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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.

Although found all over Britain, glow-worm colonies are most abundant in southern England. They can be seen in grassland of every type, apart from sites that have been ‘improved’ with fertiliser or heavily sprayed with insecticides, and also occur in moorland, heathland and occasionally woodland. The spectacle is mostly a rural phenomenon, and country anecdotes abound of glow-worms being put in jam jars to read by at night!

The last 50 years are thought to have seen a steady fall in the number of colonies thanks to the usual lethal cocktail of habitat destruction, fragmentation and pollution. Artificial lights may well also present a problem as the males could be distracted by the lights and find it difficult to spot the females in the glare. Let’s hope, however, that these magnificent insects continue to bring light into our lives for many years to come.