A Review of Predation in Photuris, and its Effects on the Evolution of Flash Signaling in Other New World Fireflies

Abstract

Predation by aggressive flash mimicry in fireflies, indeed in any insect, is only known in the Photuris 
species, primarily in the females.  This involves responding to the flashes of heterorspecific males who are 
searching for mates, deceiving them into approaching, and then eating them.  Some Photuris species are 
better at this than others, for example, Photuris versicolor is known to prey on eleven species of firefly, 
and twelve other Photuris species prey on at least two, or more, species.  On the flip side, one prey 
species in Florida has 6 predators (Lloyd, 1984c), therefore there is overlap between Photuris species in 
their flashing.  Photinus,   and other fireflies on the prey list, generally only have one flash signal and, 
traditionally, taxonomy has been based on species-specific mating flashes.  Photuris is one of the 
difficulties taxonomist have had to overcome when using this method.  To compound the problem, it is not 
only the females that mimic other species, the males are good at mimicking as well.  It is not known, 
however, whether this is a strategy for locating and seducing their own hunting females, or an attempt to 
lure males of other species in for dinner, or even as a counter-measure against larger species of Photuris.  
There is evidence to suggest that aggressive mimicry is not all that effective.  The prey species have 
adapted many novel behaviors in terms of changing their flash code and how they ultimately approach the 
female.  So, Photuris will also target on the flash of their prey as a guide for an aerial attack, this is called 
hawking.  Many times females will use mimicking in conjunction with hawking, they will signal, luring 
them in and then home in on the males' response in a aerial attack.  Male and female Photuris will also 
hover around a female of any species, in the "luring mode", and attack any males closing in on her.  
Predation is arguably one of the more important pressures on the evolution of signaling in the New World.  
The differences between Old and New World fireflies is fascinating.

	Introduction

Fireflies in both the Old and New World use flashes of light from their abdomen for sexual 
communication.  In Old World fireflies, the Pteroptyx species aggregate in synchronously flashing 
swarms.  The males are usually stationary on what are known as "firefly trees" and females come to them 
(Lloyd, 1984a).   In the New World, however, this is rare.  Generally, males fly and flash over an open 
space, like a forest edge or lawn, searching for a female on the ground, or in low vegetation, that offers a 
proper response.  A "flash dialogue" ensues to help him orient to her location and make sure she's the 
right species, usually this takes between one and ten flash exchanges.  After finding her, he drops and 
walks to her. Mating follows (Lloyd, 1966).

 	Most of the time, males will emit a single type of flash pattern that is species-specific and females 
emit a stereotyped response.  The response latency, the delay between call and response, is a critical factor 
for species recognition (Vencl, 1994).  This has frequently been used as a handy taxonomic reference.  
However, Photuris females have learned to "mimic" females of other species to attract heterospecific males 
and eat them (Lloyd, 1980).  Predation in adult Photuris species is apparently unique in fireflies, other 
species will either feed on plant fluids or use larval fat storage (Lloyd, 1984a).  The term "aggressive 
mimicry" has been coined to describe this type of predatory behavior.  Photuris females generally have 
repertoires of several flash responses that duplicate heterospecific females.  This has resulted in what is 
ultimately an "arms race" that has been important in the coevolution of flash communication and novel 
courtship behaviors in New World fireflies (Vencl, 1994).  Even male Photuris locate their females by 
mimicking her prey, at the times and locations where she is hunting, in hopes that he can seduce her and 
make her receptive to mating.  Some species have lost their species-specific flash entirely (Christy, 1995).  
If the female does not change the rate of her flashing, it usually means the answer is no, and he had better 
leave to avoid becoming dinner (Copeland, 1995).

	General Flashing

Flashing in fireflies has many different functions.  The most important of which is sexual communication.  
Flash signals have been classified into two systems.  System I is the simplest, it is where a continuously 
glowing female attracts a non-glowing male, this is the system generally found in the Old World.  System 
II is when a female flashes a response to a flying male, who is broadcasting a pulsating signal.  He will 
then orient on her, using the repeated flash dialogue between them, this system is found in New World 
species (Suzuki, 1996).

Of the System II flash signals in Photinus, the flash-answer signal is generally the most efficient, it is as 
short and simple as it can get (Lloyd, 1966).  But , it is well known that a "receivers' perception of, 
processing of, and responses to stimuli, strongly affect signal design" (Christy, 1995).  The design 
determines the response, if there is one, and how well it is transmitted and perceived.  Even the physical 
structure of the eyes in the firefly has been enhanced to increase accuracy at night. They are about 20,000 
times more sensitive than in the day, to help in the perception of the signals (Lall, 1993).  Many test have 
been done to females of  Photinus species to demonstrate their ability to discriminate on the bases of pulse 
length, and the timing between pulses (Lloyd, 1966).

There are other functions of flashing as well.  In Photuris species, both males and females have been 
observed using their luminescence for illumination during landing and walking on the ground.  Fireflies of 
many species also flash when captured or confined, this is thought to intimidate predators (Lloyd, 1969).  
In a Old World species, a male will bend his light organ forward and put it in the faces of the female he's 
mounted, while flashing, to blind her dark-adapted eyes to decrease her receptiveness to rivals (Lloyd, 
1984a).

Aggressive Mimicry

Females of the Photuris species have "cracked" the species-specific signal codes of their prey to use their 
own sexual signals against them (Lloyd, 1990).  This involves flashing variable (and apparently proper) 
responses to the flashes of heterospecific males.  This is called either aggressive mimicry or predatory 
responding (Copeland, 1991).

Twelve Photuris species have been observed preying on between two and eight species, but the Queen of 
aggressive mimicry is Photuris versicolor which preys upon as many as eleven species.  In fact, except for 
the congener group and some idiosyncratic exceptions, all Photuris prey on at least one species of firefly.  
The idiosyncratic exceptions include those who prey on other insects, for example, one Mexican species of 
Photuris prey on mosquitos and beetles that they capture on a common food source, sticky grass seed 
heads (Lloyd, 1984c).  It is theorized that these species have abandoned aggressive mimicry either due to 
the danger of larger species of Photuris preying upon them, or to the lack of prey fireflies in a particular 
habitat.

The fact that  predator and prey species vary in geographic and seasonal distributions, and that some 
Photuris species dominate others, and most Photuris have multiple prey, suggests that conditions must be 
extremely variable for both predator and prey.  This also suggests that aggressive mimicry is not a 
successful technique during most seasons.  Photuris generally have larger populations than their prey 
species and observations of capture rates are low which contributes to this theory (Lloyd, 1990).  Also, 
most Photuris use secondary attack methods, either in conjunction with aggressive mimicry, or by itself.
	
Mechanisms of Aggressive Mimicry

Studies on Photuris versicolor females have shown that the elicited response latencies, the timing of the 
dialogues, is the same for conspecific and heterospecific males.  This suggests that the same flash control 
mechanisms are used for both courtship and predation.  It has been proposed that the predatory response is 
based on a entrainment mechanism where a periodic flash output is produced by a central oscillator 
resulting in a interactive exchange with rhythmic photic input (Vencl, 1994).  This would explain how 
Photuris species can mimic from two to eleven different species through a single neural circuit, and still 
have relative success.  They properly mimic other females by lumping them into one group within a certain 
variable range (Lloyd, 1984a).  The plasticity of the response in Photuris females is on a graded 
continuum to permit adjustments to the local availability of prey, who exhibit seasonality and are only 
temporarily abundant.  Therefore, Photuris species generally do not possess a discrete response to 
conspecific courtship patterns and response latency is not as important in their own courtship as in other 
fireflies (Vencl, 1994). 

Female Photuris also exhibit a change in response behavior based on mating status and seasonality.  
Before mating, females generally respond to conspecific signals, which is a triple pulse for some species.  
After mating, females respond more to the single and paired flashes of heterospecific males.  The change 
also occurs towards the end of the mating season.  This seems to be a shift of the variable range of 
response latencies from a receptive range to a predatory range (Vencl, 1994), and also appears to be 
coincidental with changes in the female internal state and parallel to the changes in the sex ratio (Copeland, 
1989). 

Although using sexual signals in predatory attacks is considered "deceitful" and antithetical to the 
development of "honest" signals.  The trap, the lie, is favored because it addresses preexisting responses, 
and is not all that dissimilar from "true" responses (Christy, 1995).  The flashing courtship displays of 
males in large congregations gave Photuris species a rich, and largely uncontested source of food and the 
ability to steal toxic defensive compounds from other fireflies in the New World. 

	Problems with Firefly Taxonomy

The mating flashes of male fireflies has long been considered a valuable aid in taxonomy.  The modern 
study of firefly systematics was started in 1910, by Frank A. McDermott, a chemist with a hobby in 
fireflies.  McDermott wrote the first catalogue of Lampridae worldwide, nearly 2,000 species where 
described (Lloyd, 1990).  Indeed, many taxonomists have discovered the presence of morphologically 
cryptic species of Photinus and Pyractonene using these signals.  Mating flashes have been found to be 
very important in terms of reproductive isolation. They are generally consistent and simple signals repeated 
at fairly regular intervals ( Lloyd, 1969).

Because of this tradition, aggressive mimicry by Photuris females is an "insideous affront" to those who 
believe that the firefly species problems can be solved simply (Lloyd, 1990).  This has also caused some 
error and confusion in the firefly taxonomy.  In Nova Scotia, it was once thought that there were several 
species of Photuris, however, closer examination showed that there was only one (Buschman, 1974).
There are many reasons for the increased difficulty in taxonomy of fireflies: 1) Photuris females, and 
males, flash in other contexts than mating (predation), 2) The signal structure is very complex, 3) the 
signals structures change over the course of the evening, 4) Captive females either do not respond, or 
respond erratically to artificial or male flashing, which precludes discrimination tests and, 5) The mating 
activity itself cannot be observed readily, because Photuris species are generally arboreal, and during their 
own courtship they will fly and flash at the top of trees and presumably mate there (Lloyd, 1969).  Also, 
different populations of the same species of Photuris may use different mimicked responses according to 
local populations of prey available and appear to be sibling species; or, sibling species could be mimicking 
the same prey species and appear to be the same species.  Consider the worst possible taxonomic 
nightmare " ... a bucolic meadow of two Photuris species with polyphenism combined with 
polymorphism and sexual parasitism by a parthenogenic sibling species, with the usual seasonal, 
geographic , and ecological variations, with the species preying upon each other by aggressive mimicry, 
and with male cannibalism and an overlay of female mate choice (Lloyd, 1984a).

	Male Mimics

As if the problem with taxonomy, because of aggressive mimicry in female Photuris, where not enough .  
The male Photuris also has two, or more, patterns, mimicking the flashes of their females' prey.  Photuris 
quadrifulgens have five different flashes and individuals change among these patterns throughout the night 
(Forrest, 1995). Males can often be observed to change their signal and altitude at the same time, 
switching form a treetop courtship of a conspecific female and emitting a single, or triple,  flash pattern 
thought to be species-specific, to mimicking their females prey closer to the ground and searching for 
hunting females.  Some Photuris males only emit the mimicked pattern, and are thought to have completely 
lost their own species specific signal (Lloyd, 1980).  In the Nova Scotia species of Photuris, males will 
increase the pulse of his flash, in hopes that the female will respond with a increase of pulses in her 
response until both seem to flash continuously, then the male will land and mating will ensue.  If the 
female does not change form a single or double flash response, the male will know better that to land 
(Buschman, 1974) , this also suggests that females may not discriminate based on pulse number (Forrest, 
1995).

Males will often shift form their own signal, to one mimetic pattern, and then on to another over the 
evenings display.  This is thought to be a shift form searching for a receptive females to hunting females.  
This also could be adaption to the temporal change of prey species over the course of the night (Forrest, 
1995).

It is generally thought that male mimicry targets the females that are hunting.   Perhaps females alternately 
oviposit and hunt over several days, and mate with several males during that time.  It is possible that 
mimicking may indicate to the females a desirable trait (Lloyd,1980). 

But, being a male mimic is not without danger.  Sometimes a Photuris male will elicit a response form a 
Photuris female of another species, and if that female also eats other Photuris than he's in trouble, 
because, in general, males are smaller that females (Lloyd, 1990).  Sometimes, a female of the mimicked 
species will respond, but if she does not change her response with his pulse change, then she is probably 
treated like a Photuris female who refuses to be seduced (Lloyd, 1980).

Most people think that this is the only context for male mimicry, however, Photuris males have been 
known to eat fireflies they are confined with, and have been observed answering flashes of Photinus males 
and penlight simulations as a female would (Lloyd, 1984c).  Also, they will wait in ambush near signaling 
Photinus females to attack responding males, emit false signals while approaching males to attack in the 
air, as well as attack the attracted males to their own false, female signals (Lloyd, 1984a).

	Novel Defense Behaviors

Flash signaling already has pressure from being a reproductive isolation mechanism.   The signals must be 
clear, concise, accurate, and species-specific.  However, predation, especially aggressive mimicry, has 
driven this courtship communication to new heights.

Some species shift their mating to times or habitats that have less predation pressures.  One species has an 
unusual mating behavior because they choose nights when the temperature is less that 54°c, when most 
fireflies are not active ( Lloyd, 1966).  Others are active later at night, after Photuris is finished with their 
display.  Others, still, have abandoned nocturnal activity entirely, opting for phermones used during the 
day (Lloyd, 1984a).  Photinus macdermotti males will approach an answering light but land a good 
distance away, and will walk to where the female waits, in order to avoid predation.  

Some species' counter-measures are in the flash signals themselves, either adopting a particular flying 
pattern while flashing, or using the off transient of the signal with respect to the timing, delay, of the 
female's response.   Varying the timing of response also helps detecting predaceous females, sometimes 
males can omit certain aspects of his flash to try and confuse a potential predator.  If the female flashes the 
correct response, she is of the correct species, a predator would either follow his pattern or not respond at 
all (Lloyd, 1984b).  

The only species of firefly in the New World to use aggregative mating system is Photinus carolinus.  This 
is probably due to the lack of Photuris species within their habitat.  But in general males of all species, 
Photuris included, must be extremely cautious and only answer the most accurate and clear of female 
signals.

	Predatory Alternatives to Aggressive Mimicry

Aggressive mimicry is not as efficient as it once was, with the development of counter-measures and better 
discrimination in their prey.  In one study, 119 male Photinus where tracked and observed in the field.  
None approached the more abundant flashing Photuris females; yet, two conspecific females where 
quickly responded to and mated with (Vencl, 1994).  Therefore, Photuris has had to develop their own 
rebuttal to these countermeasures.

 	Both males and females are known to orient on the flash of a male firefly in an aerial attack, this is 
called "hawking" (Lloyd, 1983).   Many times this is used in conjunction with aggressive mimicry, 
drawing the males in then attacking them in the air before they wise up (Lloyd, 1984a).  Photuris has also 
been observed hovering over both conspecific and heterospecific females in the "luring mode" to catch any 
responding males (Lloyd, 1983).  This seems to be antithetic to the evolution of long flash signals.  In fact 
there is evidence that prey species increasingly favor males that flash less and use double flashes more ( 
Lloyd, 1984a).

	Old World and Atypical New World Fireflies

Old World and New World fireflies are generally as different from each other as night and day.  Because 
of predation, which is the dominant pressure on New World fireflies (Lloyd, 1984a, 1990), many species 
have developed more cryptic signals and behaviors to avoid being eaten.  Old World fireflies do not have 
such pressures on them and have developed their flash signals along a different path.

In the swarming males of Old World species, other courtship behaviors have evolved.  Some species have 
developed clamps to hold onto females (Lloyd, 1990), others use chemical signals in addition to flashes 
for close range courtship and reproductive isolation, which is impossible in the new world if at close range 
you could get eaten.  Both females and males of Old World species seem to be attracted to swarms emitting 
synchronous luminescence.  Males of the Pteroptyx species will aggregate in mangrove trees, which are 
also called "fireflies trees".  Some species, for example Mulaccae, are generally stationary on these trees, 
while others, like Valida, make looping flight out of the trees and are perhaps actively looking for females 
( Lloyd, 1989).

There is only one species of New World firefly that exhibits synchrony, in an annually observed 
summertime phenomenon in the Great Smoky Mountain National Park, called the "light show" (Copeland, 
1995).  It is thought that this is due to a reduction of Photuris predation in this habitat.  It is aggressive 
mimicry that more in likely "ate up" the synchronous swarms of flashing males in early America and "gave 
Photuris the taste for fireflies in the first place" (Lloyd, 1984a).

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