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Despite their name, glow-worms aren't worms at all: they're beetles. They use their bioluminescent bodies to communicate with other beetles, in an attempt to attract a mate.
Some animals on land have evolved to produce light, too. Glow-worms are a group of beetles that use their luminosity to help them flag down an appropriate mate.
Glow-worms belong to the family Lampyridae. The beetles in this group are commonly known as fireflies or lightning bugs. There are over 2,000 lampyrid species currently known to science.
Generally, the term glow-worm is applied to species where adult females look like their larvae (known as larviform females), are wingless and emit a steady glow of light. The females' larval looks are likely why these beetles are labelled as 'worms'.
Lampyris noctiluca is the glow-worm species most often seen in the UK. These nocturnal beetles, known as common glow-worms, are found across Europe and Asia.
It can be tricky to tell common glow-worm females and larvae apart, as they both have similar-looking segmented bodies. But the larvae have distinct reddish spots on the outside edges of each segment, which don't occur on adult females.
The larvae prey on snails, whereas the adults don't eat anything. Adult females can reach around 20 millimetres in length.
Males can fly and are distinguishable from females by the hard wing case covering their bodies and their far smaller size.
Adult female glow-worms have a large, light-producing organ at the end of their abdomens. At night they use a bright, steady stream of yellowish-green light to attract flying males. During the day they burrow underground to avoid predators.
A female will climb to a high point, such as a grass stem, and turn her glowing light upwards. This ensures that she is as visible as possible to flying males. Adult female L. noctiluca only live for a few weeks, until they mate and lay their eggs.
Their lights are bioluminescent, which is the natural production of light by an organism created by a chemical reaction.
In glow-worms, a molecule called luciferin is combined with oxygen to create oxyluciferin. A chemical reaction with the light-emitting enzyme luciferase produces their illuminations.
But glow-worms can't easily control the supply of oxygen, so they can't flash their lights on and off like some other firefly species. Instead they produce a sustained stream of light.
In non-glow-worm fireflies, males and females communicate by both flashing their lights in specific patterns and tempos to identify themselves. This is one of the main ways for fireflies to tell if they're receiving a message from the same species.
Fireflies in the genus Photuris, found in North America, are often called femme fatale lightning bugs. The females mimic the light patterns of other species, attracting males to kill and eat them.
In species of firefly where the larvae glow prominently, this is probably for defence. Their glow may be being used to signal that they wouldn't be a pleasant meal for predators.
Fireflies' lives revolve around light. Artificial light in night environments is increasing, even in countryside areas. This can lead male fireflies into thinking they have found a colony of brightly glowing females. Artificial light can block out female bioluminescence and make it difficult for males to locate females.
A study in 2014 found that even very low levels of light pollution could interrupt the reproductive behaviour of male L. noctiluca that were searching for mates. The authors suggested that in areas where glow-worms are in decline, light pollution should be looked at as a possible cause.
In the UK, adults of L. noctiluca are active between May and September, usually with a peak of activity in June and July. The larvae are often active for a little longer, seen between April and October.
Glow-worms are usually found in locations where there is a good supply of small snails for larvae to feed on. The beetles also tend to be associated with limestone areas.
In 2020 a new study that recorded glow-worms in the UK over the last 18 years found that glowing female L. noctiluca at sites in southeast England have declined in number by about 3.5% per year.
The lesser glow-worm (Phosphaenus hemipterus) can also be found in the UK. But both the males and females of this species are flightless and neither glows with much intensity or regularity. They usually only illuminate when disturbed.
Their small size makes this species easy to miss. Unlike the majority of other fireflies, the lesser glow-worm is active during the day, removing the reliance on bioluminescence. To find each other, females may release pheromones that males sense with their long antennae.
Glow-worm sites are found widely across Britain - although according to the National Biodiversity Network Atlas, P. hemipterus is currently only known from Hampshire, Surrey, East and West Sussex.
During their active season, a number of glow-worm walks and events are organised by local nature groups in the UK.
The term 'glow-worm' is also used to describe another group of insects. The fly genus Arachnocampa comprises five species of fungus gnat which emit bioluminescence during their larval stage.
But unlike glow-worm beetles, the gnats use their luminosity to entice prey. They produce sticky webs that hang from the walls and ceiling of the caves, and use their lights to attract and ensnare moths and other prey.
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