midday sun and so absorb the maximum amount of solar energy. Simple actions of opening and closing entrance holes control heat, but more sophisticated methods are used by the workers acting as storage heaters. They will 'sunbathe' and then go to the nest brood chambers to release the excess heat. Often the nest is built over a decaying tree stump which, along with other decaying organic matter, provides heat. Damp nest material is carried to the surface from the lowest part of the nest to dry out.

Wood ant workers spend much of their time collecting food either by hunting or scavenging dead insects. In the morning, columns of ants can be seen setting out on a half dozen or so regular trails that radiate from the nest. They detect their prey by vibration and scent, though they can see up to about 10 cm. Small invertebrates are stealthily stalked and then suddenly pounced upon whereas larger victims will be subdued with the aid of nearby fellow workers. As well as pincer-like mandibles, wood ants have another formidable way of overcoming their victims. In the bulbous end section of their bodies (the gaster) they have glands that contain formic acid which can be sprayed accurately to a distance of 5 cm. This behaviour is used not only help to subdue prey, but also as a defensive measure when threatened. Each foraging ant brings back about one and a half times its own weight in food each day.

Wood ant tracks are not only on the ground, but are three dimensional, leading up trees into the canopy above. The ants not only hunt in the trees, they are known to farm as well. Wood ants have a symbiotic relationship with aphids (for example Symydobius oblongus). The aphids are `milked' by the ants who gently stroke them to get them to release droplets of honeydew, a food that is rich in sugars, acids, salts and vitamins. To the aphids this is their waste product as they have to suck a lot of tree sap to get the protein they require. In return for the honeydew the ants protect their precious sugar source from predators and competing sap sucking insects.

Within the nest, running into the ground, are a series of chambers and galleries which vary slightly in temperature and humidity. The workers constantly fuss over the eggs, larvae and pupae, moving them around the nest to keep them in the most favourable growing environment. Eggs hatch into helpless larvae that totally rely on liquid food provided by the workers. They moult several times as they outgrow their skin, eventually going into a period of pupation within a cocoon when they undergo complete metamorphosis, emerging as adults.

In the spring winged sexuals are raised – queens and males. On calm, balmy evenings in April to October these sexuals take to the air to mate. During this nuptial flight one mating provides enough sperm for the queen's lifetime. Hundreds of ants swarm at once but many perish, being eaten by birds and other predators. The fertilised queens of Formica aquilonia and F. lugubris, usually return to their home nest or a neighbouring one and at this point shed their wings. A nest may contain many queens. A newly mated queen may stay in the nest where she was raised or establish a new colony nearby with some loyal workers. Often the daughter colony is very closely connected to the mother nest by trails both above ground and underground.

Wood ants have fascinating associations with other animals at various levels, from having guests, to parasites, to scavengers that live in the nest, ignored by the ants. The worm, Dendrodrilus rubidus, is found more readily in the nest of Formica aquilonia than in the surrounding soil, as conditions within the nest are very worm friendly, with an abundant food supply. As the worms help prevent the mounds from becoming overgrown with moulds and fungi, this is a mutually beneficial relationship for ants and worms. Outside the nest, relationships continue as exemplified by the farming of aphids described above.

Wood ants play a very important role in the ecosystem of the forest. Where ants have been removed by forest practices, many herbivorous insects can reach numbers out of balance with the rest of the ecosystem and so become damaging to forest trees. The ants are known to predate upon caterpillars of species such as the sawfly (Neodiprion sertifer) and pine looper moth (Bupalus piniaria) that eat Scots pine needles. But the truth is, little is known about them and the extent of their importance. This however is being rectified with several organisations and universities undertaking extensive research to aid future conservation measures.