Nature’s Top 40: Britain’s Best Wildlife

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.

38 Displaying glow-worms (#ulink_effa7f54-7558-5cda-af62-a6f8e3311faf)

There can be few more delightful surprises for a naturalist than to walk along a country lane on a summer’s evening and to chance upon small pinpricks of luminous light emanating from the fields flanking the hedgerows. In addition to these unique biological lights being one of Britain’s most underrated wildlife spectacles, their function plays a vital part in the mating game of a fascinating insect.

Glow-worms

WHEN

Mid-June to mid-July

WHERE

More common in the south. Slapton Ley NNR, Devon; Barnack Hills & Holes NNR (Natural England), Cambridgshire; Aston Rowant Nature Reserve, near Stokenchurch, Buckinghamshire

The flightless female puts on one of nature’s finest light shows … and all to attract a male!

Stephen Dalton

The glow-worm is Britain’s only bioluminescent representative of an order of insects called fireflies, a group that reaches its maximum diversity in the tropics. However, both their name and the order to which they belong can be very confusing, as glow-worms are neither worms nor flies, but members of the order of beetles.

While those lucky enough to have seen this miniature version of the Blackpool lights are few, even fewer people realise that the light show is merely the final two per cent of the glow-worm’s intricate life cycle. For the previous 15 months, they have been quietly transforming from egg, to larva, to pupa and finally to adult as they prepare to go out in a blaze of colour.

In common with the vast majority of insects, the glow-worm starts life in the autumn as an egg. Batches of between 50 and 100 eggs are laid under vegetation or stones by a female from the preceding generation, a task so arduous that the exertion is her last task before she dies.

Each egg takes around 35 days to hatch, after which out pops a larva. The creature that emerges is a lean, mean, killing machine, and, once its skin has hardened, it has only one thing on its mind: food. The glow-worm larva is a specialist eater, restricting itself to a diet of snails and slugs, which are tracked down at night with its sensitive antennae and palps (feelers). Once potential food is located, the larva is able to tackle prey, such as snails, much larger and heavier than itself. It does this by giving the snail’s foot a series of nips with its sickle-shaped mandibles (or jaws). These mandibles are hollow and deliver a powerful poison, which firstly acts upon the snail’s nervous system to cause paralysis, and secondly starts to digest the snail into a ‘soup’ that is then lapped up by the larva.

During this process, the snail is paralysed but still alive; very occasionally, partly eaten snails have been known to recover as the toxin wears off and crawl away to fight another day. The larva is capable of eating substantial meals at one time meaning a rapid growth rate. However, this presents a problem as the growth occurs inside an inflexible exoskeleton. The larva solves this like many other immature insects, by undergoing a moult, in which the old skin is shed and replaced with a larger version.

Even though it is the adult females that are associated with the light show, the larva is also able to produce a faint light, which often pulsates on and off. The reasons for this are not entirely clear, but this technique is thought to be used to scare off potential predators. After a couple of moults and as winter takes hold, the larva becomes more lethargic and then hibernates under logs, stones or leaf litter. In the spring of their second year, the larva emerges hungrier than ever and eats voraciously over the spring and summer, undergoing moults along the way in preparation for breeding the following summer. Towards the end of the summer in their second year, as they curl up to see out their second winter before breeding, the much larger females can easily be identified from the males.

By the following spring, the larva is virtually full-grown and may not even need to moult again before it pupates. Once early summer arrives, the larva often loses its nocturnal habits as it actively searches for a suitable place to pupate. This is particularly important for the females: they are remarkably sedentary as adults so use this period to colonise new areas.

The larva then picks a secluded area and begins to pupate, a remarkable process in which the larva’s body, including all the organs, is completely broken down and then reconstituted as the adult form takes shape. This process takes just over a week for the females and slightly longer for the males as they have to undergo a more drastic alteration due to their need for the power of flight.

The first glowing females of the season are usually seen from mid-June, and, with their segmented bodies and absence of wings and wing cases, retain more than a passing resemblance to the larvae. The light organ is positioned on the underside of the abdomen and consists of two luminous bands and a couple of luminous spots set either side of the ovipositor, the organ from which the eggs will be deposited. The glow-worm’s light is produced by a string of chemical reactions between a small protein called luciferin and a large enzyme called luciferase. Though the complex reaction is still not totally understood, it is an incredibly efficient process: 98 per cent of the energy is released as light, compared to a measly 5 per cent in a light bulb, in which the vast majority of the energy is wasted as heat. This means that the light organs of the glowing females are completely cold to the touch.

Rough grassland with little light pollution is essential if you are a female glow-worm desperate for attention

David Woodfall

Andy Rouse

It is only virgin females that glow; once a female has mated and begins egg laying, the light organ has served its purpose and is switched off.

The glowing performance usually commences soon after dusk at around 10–10.30 p.m. It is thought the turning on of the light is triggered by a drop in light intensity below a certain threshold level, which explains why glow-worms that advertise in the darkness of a woodland edge will begin glowing earlier than populations in predominantly grassland locations. The female will generally display close to the ground, or up to a maximum height of around 40 to 50 centimetres if she feels a higher vantage point would be more beneficial. Females will even glow during rain, but usually stay closer to the ground during inclement weather.

Because the light organs are set on her underside, the female also has to twist her abdomen around to make sure that any males flying past will see her lights. This twisting is often accompanied by a swinging of her abdomen from side to side like a metronome, which, to complete the exhibition, gives the effect from a distance of the light brightening and dimming.

The display usually lasts for a couple of hours, after which, if she hasn’t been successful in attracting a mate, she turns off the light and retreats back into the grassy tussocks to prepare for a repeat performance the following night. The females are remarkably sedentary and are often seen displaying from the exactly the same spot until they either snag a mate or die an exhausted spinster after around ten consecutive nights of lighting up the night.

The numbers of glowing females can vary enormously from just a few females at small sites, to Brush Hill in Buckinghamshire, the location thought to contain the largest colony of glow-worms in Britain, with 320 females counted in one visit in 2007.

The male glow-worm looks very different to the female, because he has to be mobile in order to track down the displaying females, and so has a fully functioning pair of wings tucked away under his wing cases. He normally takes flight shortly after the females have started glowing and flies a couple of metres above the ground until he spots a virgin female. He will then drop out of the sky with unerring accuracy next to the female and attempt to climb on her back. It is not uncommon to have a number of suitors chasing a single female, meaning a form of rugby scrum can sometimes ensue as they jostle for position.

Like the adult female, the male cannot eat and so has a very limited adulthood. Once mating is over, the exhausted male dies, usually no longer than a week after emerging from his pupa for his date with destiny. His death is closely followed by the female’s after she has dutifully discharged her eggs to produce the next generation.