Unusual facts about glow worms
Authored by Hannah Thomas - a conservationist and
Wildwood Devon volunteer, based in Somerset
William Shakespeare, Roald Dhal and Wordsworth have all been captivated by glow worms softly luminescent appearance. Wordsworth poetically named them ‘Earth-Born Stars’, but what are they exactly?
Glow worms are not actually worms at all; they are beetles! The fact that they spend most of their lives as larvae and the wingless, glowing adult females don’t look much like beetles, may be why they have such a misleading name.
Male glow worms are smaller than the glowing females. They have black wing cases and can fly, so look like typical beetles. It is the females that have made glow worms famous by emitting a greeny-orange glow, about the strength of a single LED (like the ones on TV standby).
Glow worms climb up plant stems and emit the light from their abdomens at night in order to attract males, who have large, photosensitive eyes - perfect for scanning vegetation at night.
Sometimes people mix up glow worms and fireflies, as they both are known for their ethereal glow. Although they are both members of the same overall family, fireflies are a different species from glow worms. The closest to Britain that fireflies are seen is Belgium. Unlike glow worms, fireflies can glow whilst flying.
- Glow worms belong to the family Lampyridae = Greek for the shining ones. Lampyris noctiluca is the glow-worm species most often seen in the UK.
- Glow worm larvae spend 2 years hunting and putting on body mass and storing fat. In the winter the larvae hibernate under rotten logs and fallen trees.
- The larvae are voracious predators and use their sharp mouth parts (mandibles) to inject paralysing toxins into their prey- slugs and snails (up to 200 times their bodyweight).
- Whilst they are waiting for their toxins and digestive juices to take effect on their prey, the larvae will often ‘hitch a ride’ on the shell of the snail. The larvae then ‘drink’ up the digested remains of the snail or slug.
- Young glow worms look very like the wingless females, they can produce a weak intermittent light that is easily overlooked.
- Adult glow worms can't feed, so they can live only for 14-21 days or so.
- Only adult females glow brightly, through chemical reaction called bioluminescence it is used to attract the flying males.
- Females climb up stalks of grass towards the horizon and moonlight and twist and turn their glowing abdomens for 2 hours a night.
- Females can only glow for about 10 nights to attract a mate, once she has mated, she turns out her light, lays her eggs and dies.
- Females can lay up to 100 eggs which are laid on the ground. The hatched larvae feed and grow for about two years before fully maturing.
Photo credit: David Evans
Glow worms in danger
You may be wondering why you haven’t seen more of them. Our modern well-lit existence makes it less likely that we will notice them. However, like many species the abundance of glow worms has gone down considerably.
It is estimated that their numbers have fallen by 75 per cent in the last 20 years meaning for every four glow worms that could have been spotted in the 90s, there is only one to be found now. Human impacts on our environment are likely to be causing much of that decline.
Light pollution has been shown in a 2021 scientific study to confuse males and dim the lights of females, who being flightless are unable to move away to darker areas. Changes in land use, habitat destruction and use of pesticides are all thought to be very damaging to numbers of glow worms.
Although churchyards have historically been hot spots for glow worms, ‘tidy’ churchyards = no glow worms. With more intensive agricultural practices, meadows are more frequently cut for hay and silage. As glow worm larvae take 2 years to mature colonies are more likely to be wiped out and as females are unable to fly, they can get ‘marooned’ as habitats get fragmented by increased urbanisation.
The recent trend for laser decorations projected from people’s houses has also led to confusion over glow worms or fire flies being mis-identified here in the UK. Glow worms are not found in trees and we do not get fire flies in the UK. So if you spot lots of green lights in a tree, sadly the most likely explanation is lasers rather than glow worms.
When and where can you find them?
The best time to spot glow worms is between June and July. Glow worms have been spotted in all areas of Britain and Scotland, however they are not present in Northern Ireland. Limestone areas are popular spots for spotting glow worms. They are most frequently reported close to water, especially along canal paths and river banks.
Other good places to spot glow worms include gardens, disused railway embankments (stay safely away from active railway lines though), meadows, churchyards, woodlands (especially rides and woodland edges) and roadside verges.
Look for them on a night with a crescent moon, as even the light of the full moon can make them difficult to spot. Let your eyes adjust to the darkness and avoid the use of torches if possible. They are most easily found in ‘un-improved’ or messy looking grasslands, where the stalks can grow long and there are no pesticides in use.
The best locations are those that have a good supply of snails as this is the favoured food of the larvae. As females are unable to fly when matured they will likely be found close to where they were living as larvae.
Fancy some help in spotting them?
Local nature groups often organise Glow worm walks and events. Glowworms.org.uk also provides a calendar of walks around the country.
Wildwood taking the lead
The Wildwood Trust has partnered with Derek Gow Consultancy (DGC) on a glow worm breeding project to try and increase numbers in the wild. Derek Gow is leading on the plans to boost glow worm numbers nationally.
We are working closely with Derek’s associate, Pete Cooper who is taking the lead for this initiative, having perfected his method for breeding glow worms during the pandemic. Wildwood is one of two facilities Pete is overseeing that are taking part in captive breeding for release - the other being Manchester Museum.
DGC provided Wildwood with glow worm larvae in October 2022. Some of the larvae has now hatched into adults and we are excitedly monitoring the breeding season in the hope of new clutches of eggs here at Wildwood. The plan is for the newly increased numbers of larvae from Wildwood will be released into the wild.
DGC will seek out suitable locations across the south before we release them. As the glow worms are in their larvae state for two years, they will have the time to disperse in these suitable areas before maturing into non-feeding adults and reproducing to further their numbers during their short adult stage life-span.
What Can you do to help Glow worms?
- If you are lucky enough to spot glow worms, be sure to leave them where they are – they know best where they like to be, females will return to the same spot night after night to find a mate
- Help record the sightings though citizen science at the UK glow worm survey https://irecord.org.uk/enter-glow-worm-record
- Don’t take glow worms home – they need specialised habitat
- Encourage your local area to allow habitat restoration and avoid pesticide, slug and snail pellet use so that churchyards, playing fields and community meadows can become wildlife rich and glow worm friendly spaces.
- If you have glow worms near you, avoid using low level solar lights and perhaps turn off garden lights at night from June to August.
- Find out more and join a community of glow worm supporters on facebook
- Donate to Wildwood to support our re-wilding mission
Reviving glowworms will help other species
As Pete Cooper explains “These insects are driving habitat restoration which benefits a multitude of other species who love that messy mix. Where you get glowworms, you get slow worms, and reducing artificial lighting will help bats as well.”
Main image photo credit: Pete Cooper