The lesser glow worm

Britain’s smallest, rarest and most mysterious glow worm rediscovered at the Bursledon Reserve, Hampshire

Raphaël de Cock, John Horne and John Tyler

The so-called ‘common’ glow worm, Lampyris noctiluca, is Britain’s best-known species of firefly. Along with over two thousand other species spread around the world it belongs to a family of light-producing beetles called the Lampyridae. The males of many fireflies are able to send out brilliant flashes of light as they fly, whereas in our glow-worm the male produces little more than a feeble glow, and then only if he is threatened or disturbed. Instead he leaves the glowing to the caterpillar-like female, who being wingless has to wait patiently on the ground or among vegetation, using her bright yellow-green beacon to flag down passing males. 

Towards the end of the 19th century a second firefly species, known as the lesser glow worm, Phosphaenus hemipterus, was discovered on a handful of sites in Hampshire and Sussex. In the 120 years or so since then it has been seen in Britain fewer than 20 times, with decades passing between sightings, fuelling speculation that it might have become extinct in the UK.

As a firefly the lesser glow worm is quite exceptional. In fact at first sight it hardly looks like a glow worm at all. The males are tiny, no more than 10 mm from head to tail, and have relatively large antennae, runMale Phosphaenusning about half the length of the body. What makes him particularly unusual - and possibly unique – among fireflies is that he is flightless: there are plenty of firefly species in which the female is wingless (earning them the name ‘glow worm’) but as yet the lesser glow worm is the only known species in which neither sex is able to fly. The wings that his distant ancestors would have had are now reduced to small vestiges covered by equally small wing-cases, giving him the appearance of a rambler carrying a rucksack. This also makes him look very similar to numerous species of common rove beetles (in fact both types of beetle, though only distantly related, may have lost their wings for the same reason, allowing them to move freely amongst soil and leaf litter). This resemblance, together with his diminutive size, makes the lesser glow worm spectacularly easy to miss. 

The male is also extraordinary, at least within the European fireflies, in being active during the day, using its large, sensitive antennae to sniff out the airborne scent (pheromone) produced by the even more inconspicuous female. No more than 13 mm long, she is a miniature version of a common Glow-worm but she lacks the well-developed light organs that make the latter so visible. What feeble light she can muster comes from two small dots at the tip of her abdomen which, like those of the male, are inherited from the larval stage and are switched on only in emergencies, if she is attacked or disturbed. This makes her extremely difficult to find (at least for humans), so in most places the majority of sightings are of males, which often roam about on bare surfaces such as footpaths, pavements and the bases of walls on sultry June afternoons.

As adults neither of the British glow worms is able to feed and few will survive for more than a week or two. Instead they must rely on the internal larder of fat built up during their larval stage, which may last up to three years. The larvae of the two species differ in their tastes. The common glow worm, like the majority of fireflies, hunts slugs and snails. The lesser glow worm, again an exception to the rule, prefers a diet of earthworms, a habit which it shares with just a handful of its American and oriental relatives. The tiny larvae are capable of overcoming prey a hundred times larger than themselves by using their sharp, hollow jaws to inject a powerful toxin that paralyses the victim and digests it from within.

In 2002 John Horne discovered that he had common glow worms on his nature reserve at Bursledon near Southampton. They had probably benefited from the decades of careful and sensitive habitat management which John had been carrying out and which had also encouraged several rare dragonflies, amphibians and many other creatures, some listed in the Red Data Book of endangered species. As well as managing his reserve John has been conducting a long-term study of common glow worms there, which involved putting out a large number of boards, concrete blocks and even pieces of carpet under which he could observe the glow worms more easily.

In 2007 this study also led to the totally unexpected discovery of a female lesser glow worm under one of the boards. Larvae and adults of both species seem to like hiding under these artificial shelters, giving John a rare opportunity to photograph them feeding, mating and laying eggs. The lesser glow worm is another Red Data Book species and Max Barclay, the Beetle Curator at the Natural History Museum, says that the Bursledon colony is providing a valuable insight into its way of life. ‘Without that information’ he says ‘we don’t have a hope of conserving it, so [John’s study] is a massive contribution’.

At the end of May the Bursledon Reserve was visited by British glow worm specialist John Tyler and the Continental lesser glow worm specialist Dr. Raphaël De Cock. During their two-day stay over a dozen lesser glow worm larvae were found, which suggests that the reserve supports quite a sizeable population. At present John’s reserve is the only known British colony of the UK’s rarest firefly. What makes it particularly valuable is that until now virtually all records of this species have been one-off sightings, with no opportunity for follow-up research to learn more about its biology and habitat requirements. Future surveys using John’s ‘board technique’ or live traps ‘baited’ with females, used in Europe by Dr. De Cock, may reveal whether other, formerly unnoticed lesser glow worm populations exist on nearby sites.

Back to the UK Glow Worm Survey