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Ants are a subject to which many people have never given much thought, unless it is how to extinguish them when they invade our houses. The best thing to do, though, is to take a moment to watch ants, rather than exterminate them. There is much to be admired about an organism with levels of societal organisation that are the envy of the natural world.

Wood ants

WHEN

Most active from early spring through to late autumn

WHERE

In scattered woodlands and coniferous plantations throughout Britain; distributions depend on species. Coed y Brenin, near Dolgellay, Gwynedd; Glen Affric NNR (SNH), Inverness

A foraging party of wood ants. Something to stay well clear of if you are a juicy aphid.

Stephen Dalton

Britain’s ant fauna of around 50 species is impoverished when compared to the tropics; the famous biologist and ant expert EO Wilson once stated that he could find as many species of ant on one tree in the Peruvian Amazon as exist in the whole of the British Isles. However, what Britain lacks in quantity, it makes up for in quality, with the wood ants being among the most charismatic of this immediately recognisable, but little understood, group of insects.

Of the six possible species of wood ant in Britain, only three are commonly encountered. The Southern wood ant, the Northern hairy wood ant and the Scottish wood ant all look quite similar and are identified by the shape and amount of hair on their heads or, rather more easily, by geographical location, with the exception of a few places where two of these species will occasionally overlap. Moreover, none of these species will come anywhere near your larder!

With the largest individuals being around ten millimetres in length, wood ants are not only the biggest British ants, but they also have the most populous colonies, with some nests reaching close to half a million individuals. They are also the only true woodland ants in Britain, as all of our other native species need warmer temperatures and so are invariably found in more open and warmer habitats such as heathland or grassland. The ability of the wood ants to conserve heat in the cooler forest environment make them the ideal candidates for living in what might be considered an unsuitable habitat.

A large wood ant nest on an otherwise fairly bare forest floor is an impressive sight. These domes can easily reach to over two metres in height and are, moreover, only the above-ground part of the nest; the structure can extend at least as deep again underground. If one of these nests were to be sliced in half, it would reveal a citadel of complex interconnecting tunnels and galleries that would surpass even the wildest imagination of today’s science fiction writers.

During the summer months the surface of these nests can be a wriggling mass of thousands of ants busily carrying out their specific chores. The vast majority will be wingless workers, essentially immature females that take no part in reproduction. These differ in size between five and ten millimetres, according to the different jobs that have been preordained to carry out from the moment they emerge from their pupae. Some will spend their lives collecting honeydew – a saccharine substance found on the leaves of plants – or catching insect prey, while others will tend to the eggs, larvae and pupae, and a still different caste will have the responsibility of building and repairing the nest.

The sole egg-laying machine in the wood ants’ nest is the queen. She can usually be identified by her much larger thorax that houses the flight muscles, and an outsized abdomen, which contains the ovaries and a sperm sac from her single mating. Although the vast majority of her eggs will develop into the different worker castes, during spring, winged queens and males begin to emerge from slightly larger pupae. These ‘sexuals’ will take to the air and mate, after which the queens disperse to form new colonies away from the mother nest. Separate nests in the same wood are able to synchronise the release of their sexuals on the same day so that new populations have a shuffled combination of genes to avoid inbreeding. These synchronised flights of the winged wood ants will only occur during the right climatic conditions and the number produced can be vast as they rise into the canopy to mate on the wing or in the treetops. While many will be eaten, for example by insectivorous birds, the local predator population often becomes quickly swamped by this sudden surplus of food meaning that there is negligible overall impact.

An ant citadel, with a population slightly larger than Bristol and, arguably, far fewer traffic jams!

Laurie Campbell

After this mass aerial ant orgy, the males die and the females return to the ground, shed their now-obsolete wings and look for suitable nest sites. In the case of the Southern wood ant, the queen will track down a colony of a closely related species, the common black ant. She will then gather a few common black ant pupae and construct a special cell within the colony for them; upon hatching they will join their ‘stepmother’ in killing the original host queen. They will then look after the queen Southern wood ant’s eggs but, once the queen’s own eggs have hatched, the new Southern workers take over the colony and extinguish the original black ant colony!

The one and only mating from her nuptial flight will have provided the queen with enough sperm to last her entire life, which, in the case of wood ants, may be an astonishing fifteen years, during which time she may well lay hundreds of thousands of eggs. These eggs are placed in the warmest part of the nest until they hatch into hairless larvae. The larvae are then fed on a part-digested liquid meal by the workers, before they finally graduate on to prey items collected from the forest. As the larvae grow, they are meticulously looked after by the specialised workers while they undergo a number of moults until they are ready to pupate into either the workers or the sexuals.

Wood ants also use their incredible social organisation when, after a period of winter hibernation, the first few days of spring will be marked by an increase in activity and the workers can be seen pouring away from the nest along special pathways that they meticulously keep obstacle-free. In contrast to many ant species, which use chemical signals or pheromone trails to find their way, wood ants exploit their excellent eyesight; they use a combination of the shapes of the surrounding vegetation against the sky and the orientation of the sun. Foraging parties will then retrace their route back to the nest with their spoils.

Diligent ant researchers have extrapolated that, on a single warm summer’s day, a typical wood ant colony can bring back as many as 60,000 individual items to the nest; these include aphids, caterpillars, beetles, flies, harvestmen, woodlice and plant material. A daily haul of around 140 grams of solid food can have a serious impact on the wood, leading to so-called ‘green islands’ around nests, where the vast majority of the defoliating insects have been removed, meaning that the leaves remain virtually unblemished. A very important additional food source for wood ants is honeydew which is collected by the ants from aphids that have tapped into a plant’s sap. The wood ants will often protect their aphids from attack by ladybirds and, in return, effectively milk them, like we would our cows, for a reward of a tiny drop of sugar solution.

A substantial proportion of the daily items brought back will consist of leaves and twigs, or needles if the nest is under conifers, which will be used for running repairs to the nest. The nest is designed to keep out the rain, but is also constructed to catch heat to enable the ants to operate under conditions much warmer than that of the surrounding ambient air temperature. On the surface of the mound the temperature may vary by as much as 15°C in summer, but the heart of the nest can be kept at an incredibly constant and snug 25°C. Particularly in the first cold but sunny days of spring, it is thought that many ants will emerge to sunbathe before carrying the heat below to warm the nest.

The wood ant workers also have three formidable weapons to defend the nest against potential intruders and to subdue their insect prey. Like all ants they have relatively powerful jaws and huge supplies of formic acid, which can both repel borders and act as an alarm signal to the colony. Finally, the wood ants are able to rely on sheer numbers to help them overrun and subdue the enemy. If the surface of the nest is stirred up, large numbers of ants run to the disturbed point and curl their abdomens around to squirt the acid at the potential threat. Despite these defensive measures, significant numbers of ants will still often be consumed by green woodpeckers and badgers.

Away from the nest, territories may spread for close to 100 metres in all directions depending on the local competition. Ants can recognise colleagues from their own colony by a ‘nest odour’ and may viciously attack strangers; spring battles sometimes occur between some of the separate wood ant colonies as they delineate territories. Like all civilised societies, diplomacy will usually predominate if there is more than enough territory to go around, resulting in nests of many sizes and stages of maturity in favourable woods.