The Great Gloworme - Llandudno

There’s a road in Llandudno known as ‘Millionaire’s Row’, so named because of its luxurious houses with huge gardens that overlook Conwy Bay.  But it’s flush for another reason, there’s a resident with more bling than Joan Collins, who prefers to ‘shake it’ with bright green bioluminescence! 

Photo by Geoff Wedge

As dusk falls on the Great Orme, hundreds of little lights appear on the western slopes of this prominent limestone headland.  The glow-worm or tan fach diniwed [innocent little fire], does indeed produce a green/yellow fire in the tail segments of her abdomen, but innocent they are not!  Glow-worms only feed during their larval stage [about 90% of their life-span], feeding on slugs and snails which they paralyse with poison.  Their jaws are sharp and sickle-shaped and each time they pierce the skin of its victim, brown toxic fluid is pumped down the hollow mandibles into the snail's body.  The poison is produced in the larva's intestine and digests proteins, a single bite may be enough to halt a Grove snail, but larger Garden snails may need ten times as many bites.  As the poison acts on the snail's nerves and muscles, the victim is slowly digested into a 'soup' which the larva can lap up.  Throughout most of the meal the snail is paralysed but still alive; its heart rate rises rapidly after the first bite and begins to fall as the poison takes effect.  But glow-worms are merciful, the partly eaten snail has then been known to recover and crawl away.

Photo by Geoff Wedge

In June, a glow-worm workshop was held on the Great Orme by the North WalesWildlife Trust and Robin Scagell of the UK glow worm survey.  It was a fascinating insight into the ecology and conservation of one of our least known insects; its presence being an important indicator of old-growth grassland.  The UK glow worm survey began in 1990 and before the survey started, it was said that there were fewer than one hundred sites where glow worms could be found in the UK.  The survey has shown that there are in fact hundreds of sites throughout the UK where they can be seen, and more sites are reported every year.  The workshop was followed by a field survey, undertaken by four groups who surveyed different areas of the Great Orme.  74 glowing females were recorded in one area and one group reported males and females.  It is only the females that glow, she crawls up a grass stem and gently shakes her abdomen from side to side, hoping to attract a flying male.  She will do this for 2 to 3 hours, every evening for a few weeks in June and July until she succeeds in attracting a mate.  She will then move underground, lay her eggs and then die.

Orme at dusk  - photo by Jenni Cox

According to the survey, glow-worms are widespread and relatively abundant in the UK, but I regularly hear old-timers saying ‘When I were a lad there were glow-worms everywhere’, but Robin Scagell points out this may not always be accurate; “They may have been seeing just the good years. Historical records of large numbers are few and far between. But in our village, we have recently had reminiscences of glow-worms in hedgerows where they are now absent. The place has become more urbanised, and streetlights have increased.  Streetlights are often blamed. But I know of continuing populations in places which are quite brightly lit, even right below streetlights. That is not the full answer.   Changing land use is probably the most significant factor, plus the use of chemicals on the land. But railway lines are popular with glow-worms, despite being regularly treated with herbicides.  Desiccation of the landscape may be another factor.  In a drought, larvae will find few snails at a crucial time, leading to a population drop in the next and subsequent years, which they could take a long time to recover from, if at all.”

Over two hundred have since been recorded on one slope and there are recent reports of activity on the Little Orme, Holyhead and the Lleyn Peninsula. “The nature of the Great Orme itself, which is essentially an unmanaged landscape, is important. Most other sites where glow-worms are found are subject in some way to human interference such as changes in land use or grazing patterns. But paths probably look now much as they did in the past, so any changes in glow-worm numbers are more likely to be due to natural causes than humans -- who are constrained in this case to a path only a few inches wide!”  Records of glow-worms are extremely useful in determining population trends, identifying ‘hot spots of biodiversity’ and informing conservation management plans.  Records can be submitted online via the official survey website - www.glowworms.org.uk

This article was contributed by Jonny Hulson, secretary of the Clwydian Branch of North Wales Wildlife Trust.