It was just before 10pm when I spotted the first vivid green spark in the understorey – a female common glow-worm. She had climbed a tall blade of grass and was advertising her availability to males on the wing, curling her abdomen to show off the bioluminescent rear segments to their best effect.
The ethereal illumination – which can also act as a warning – is generated within the glow-worm’s light-producing organ, known as the lantern. A chemical reaction occurs between oxygen, the light-emitting compound luciferin, an energy-transporting molecule called adenosine triphosphate, and the enzyme luciferase.
At the end of the Ghost’s speech in the first act of Hamlet, Shakespeare writes: “The glow-worm shows the matin to be near/ And gins to pale his uneffectual fire.”
Uneffectual, indeed: it is an astonishingly efficient process, with 98% of the energy input to the reaction emitted as “cold” light. Compare that with an incandescent lightbulb, where 90% of the energy used is lost as heat.
For a closer look I picked up the glow-worm (Lampyris noctiluca) gently grasping her between finger and thumb. She was just under 2cm long and looked like an elongated woodlouse. I was surprised by the marshmallow softness of her body, which was quite unlike the shell-like exoskeletons of other beetles.
By the time we had returned to the car park we had counted 14 displaying females. Some of them were mating, with the smaller, dull-brown, hard wing-cased males clinging to their backs.
The status of the glow-worm in Britain is poorly documented, but there is evidence to suggest population decline. The flightless females don’t move more than a metre or two in their adult life, so habitat fragmentation or loss of chalk grassland and hedgerows poses a threat, while increased light pollution disrupts mating behaviour, with males sometimes being attracted to street lamps instead of females.
I found another female glowing on the mown verge in front of my car. As the headlights flared, her lamp faded like a torch with dying batteries.
Living in a perpetual blaze of artificial light and rarely venturing along unlit country paths at night, it is perhaps unsurprising that most people are oblivious of the presence of these radiant insects.
Follow Country diary on Twitter: @gdncountrydiary
