Summer flings: firefly courtship, sex, and death

Natural History, July-August, 2003 by Sara Adler, James E. Lloyd

The fireflies, twinkling among leaves,/make the stars wonder. --Rabindranath Tagore (1861-1941)

As light slips from the summer sky, an army of male fireflies awakens from its daytime slumber. One by one, the insects march up blades of grass, waiting until dusk to lift off like miniature helicopters into the night. Yet these fliers aren't bent on military conquest; their goal is simple evolutionary survival. The fireflies we study--bioluminescent members of the genus Photinus--devote every night of their short adult lives to courtship, first broadcasting their amorous intentions with flashing light signals, then seeking to mate with responding females.

Few insects are considered charismatic, but fireflies are a clear exception. All over the world their spectacular courtship displays have long delighted children and inspired poets. On long summer evenings throughout the United States countless children chase fireflies through fields and backyards. In Japan, where a broad respect for nature is both traditional and deeply felt, fireflies--hotaru--are particularly revered. School graduation ceremonies feature the song "Hotaru no Hikari," which means "fireflies' light," and many cities celebrate communal firefly watching with annual festivals known as hotaru matsuri ("fireflies' festival"). In the popular Japanese cartoon Sailor Moon, the heroine is Tomoe Hotaru, a name that means "firefly of earth." And in Japanese poetry the firefly serves as a metaphor for silent yet passionate love.

As biologists, the two of us still fall under the spell of fireflies. In particular, it is their single-minded focus on procreation that has inspired us, as students of the evolutionary process of sexual selection, to spend countless nights for the past several decades observing their drama of love and death. We, along with our colleagues, have been keen to learn what makes certain individual fireflies more likely than others to find mates and insure that their genes are passed on to future generations. And our observations, both in the wild and in the laboratory, have led to new insights into how fireflies (and other species as well) play the game of evolutionary survival--how they live, love, and die.

Fireflies are not flies at all, but beetles, belonging to the family Lampyridae. To date, entomologists have formally described some 2,000 firefly species worldwide. The family includes some non-luminescent (and often diurnal) species that rely on pheromones to locate mates, as well as some species that merely glow rather than flash. In North America the flashing fireflies fall into three main genera: Photinus, Photuris, and Pyractomena.

To the uninitiated, the adults of the three genera look almost identical. Like all beetles, they have elytra, the hardened front wings that form a protective sheath above the hind wings; it is the latter that are used for flight. Most of the fireflies in the three genera have black elytra edged with yellow, and a shieldlike head covering, typically with red markings. Subtle morphological differences separate the genera; within each genus, entomologists distinguish species by differences in coloration, in the shape of male genitalia, and in flash behavior.

Fireflies themselves generally have no trouble determining whether another firefly belongs to their own species, or to their own sex for that matter. To do so, they rely solely on species-specific flash patterns--one or more short pulses of light. To identify males of their own species Photinus females key in on several flash characteristics, including pulse rate, duration, and the number of pulses in the overall flash pattern. Photinus males, in turn, usually focus on the length of the time delay before a female responds with a flash of her own.

By mimicking the signals of each sex with a penlight, you, too, can attract males and get responses from females. With one in hand, you can distinguish males from females by the size of the light-producing lantern on the underside of the abdomen. In the Photinus male, the lantern takes up the entire last two abdominal segments. In the female, the lantern is much smaller, restricted to the middle of the penultimate segment.

However conspicuous it is, the adult stage of the firefly makes up only a small fraction of the life cycle. In North American fireflies the adult stage lasts at most a few weeks. The life cycle begins when the female lays her eggs in moist soil or moss. After about two weeks the eggs hatch, and minute, carnivorous larvae emerge. Firefly larvae live underground or beneath leaf litter, feeding on earthworms, snails, slugs, and soft-bodied insect larvae. In the northern United States fireflies probably spend between one and three years as larvae; farther south they can complete their development within a few months of hatching. Firefly larvae pupate in late spring within an igloo-shaped underground chamber. They emerge a few weeks later, having assumed their familiar adult form.

Once fireflies reach adulthood, the race for reproduction is on. Photinus fireflies devote their entire adult lives to reproduction; most of them do not eat after they become adults. Triggered by dusk's fading daylight, male fireflies lift off into the air and begin their courtship. The roving males fly slowly, between three and six feet above the grass, advertising their availability with a flash pattern of one, two, or several short light pulses, repeated at regular intervals [see diagram at bottom of opposite page].