Described by poet David McCord as “little lanterns sailing by, like stars across a mimic sky”, the sight of fireflies lighting up the night as they search for mates is a shining example of a wildlife spectacle.
What are fireflies?
Fireflies are also known as ‘lightning bugs’ and ‘glow worms’, but these alternative names are as taxonomically illiterate as ‘fireflies’, since these illuminating insects are in fact beetles from the family Lampyridae.
More than 2,000 species are currently described and many of them are capable of emitting light, with aerial displays recorded from Mexico to Malaysia and Japan to the Phillippines. However, fireflies are perhaps best known and have been most comprehensively studied in the USA.
What causes fireflies to glow?
The magic trick of bioluminescence is, of course, not confined to beetles, with certain fungi, jellyfish, shrimps and plankton also capable of manufacturing light, at least in some capacity.
The beetles’ bioluminescence is created by a highly efficient chemical reaction that results in the release of light with little or no emission of heat. Referred to as a ‘cold light’, this contrasts with incandescent light bulbs, where 90 per cent of the energy used to create the light is in fact wasted as heat.
The beetles’ light is created when oxygen breaks down the chemical luciferin, with the help of the enzyme luciferase, in a section of their abdomens called ‘the lantern’. The beetles can effectively turn their glow on and off like a light switch, simply by restricting the supply of oxygen.
This ability to control their light so effectively can only be achieved when the beetles metamorphose into adults, the final chapter of a life-cycle that can last up to two years.
In order for the various species of firefly to differentiate between each other, they have evolved different flash patterns. For example, the US species Photinus brimleyi produces a single flash of yellow light at intervals of 10 seconds or longer.
How long do fireflies live? A guide to a firefly's lifecycle
Upon hatching, the larvae hide out in leaf litter on the forest floor, where they feed voraciously on worms, snails and slugs, before hibernating in winter. Some even spend a second winter in their larval form, before pupating in late spring and emerging as adults a few weeks later.
Lasting no longer than three or four weeks as adults, the beetles’ sole raison d’être is to mate, with the males typically communicating by flashing on the wing near the ground, while the stationary females flash back once they detect that the male belongs to the same species.
Some fireflies in the genus Photuris use and abuse the mating game in an attempt to gain a quick and easy meal. These predatory fireflies are able to mimic the flashes of Photinus females in order to attract and devour any Photinus males drawn in by this devious deception.
Where's the best place to see fireflies?
The most celebrated place to watch fireflies is arguably the forests of North America’s Great Smoky Mountains National Park, which straddles the border between Tennessee and North Carolina.
Of the 19 known firefly species within the park, one, Photinus carolinus, is known to take the show a step further by synchronising its flashing. Scientists studying the flash patterns of this species have indicated that the males flash in unison to ensure the females know they are indeed watching members of their own kind.
When visiting firefly hotspots, take care where you’re walking to avoid trampling on both mating adults and the developing larvae. If using a torch, it’s a good idea to cover it with a red filter, but it’s preferable to arrive at your viewing spot in daylight, negating the need for artificial light.
The timing and pattern of the flashes of Photinus carolinus consist of a series of five to eight flashes, followed by a pause of around eight seconds, before the pattern is repeated. Initially the flashing appears
to be random, but as the evening becomes steadily darker and more males join the fray, the flashes begin to synchronise, and entire sections of the forest pulsate with light.
The mating of this species only lasts for two to three weeks each year, peaking from around the third week in May to the third week in June. The precise dates tend to vary from year to year according to the ambient temperature and soil moisture, with warm, still evenings undoubtedly producing the most spectacular displays.
Firefly fiestas may have been celebrated for generations, but this hasn’t stopped their lights from continuing to be extinguished, with populations of these bewitching beetles sadly declining around the world. The chief threat currently facing fireflies is loss of habitat, but light pollution in our cities and suburbs can also be a romance killer, making it far trickier for males and females to make each other’s acquaintance.
In essence, the loss of this spectacle would not only turn North America’s forests a touch darker, it would turn our lives a touch gloomier too.
Other great places to see fireflies in USA
Oak Ridge Wildlife Management Area, Tennessee
The old-growth forests in this 13,000ha reserve are home to a species of synchronous firefly called Photuris frontalis, also known as ‘snappy syncs’
Allegheny National Forest, Pennsylvania
Biologists first discovered Photinus carolinus in the deciduous woodlands here
as recently as 2012
Congaree National Park, South Carolina
The synchronous firefly Photinus carolinus puts on a fabulous light show among the trees in this floodplain forest
