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Each summer, there comes a moment when we suddenly notice that the ground is crawling with large winged ants. Then the air seems to be full of them and we find ourselves ducking and diving, trying not to get them in our mouths or hair.
So what causes this simultaneous taking to the air, and is there really such a thing as flying ant day? Find out when and why ants grow wings and swarm.
Flying ants are known as alates. In the UK, particularly in urban areas, the winged insects you see are almost always the sexually mature queens and males of the black garden ant, Lasius niger. The larger ants are the queens. They can be up to 15mm long.
This annual swarming event usually occurs in July or August and coincides with a period of hot and humid weather. Winged ants appear at different times around the country and local weather conditions are critical for the coordination of swarming activity.
Ants tend to fly earlier in urban areas than rural areas, probably because temperatures are generally warmer in urban environments, known as the urban heat island effect.
It can be unfortunate timing for tennis players at Wimbledon. There are years when flying ants plague players during their matches, causing so much disruption it makes the news. However, swarms of flying ants can appear any time between June and the start of September.
A multi-year citizen science project by the Royal Society of Biology found that the widely held idea of a 'flying ant day' is actually a misconception: there is no single day when ants fly all at once. Rather, there is a 'flying ant season'. Winged ants actually emerge over several weeks, although there are often several peaks in appearances, each lasting only a few days. The precise pattern of swarming varies from year to year.
Swarming is triggered by the weather: the study found that ants only flew on days when it was warm, not windy and conditions had improved compared to the previous day.
There is also anecdotal evidence that flying ant days often occur after some summer rain.
Prior to swarming, ants are going about their everyday business and living in a colony in a nest.
Black garden ants nest in dry soil. You'll often find them in flower beds and lawns, and under paving slabs or stones. Patios are a favoured location. They are common in almost any dry, open area that is warmed by the Sun - including gardens, pavements, brownfield sites, heathland, grassland and coastal areas.
In the few weeks before the swarming event happens, you may see heaps of soil appearing above the nests.
Ants live in a caste system, where individuals have specific jobs. The queen lays the eggs while female workers look after the queen, eggs and larvae. They also gather food, enlarge the nest and otherwise ensure the colony runs smoothly. Most of the eggs develop into workers, but when the colony is ready, the queen begins to produce virgin queens and males.
When the winged males (drones) and virgin queens (princesses) emerge from the nest, they scatter to maximise the chance of mating between different colonies and reduce inbreeding.
An ant colony can only expand so much. At some point a new queen will need to strike out on her own to begin a new colony. She needs to meet and mate with a male from a different colony and find a new area in which to start building her nest. Growing wings and flying enables her to do this.
So each year, alates emerge from nests and take flight. They aren't interested in people or picnics - they are just looking for a mate.
The large winged females and the smaller winged males are often seen flying joined together. This is known as the nuptial flight.
Why do flying ants appear in such large numbers all at once? One reason is that this gives them protection from predators. There really is safety in numbers.
Another reason to swarm is to increase the chance of reproduction - with larger numbers of their species around the ants won't have far to look for a mate.
During this brief, once-in-a-lifetime mating period, a queen usually mates with several males.
Once ants have mated, the role of the males is over. The mated queens quickly chew off their own wings and begin looking for a suitable site in which to nest and set up a new colony. This is why you often see large ants walking around after a 'flying ant day' and may even see discarded wings scattered over pavements.
The ants you see the rest of the year are female workers, gathering food for the colony.
Once the queen has found a suitable site, she digs herself an underground chamber and lays her first few eggs, which she rears to adulthood. She won't eat for weeks - not until her first brood of daughter workers are ready to forage for food for her.
The stock of sperm the queen received during the nuptial flight will enable her to lay fertilised eggs for the rest of her lifetime. And she has many egg-laying years ahead of her, often reproducing until a colony is thousands strong (large nests can have more than 20,000 workers).
Males don't do any work in the nest and are only produced by their colony during flying ant season. They develop from unfertilised eggs. After the nuptial flight, the male ants usually only live for another day or two, so not much more than a week in total. Their sole reason for existence is to mate with new queens.
Lasius niger queens can generally live for up to 15 years (although L. niger queens in captivity have reached 28 years). But they spend most of their lives in their nest. They only spend a small portion of their lives as winged or flying ants - when they are young queens that need to establish a new colony of their own.
These flying insects may seem annoying to some people, but their tunnelling activities play a vital role in improving soil quality.
Their swarming events also provide a vital food resource for many species of birds. Swifts and gulls can often be seen feeding frenziedly from rising swarms of ants.
The black garden ant and the related cornfield ant (Lasius alienus) are particularly important to the continued survival of the silver-studded blue butterfly (Plebejus argus) on heathland. Populations of this attractive butterfly have declined across most of its range.
The ants and the butterfly have developed a mutually beneficial relationship. The ants tend the caterpillars, protecting them from predators. In return, the ants feed on secretions produced by the caterpillars.
Ants, along with bees and wasps, belong to the insect group called Hymenoptera. They are recognised by their three fairly distinct body parts.
There are about 60 species of ant in the UK, and they all live in complex colonies.
The nuptial flight is an important phase in most - but not all - ant species' reproduction. Red ants (Myrmica rubra) are among the other common ant species in Britain that grow wings and swarm. However, in the flying ant surveys conducted by the Royal Society of Biology, nearly 90% of the winged ants observed were black garden ants (Lasius niger), also often called the common black ant.
The types of ant you see flying will depend on where you are. For example, in woodland you may see wood ants. All ants require good weather to fly, with no rain or wind. The temperature and humidity that triggers swarming and flight is different for each species, so the timings of their nuptial flights will vary.
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