Summer flings: firefly courtship, sex, and death

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

The traditional theory of sexual selection holds that males, whose gametes are relatively small and energetically inexpensive to produce, will mate as often as opportunity allows. Females, in contrast, whose gametes are large and costly, will be much more selective than the males about their mates. For at least some of the Photinus firefly mating season, that pattern seems to hold. Early on, males compete for access to females, whereas females respond to flashes only if they're impressed.

But because Photinus fireflies don't eat once they become adults, the males can produce only a limited number of spermatophores in their lifetimes. As the availability of spermatophores dwindles, the males become increasingly selective about which female they mate.

Firefly flashes may be a natural form of poetry, but they are also highly visible signals that make courtship a risky business. Flashes are readily intercepted by predators. As protection, Photinus species and some other fireflies contain noxious, extremely bitter compounds known as lucibufagins, which deter many potential predators such as spiders, primarily, as well as birds, lizards, and ants.

But not even noxious chemicals can protect Photinus fireflies from their archenemies, the larger, quicker Photuris fireflies. Female Photuris fireflies are leading ladies among the insect world's infamous troupe of femmes fatales. Photuris females are highly specialized predators that spy on Photinus courtships, then imitate the flash responses normally given by Photinus females. Those false signals lure unsuspecting Photinus males into the clutches (and guts) of their predators. The Photuris female is a voracious predator that can devour several Photinus fireflies each night. And, in the process, she gets much more than just a nutritious meal. The ecologist Thomas Eisner and the chemist Jerrold Meinwald, both of Cornell University, discovered that Photuris females can co-opt the bitter chemical deterrents produced by their Photinus prey to deter their own predators.

In addition to such natural hazards, human activities pose problems for fireflies. Pesticide use takes a toll, and urban sprawl increasingly threatens the open fields and woodlands inhabited by various firefly species. Fireflies tend to be highly site-specific, gathering and mating year after year in the same spot. If a population's breeding site is disturbed, migration to nearby areas is unlikely, and local extinction is almost certain.

Bounty hunters, too, may have been contributing to declining firefly populations. For about forty years the Sigma-Aldrich Corporation in St. Louis, seeking luciferin and luciferase, sponsored a firefly-collecting club. The company paid a network of collectors nationwide a penny a firefly (with quantity bonuses that total $600 for 200,000 fireflies). Millions were collected. Although a few firefly species might be abundant enough to support such harvesting, many less-abundant species (and species are collected indiscriminately) could readily be snuffed out. Fortunately, there is no longer any reason to collect fireflies from the wild. Synthetic luciferin has long been available, and the firefly luciferase gene has been cloned. Sigma-Aldrich ended the collecting club a few years ago.