Lampyris noctiluca

Geography

The Lampris noctiluca has been found mainly in valleys in Wales and Scotland with other appearance in Europe, such as Estonia, Belgium and Finland. They typically favour chalky and limestone areas, open grass, hedges, and woodland and will rarely be found on land which has been improved for agricultural purposes. They prefer open air, especially for evolutionary purposes for the female to be able to attract a male through her light display during the months of June, July, and August.

Physical description

The adult females range in size from 12 to 20 mm while males are significantly smaller. The larvae are only a few millimetres in length (1). They also have segmented bodies such that females may emit a bioluminescent glow (2). They vary from brownish to blackish. Males are equipped with two pairs of wings, using the second pair for flying and the first pair, known as elytra, to cover the second. Females, however, do not fly. The insects are soft-bodied and elongated in form, with a head concealed by a pronotum and threadlike antennae. Luminosity is confined to the last few abdominal segments. Among them, the adult female is notably the most luminous, using her glowing ability to attract males, while larvae also glow but more faintly and only intermittently. The light produced is cold light which benefits the species as they waste little heat. They transform only about 3 percent of their chemical energy into waste heat which is relatively efficient compared to other glowing species. Despite the larvae's similarity to adults in having segmented bodies and six legs near the head, their mode of movement can make them resemble caterpillars (1).

Light production

Light production is vital for Lampyris noctiluca. With it they have greater chances of mating, so there is a complex process involved.

They emit yellowish-green light from the translucent underside of their last three abdominal segments to attract the smaller males, which are winged. They will glow for two hours and then retreat back into their hiding place until the next night, or stop glowing as soon as they find a mate. The females can survive for at least 10 consecutive nights doing this.

The light can be seen by males up to 45 metres away. The light is emitted continuously, although they will wave their abdomens from side to side, which gives the impression of it brightening and dimming. Larvae are sometimes seen to glow, although they seem able to turn off their light more easily than the female, especially when disturbed. The pupae glow when disturbed.

The insect can regulate its light production by controlling the oxygen supply to the light emitting membranes containing luciferin. The chemical reaction producing the light from luciferin is catalysed by luciferase, the exact chemical structure of which is determined by the glow worms' individual genetic structure, which in turn determines subtle differences in the light produced from individual to individual. Virtually all of the energy produced is light; only 2% is heat.

The brightness of the glow is used by the males as an indicator of female fecundity. Males are more likely to fly to a brighter female, because it is larger and has more eggs. Due to this, females compete against each other for matings.

Reproduction

In the reproduction process of Lampyris noctiluca, the female plays a crucial role in attracting mates. To catch the attention of male glow worms, which fly about a metre off the ground, a female climbs a plant stalk. By positioning herself above most ground-level vegetation and bending her abdomen upwards, she showcases her glowing organs, signaling to any males passing by. This display is part of her adult life, which spans only a few weeks dedicated to glowing, mating, and laying eggs before her death. The eggs hatch into larvae after a few weeks, undergoing a larval stage that lasts for one or two summers, during which they feed on small snails by paralyzing them before consumption. This lengthy development cycle contributes to the observed "boom or bust" population cycles of the species, with abundance fluctuating significantly from one year to the next.

Unique to Lampyris noctiluca is its method of sex determination, which is influenced by hormones, a rarity among insects. Sexual differentiation between males and females begins during the fourth larval instar, with distinctions being initially subtle. Male cells in the apical tissue of the gonads start to divide at this stage, contrasting with female cells, which divide in the basal tissue. Remarkably, transplanting testes into females before the fifth instar leads to masculinisation, whereas similar procedures performed later or attempts to feminise males do not result in sex reversal. This demonstrates that while testes can transform pre-fifth instar female gonads into male ones, ovaries do not have a feminizing effect on males, highlighting a unique aspect of the species' biology (2).

Food

They undergo their entire feeding phase during the larval stage, The larvae are fierce predators and roam leaf litter in search of slugs and snails (1). They do this by injecting their prey with digestive enzymes and then consuming the liquefied internal contents. In stark contrast, the adult glow worms lack mouthparts entirely, rendering them unable to eat (2).

Light pollution problems

These beetles rely heavily on light for their reproductive processes. However, the rise of artificial lighting, even in rural settings, poses a significant challenge. Such light sources can complicate the males' search for partners. A 2014 study highlighted that minimal levels of light pollution can disrupt the mating behaviour of male L. noctiluca, as they struggle to find females. This research underscores the potential link between declining glow-worm populations and light pollution, suggesting that it warrants consideration as a contributing factor (5).

Habitat

They are found in old-growth grassland, especially on chalk and limestone soils. They are also found in verges, hedgebanks, and on heaths. The larvae live in sheltered places, under rocks and wood, but do crawl over different terrain.

Life cycle

Once the females have attracted the males with their glow, they mate, lay eggs, and die.

Typically, 50–100 eggs are laid over three days, with the adult not surviving to see them hatch. They are laid in fairly moist places, e.g. stems of grass, under moss and under leaf litter.

The eggs are pale yellow, 1 mm across. They may glow faintly yellow.

The eggs hatch after two or three weeks, possibly longer in colder climates. Larvae and adult females look similar, but larvae have bright spots on each of 12 segments, whereas adult females have a completely black back.

The predatory larvae feed for two or three years on slugs and snails which they inject with a brown, toxic, digestive fluid, delivered by a series of nips and bites. The poison takes time to work, and the larva must be careful not to become stuck to any protective mucus that its prey may secrete. The prey is gradually paralysed and the digestive fluid turns part of it into a brown broth that the larva can lap up. The prey remains alive, and some partially eaten victims have been known to crawl away after the meal.

The slugs and snails are often 200 times the weight of the beetle larva. The larvae are nocturnal, and are most active during moist conditions, when their prey are most active.

The larvae may moult four to five times in their lifetimes. They spend winters under logs, stones, wood holes, or leaf litter, their bodies drawn in like concertinas, hibernating as food becomes increasingly difficult to find. They awaken in spring, and the cycle is repeated for one more year, or possibly two.

They become adults from May to July or sometimes later, surviving on food reserves accumulated during time as a larva, and dying after reproducing.

Threats to species numbers

Threats include habitat destruction, habitat fragmentation, pesticide use, pollution, distraction by artificial lights, insufficient grazing and climate change.