Photic Signaling in the Firefly Photinus greeni

What We Already Know

Most male and female fireflies use their flashes to come together for mating purposes. In a field with many other types of fireflies also trying to attract a mate, each species has its own specific flash pattern that allows them to locate and approach only fireflies of the same species. For many members of the genus Photinus, the male gives his flash signal while in flight, repeating the signal at fixed intervals. The female does not flash spontaneously, but only in response to a recognized male pattern. The female flashes from a perch, and almost never flash while in flight. Her flash is emitted at a certain time interval after the male's flash.

Although many aspects of the flash pattern are fixed, other aspects such as color, intensity and duration of flash can vary within certain limits without affecting the usefulness of the signal. Flashes of artificial light can be substituted for either firefly in the dialog.
The rhythm of the flash in the male is controlled by a pacemaker in the brain, and the timing of both the male's cycle and the female's response can be influenced by light signals received through the eye. The minimum time for the signal to reach the brain once light has stimulated the eye is about 0.2 seconds. Almost never, however, does the female respond to the male the instant his light signal arrives at her brain. It is clear, therefore, that some type of a delay is built into her nervous system.
For many Photinus fireflies, like P. pyralis* with both males and females emitting only single flashes, there is no ambiguity about time relations within the code. The female's time delay is a set number of seconds after she receives the male signal. In other fireflies, like the consanguineus group (P. consanguineus, P. macdermotti and P. greeni) with the male emitting a double flash, some interesting questions arise. The males of this group emit flashes in pairs. It is clear that both flashes in the male's "phrase" are necessary for the female to recognize him since females very rarely respond to a single flash in a phrase. It is not known why fireflies in this group evolved double flash phrases. Does the female time her response to the first flash in the male's phrase or the second? How much variation in the male's flash pattern can still elicit a response from the female?

Materials and Methods

The species studied P. greeni, which is active in Woods Hole, Massachusetts in late July and the first three weeks in August. The male begins his activity around 8:00 p.m. while it is still quite light. The flight of the male lasts for about one hour. The experiment concentrated on exploring which parts of the male flash signal can be changed and by how much, while still eliciting a response from the female. An ordinary flashlight was used to represent the male flash. A light meter was used to record the various flashes.

Discussion of Results

The male P. greeni flies a meter or so above the ground, repeating a two-flash phrase every 5 seconds (at 80°F). When he flashes within range of the female, she replies with a single flash timed 1 second after the second flash in the male's phrase. The male approaches in flight, continuing his flash pattern, and eventually lands close by and continues the journey on foot.
The time between the male's phrases — 5 seconds — is not critical. The female responds if the male repeats the phrase every 3 seconds, or she remains receptive to the male for many minutes even if the phrase is not repeated. It is clear that the female is keying in on the time period between the two flashes of one phrase. In a number of tests, females responded to the male's phrases when the two flashes were separated by an average of about 1.3 seconds, varying between 1.1 and 1.7 seconds. Females responded to the male about 1 second after the male's second flash in the phrase.
The study showed that, within limits, the length of each individual flash in the male's phrase was not critical, nor was the color of the flash. Even if an extra flash was inserted in between the two flashes of the phrase, as long as the timing of the two remained within the time constraints, the female would respond. Although not part of the study, observations suggest that while the female P. greeni needs to see both flashes in the male's phrase, she times her response from the second flash in the male's phrase, not the first.
Physically, fireflies in the consanguineus group look identical. Clearly the fireflies use flash patterns unique to each species to locate a prospective mate of the same species. Although flash pattern timing changes according to temperature, the interval of the two flashes in the phrases of the three members of this group are different enough for species recognition. This interval is about 1.3 seconds for P. greeni, 0.5 seconds for P. consanguineus, and 2 seconds for P. macdermotii.
* Scientists often abbreviate a scientific name, giving just the first letter of the genus name. Photinus greeni would be abbreviated to P. greeni.