ABSTRACT
Fireflies are members of the order Coleoptera and consist of the family Lampyridae. Male fireflies flash in order to convey the message of location and to state that they are sexually mature. Female fireflies flash a response to convey their location and that they are receptive. It has been hypothesized that fireflies also use their flash to locate a safe landing site. It has also been hypothesized that the female firefly uses the flash to locate a suitable area in which to lay her eggs. It has been proven that the different species of fireflies exhibit different flash characteristics. This is accomplished by flash intensity, flash shape, and the timing of the flash. Different species of fireflies avoid extra-species mating by these factors. This enables researchers to identify the species and sex of firefly by it's original flash signal. It has been observed that the male firefly exhibits many variations of flash patterns. The flash patterns vary in that the male firefly makes subtle changes in the timing of the flashes. The male exhibits a trolling or roving flash pattern in comparison with the stereotyped courtship flash pattern. The two differ in that when the male is looking for a female he employs the "trolling" pattern and when he has found the female he employs the courtship pattern. Fireflies both male an female also flash differently at different levels of arousal, history and temperature. Flash signaling is used as a means of competition for male fireflies. Rival males will either synchronize or they will employ "transvestite" asynchronous flashes in order to draw the female's attention away from other competing male fireflies. North American species of fireflies use synchrony to compete, while South American species aggregate on trees in order to pierce the dense vegetation with their flashes. Flash behavior of one species has also been observed to be mimicked by another separate species. Thus mimicking is performed by "femme fatales" that eat other species of fireflies.
NATURAL HISTORY and BASIC BIOLOGY
The members of the family Lampyridae display the normal life cycle typical of a Coleopteran. The insect transverses the stages of egg, overwintering larval stage plus a number of instars, pupation in spring, and eventually eclosion as an adult usually in the months of June or July. This cycle is typical of the North American species Photuris and Photinus (Carlson et al., Quarterly). Photuris and Photinus larvae usually take two years to fully develop into adult fireflies.
The larval stage of the firefly is characterized by the fact that the larvae are carnivorous. The larvae feed on other insect larvae; worms, molluscs, and other Coleopterans. The larvae subdue their prey by means of poison, injecting the venom through curved mandibles (Carlson et al., Quarterly). Larvae possess tiny, paired lanterns on the eighth abdominal sternite. These lanterns differ in structure, function and biochemistry from the adult's lanterns (Carlson et al., Quarterly). The larval lantern cannot produce a flash, just a glow that may last over a period of a few seconds. The glow's function is theorized to be an aposomatic warning, telling would-be predators that the insect is unpalatable.
When the larvae pupates in the spring it has a fully developed adult lantern located on the sixth and seventh abdominal sternites (Carlson et al., Quarterly). The adult lantern is composed of three layers:1. a transparent outer "window", 2. The light organ, and 3. an opaque, whitish layer filled with uric acid crystals referred to as the "reflector" layer. The light organ, the second layer, contains slablike light cells, filled with large and smaller darker granules (Evans, Life). The large granules are the source of the light emitting molecule luciferin (Carlson et al., Quarterly). The smaller, darker granules are mitochondria and therefore the source of ATP for the light reaction (Evans, Life). The reaction between reduced lucferin (LH2), luciferase (E), and Mg++ forms the (LH2-E-AMP) activated complex and pyrophosphate. Light is produced when the complex is oxidized (Carlson et al., Quarterly). This light, being extremely efficient, is a cold light. The adult lantern shows all the characteristics of a nueroeffector, a muscle. When stimulated by electrical impulse it the lantern responds much like a true muscle would. The lantern shows facilitation, summation, constant stimulus-response latency, tetanus, treppe, adaptation and fatigue (Carlson et al., Quarterly). It is theorized that oxygen is the main factor behind the fireflies's ability to produce light. The firefly appears to be able to control the flow of oxygen to the lantern, thus being able to make rapid and brilliant bursts of light (Carlson et al., Quarterly).
FLASH PATTERN/COURTSHIP
The flash patterns displayed in different species of fireflies are small variations in timing, shape, color and intensity. It has been theorized that fireflies may use their lanterns for other purposes. Both males and females may use their lanterns for illumination of a landing site (Carlson et al., Quarterly). The female may also use her lantern to illuminate or find suitable egg-laying sites. The main use of the flash mechanism is for a means of communication between the male and females. The changes in the patterns allow the many different species of fireflies present in one woodland area to distinguish members of their own species. This behavior is present so that the male of a species can locate a receptive female of his species.
The flash pattern can not only distinguish the different species but it can also be used to distinguish the sexes of the participating fireflies. The flash pattern can be used to identify sexes in that males are generally found flying and signaling from the air. While the females are usually found signaling from the surrounding vegetation. This is a rule that does not apply to all species of fireflies, just most. The male firefly can vary his pattern in a variety of ways. The two main differences in the male pattern are that the male has a "roving" and a "courtship" pattern (Carlson et al., Quarterly). The male uses these different flash patterns for different scenarios. In Photinus pyralis the males have been observed to change their search flights when having difficulty finding a mate. As the evening progresses the males change their search tactic to one that is more directed and has fewer turns. This is a sharp contrast from earlier in the evening when the males display a "search-advertising" tactic where the male makes use of visual cues to concentrate their search around perches. Later in the evening when the visual cues the male changes into the "harvest-advertising tactic that employs longer, straighter search paths (Forrest et al., Variation). Another species that displays this sharp contrast in mate searching tactics is Photinus macdermotti. Flying males who have not yet located a female produce strings of rhythmically repeated flashes approximately one-second longer in flash period than the courtship flashes. This small and insignificant difference in timing produces ver different responses from the females. The longer space between the flashes induces the females to respond after every flash. This is different courtship behavior in that during courtship the female responds after every other flash. The rapid form of signaling and receiving is said to give the male more precise information. In a way the female acts as a beacon, changing her response so that the male can find her with increased confidence and quickness (Carlson et al., Quarterly).
In a way both males and females act as both receivers and senders in the complicated communication sequences. Two examples showing how species of fireflies interact and communicate is displayed in Photinus aquilinus and Photinus marginellus. P. aquilinus males would begin flying approximately twenty minutes after sunset and flew for about one hour. The males would fly generally two meters off of the ground, and flash continuously. The female P.aquilinus would signal a response with single flashes from perches on grass or small shrubs. It was observed that when the female was flashing her response she would curl her abdomen towards the male. After a dialog of variable length the male would land close to the female and proceed to walk to her location (Lewis, et al. 1991). The females were responsive to male flashes over several nights, and several were observed mating on several successive nights. However P. marginellus began flying and signaling about ten minutes after sunset and would continue for approximately forty minutes. The male generally flew less than one meter from the ground . The female perches were located in mown grass but were also located in taller grass on the fringe of the lawn. The overall average perch height of the female P.marginellus was lower than that of the female P.aquilinus. However, P.marginellus females did tend to choose higher, tree canopy, perches as the flight season progressed. Signaling males landed after a few female responses, but if the female could not induce a male to land after a few failed attempts she would move to another perch. If the female succeeded in seducing the male to land she would typically move to yet another locating, dragging the male behind them. The females were also found to respond differently to particular males. The males that eventually ended up mating elicited a higher response rate that males that did not mate. This may seem obvious, but maybe the males that mated were just better communicators; timing was better or intensity of the flash higher (Lewis, et al. 1991). The examples demonstrate that the male signals in order to display sexual maturity and the female signals back to display that she is receptive and to display her location.
In another species of firefly, Photuris lucicrescens, a much different type of courtship behavior is observed. The female still produces the stereotyped flash response pattern but the true variability and originality is found in the male. The males of this species display a "crescendo" flash pattern. The description of the flash is best stated by Barber, "Poising almost motionless in the air, its light begins to dim, grows steadily to great brilliance and dies abruptly to reappear a quarter or half a minute later as the firefly poises a few feet distant and again remains illuminated for from 1 to 21/2s" (Carlson et al., Flash). As the flash gains intensity it may rise smoothly or show fluctuations. The flash duration was very difficult to measure, but the flashes were repeated at variable intervals ranging from about two to four and a half seconds. Male and females of this species also emit short, weak flashes that to humans appear as "twinkles" (Carlson et al., Flash). This flash pattern may pass for a few seconds or for several minutes. This weak flashes induce the males to produce the crescendo flashes. Once the crescendo flash begins they inhibit the twinkling behavior in both males and females. The females of the species only respond to the crescendo flashes, and the male will only continue to produce the crescendo flash pattern when answered by a female. The female does more than just respond though, she also flies and climbs toward the signaling male. This is unique to this species in that most members of Photuris are stationary during the courtship proceedings (Carlson et al., Flash).
In P.lucicrescens as in almost all firefly species temperature is a vital key to the intensity and timing of the flash patterns (Carlson et al., Flash). Fireflies were observed at two different temperatures, the first at 22o and the second at 23o . Fireflies timed in the lower temperatures produced 102 crescendo flashes at intervals of 3.4 seconds. Fireflies timed in the slightly higher temperature recorded strings of 143 crescendo flashes at intervals of 2.75 seconds (Carlson et al., Flash) . This is a significant difference between the rate of the flashes. Temperature does pay the role of both slowing down and speeding up the fireflies in their natural environments. This is probably due to the fact that the insects body heat is governed by the outside temperature. Another interesting fact about fireflies is that the overall luminescent activity of the firefly is managed by the arousal state of the firefly. Stubborn individuals can only be induced to flash with vast amounts of time and patience. When these nervous individuals were finally induced to flash the flash was minimal and had no flash structure. Once the fireflies were aroused they produced spontaneous flashes for hours. Photinus versicolor males would emit these random flashes that were totally unlike that of the regular courtship or trolling flashes. These spontaneous flashes did not elicit a response from the females, and when the females were randomly flashing they too would not respond to a male (Carlson et al., Quarterly).
As mentioned above, the flash behavior of fireflies can be used to determine the sex of the firefly as well as the species. While Photuris lucicrescens displays the well-known crescendo flash the species Photinus pyralis displays something that is quite different. The male of this species flies low to the ground in an undulating pattern. He approaches the bottom of this pattern every six seconds and as he is rising he makes a half-second flash. This pattern of undulating and flashing writes a yellowish "J" in the air. The female responds with a half-second flash after an interval of about two seconds (Evans, Life). Flashing behavior is also apparently influenced by the color of the firefly's flash. The color can also cue researchers in on the species in question. It has been observed that yellow-flashing fireflies tend to be active only at dusk, while green flashing fireflies are not active until full darkness (Carlson et at., Quarterly).
In the final example of the difference of behavior amongst the species of fireflies we will visit Lucidota luteicollis in the dry-lands of Florida. It seems that there is little known about this species, that it is an odd and highly evasive firefly. In fact, this firefly is different from most others in that the males and the females display their courtship behaviors during the day and not at night. This may be because of predator pressures or other selective pressures but the fact remains that this is very different from most other fireflies. L.luteicollis does not glow in nature, and can only be induced to glow in the laboratory. It has been observed, and only once, that when a male did find a female he would fly down to her location. The females hide themselves in the sand and are therefore very hard to see. When the male landed he thrust his abdomen into the ground where it is assumed that he flashed and subsequently mated with the hidden female. It has been theorized that the light could come into play where ambient light levels are low. It has also been theorized that the females remain hidden because it is safer and less desiccating for them in the sand (Lloyd, Firefly).
COMPETITION
Male fireflies employ many different forms of competition, these could include; synchrony, delayed flashes and "transvestite" flashes. All of these different forms of flashing are designed in order to draw the attention of the female to one of the competing males. Synchrony has been observed in many species of fireflies, and has been especially observed in competing males of Photinus macdermotti. When a male stumbles upon a courting pair he can do two main things. The first is he can synchronize with the male, or he can enter a random flash that is seemingly timed incorrectly with the male. If the intruding male were to time his response with the first flash of the courting male, the rival's second flash would be synchronized with the original male's second flash (Carlson et al., 1988; Carlson et al.,Quarterly). This synchrony is performed in order to steal the female from the original male. In some cases the female began to orient her subsequent responses in the direction of the intruding male. Thus it would seem that the intruding male was able to distract the female. If the rival male employs the second of the two options he interjects a flash after the female has already responded. This interjected flash has been called a "delayed flash" (Carlson et al.,1988). This delayed flash has also been proven to induce the female to aim her subsequent responses at the rival male. The most effective of the two competition methods, synchrony and delayed, was the delayed method. The delay method worked 50 percent more often than that of synchrony (Carlson, et al.,1988).
Synchrony has also been observed in many different species of fireflies, but more importantly it has been employed for different situations by these species. In Southeast Asia the Pteroptyx species will congregate on trees deep in the tropical jungles (Carlson et al., Quarterly; Copeland et al.,1995; Evans, Life; Lloyd et al., 1989; Stewart, 1999). Thousands of male fireflies will congregate on these trees and display a timing that is almost perfectly synchronized (Evans, Life). The theory behind this form of synchrony is that the males congregate on these trees in order to pierce the dense vegetation with their flashes. A single firefly may not produce a large amount of light, but thousands produce enough light to be seen for miles (Evans, Life). So, by congregating, the fireflies increase their chances of breeding and passing of their genetic information.
The third and possibly the most bazaar form of flash intervals that competing male fireflies use is the "transvestite" flash (Buck, Unisex). In a few species of fireflies the rival male will actually mimic a female and respond to the original male. One of the theorized explanations for this behavior is that the rival male synchronizes with the female in order to get to her quickly. Another theory is that the rival male is attempting to distract the original male. This would allow the rival male to locate the female and mate with her while the original male is confused (Buck, Unisex).
MIMICRY
Members of the genus Photuris have learned to mimic the flash codes of other fireflies, especially the genus Photinus (Alcock, Animal). Photuris female mimic the flash signals of the Photinus females in order to draw in or seduce the male Photinus. The Photuris female is a predator, she does not intend on mating with the Photinus male, she wants to eat him. These predacious females have earned the name of "femmes fatales" (Alcock, Animal). The predacious female accomplishes this feat by perching in the activity spaces of Photinus. With this mimicry the predator is actually within the prey species set of sexual selection programs (Lloyd, Firefly). The predacious females take advantage of the completion between the males for females. It will only be a matter of time before a male cannot ignore the predacious female's signal (Alcock, Animal; Lloyd, Firefly).
SUMMARY
The flash behavior of the firefly is a complex
puzzle of timing, flash intensity and shape. The biology of the firefly
has evolved in such a way that the firefly has the ability to perform these
three factors. The flash of the firefly has been shown to distinguish
not only the sex of the firefly but also the species. This
is accomplished by varying the three factors in such ways that fireflies
are able to produce twinkles, crescendo flashes and "J" shaped patterns.
Male fireflies are fiercely competitive for the available females,
and will go to great lengths in order to reproduce. Males will
synchronize, interject, delay and mimic female signals in order to confuse
other males as well as "win" a female. Predacious females mimic
the signals of females from different species, and when the male
comes calling she grabs and eats him.
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