by Terry Lynch
Impressions and considerations with reference to the role of nitric oxide (NO) in firefly flash and the implication this has with respect to the timed delay between flashes of species and other related flash behavior. June 29, 2001, 16:40hrs.
The following primary resources were consulted and are available for reading:
To view these movies, download a QuickTime viewer. This QuickTime 4.0 digital movie runs 55 seconds and documents the response of a Photuris sp. firefly to NO gas. The first and second scenes show firefly bioluminescent courtship behavior in a natural setting. The third scene shows a firefly housed in the custom-built plexiglass gas chamber. Firefly bioluminescent response to absence and presence of NO gas (70 ppm) is shown in the fourth scene. Typical firefly flashing/glowing responses to NO are documented in this video; these observations were replicated 27 times in 13 different fireflies with equivalent results.
The following reviews and reports related to this firefly flash process research are available on-line:
Researchers Solve Firefly Mystery - AP (Jun 28, 2001)
Scientists no longer in the dark on fireflies - Chicago Tribune (Jun 29, 2001)
Scientists find switch for lights in fireflies - Boston Globe (Jun 29, 2001)
Secret formula for a firefly’s glow - MSNBC (Jun 28, 2001)
Secret to firefly light? Natural Viagra - CNN (Jun 28, 2001)
The Trimmer team presents a model of the biochemical flash control mechanism in fireflies. This is a well illustrated report and basically explains how the release of NO enables flashing by turning off the action of ATP to use oxygen in the luceriferin/luciferase reaction. Prior to this report the role of NO as it relates to light production in fireflies was not known. The Trimmer team gives an excellent model of this action on their website which I quote from:
Model for NO control of the firefly lantern flash. In quiescent mode (above dotted line), oxygen delivered through lantern tracheae is consumed by respiration in photocyte mitochondria (green) clustered in the peripheral cytoplasm: little oxygen reaches peroxisomes that contain the light-producing reactions of the luciferin-luciferase pathway. In this quiescent state, ATP produced by oxidative phosphorylation promotes formation and accumulation of the activated luciferin-adenylyl intermediate (denoted as luciferin*) by luciferase. In flash mode (below dotted line), nerve activity causes octopamine release that transiently activates lantern NOS. NO diffuses rapidly and inhibits oxygen use by photocyte mitochondria (red). Now oxygen delivered by the tracheal system diffuses through photocytes to the peroxisomes where it triggers the light-producing reaction.
Although this model provides insight into how a nerve impulse results in a flash of light, it leaves unresolved key questions relating to firefly behavior (see also Firefly Mysteries:
The later question relates to an observation I made many years ago while studying Photinus pyralis in the field (1968-1970). I observed a large hunting spider capture a firefly, then recorded the firefly's change in flash. Given that spider venom is a neural toxin, it acts to cause rapid firing of nerve impulses. Firefly flash rate increased, then gradually subsided resulting in continuous lantern glow. Yet the firefly still maintained a delay between flashes.
If the nitric oxide switch was turned "on" and left on by neural toxins, what caused the delay between flashes? I suggest that what this may involve is a chain or length of molecules, all of which must be reactivated after a flash before a new flash may occur. This is analogous to having a string of lights in series circuit; when one light bulb is removed the whole chain goes out and will not light until a new bulb is replaced.
In the case of fireflies, only when the last link in biomolecular chain is completed may a second flash occur. Thus nitric oxide is what acts to turn the switch by stopping ATP to PPi conversion and enabling the luciferyl.AMP to oxyluciferin + AMP reaction which uses up the oxygen made available when the ATP to PPi reaction was halted. But a timed delay is in place and even with nitric oxide continuously present, a second flash can not occur until the "switch" is returned to its original state. It is as if the excited luciferin* molecules are all linked in a chain or sequence, and this sequence is broken when a flash occurs, as it results in the activated luciferin-adenylyl intermediate (excited luciferin or luciferin*) converting back to unexcited luciferin.
It is then the reactivation of luciferin, a process which involves a linking of a chain of molecules or sites upon a chain or matrix, which is the delay factor in firefly flashes. Only when all the luciferin molecules in a chain or matrix are reactivated can a second firing of the lantern occur. As there are a set number of luciferin molecules or chain linked sites, and each requires a set amount of time to reform or link in sequence, then there is a delay built in by the time required for this to occur. As this delay is tied in with the lantern design and structure, it is species specific. This then is a clue to how delay between flashes varies in fireflies from one species to another. Structure of the lanterns on a biomolecular level varies from species to species. It is this structural variation that plays a key role in the period of the delay between flashes which is unique for each species of firefly.
Clearly understanding how a firefly's lantern works to produce light does not answer all the questions related to how a firefly's brain and nervous systems; i.e., how the firefly's mind works. It is understood that all behavior involves a chain or sequence of biochemical processes, and insight into those processes gives important keys or clues to the proper functioning of those behaviors under investigation. But we must endeavor to also understand how behavior is tied into the genes; how the brain of fireflies works; how behavior is genetically encoded. When this is done and the understanding of how biochemical keys and switches operate is combined with an insight into how processes such as memory work, or how behavior is genetically encoded; i.e., the relationship of structure and function to behavior on a biomolecular level, the result may enable cure or treatment for various dysfunctions or disorders, such as mental retardation, which presently have no cure.
The manner in which nitric oxide serves as a messenger to enable neural response promises to have far ranging consequences which may be of significant benefit to humanity. Hence those who have previously considered basic research into insect behavior a waste of time, resources and money, may want to reconsider the enlightenment which comes through pondering over the mysteries of the firefly and its nightly flash and mating dance.
That nitric oxide may play a role in enabling a firefly's lantern to flash fails to explain such baffling questions as how do fireflies blink in synchrony, what enables different species to have unique flash patterns and how are these behaviors genetically controlled? These behavorial questions are every bit as interesting those related to the role of NO and biochemistry in the triggering of firefly flash. It is likely that how an adult male and female firefly control the timing of their lantern flashes is not a matter of what goes on in their "tales" but what goes on in their heads! I suspect that if one were able to transplant the lantern of one variety of firefly to another variety of firefly, say to put the lantern of Photinus pyralis upon Photuris sp. that Photuris would not then exhibit the complex flight and flashing pattern of P. pyralis; rather, Photuris would flash the lantern of P. pyralis as though it were a Photuris lantern. This is because the computer which controls the complex nature of a firefly's courtship is not inside the lantern, but inside the firefly's brain.
This imagined experiment of firefly lantern transplant may not be all that fanciful. It may be possible to actually do such a transplant, using adult fireflies or firefly larvae.
Often when talking about the function of a firefly's lantern, larvae are not mentioned. This is because most people encounter only adult fireflies. Those few researchers who have studied firefly larvae usually observed only large specimens. But the fact is firefly eggs glow as soon as they are deposited and firefly larvae glow inside these tiny eggs, with glow becoming responsive to vibration prior to the emergence of young larvae. Then when firefly larvae first emerge from their eggs, their lanterns and internal structure are quite visible. Indeed these factors make firefly larvae an excellent subject for study, especially when trying to figure out fundamentals of lantern glow which is more basic in a firefly larvae than in an adult firefly.
The problem is that most researchers do not know how to collect firefly larvae or rare firefly larvae from eggs. For this reason I have published my early studies of fireflies which may aid other researchers in this area.
In pondering the mysteries of firefly behavior it is easy to make assumptions and conjectures; it is much more difficult to make the keen observations and experiments necessary to establish truth and separate wild fantasy from the reality of what is actually going on when one observes firefly courtship, synchronous flash displays or other complex sets of behavior. This is equally true with respect to the study of other insects and their behavior. Yet still discovery begins with asking questions which may be resolved through observation, experiment and research.
The Trimmer team study related to the flash trigger mechanism in adult Photuris sp. is an excellent example of how a collaborative effort yields new insight into great mysteries. If others would learn to cooperate in a similar fashion, rather than compete for fame and fortune, the benefits to humanity would be enormous! For this I commend the Trimmer team and challenge other scientists around the world to follow in thier wake!
Contact Terry Lynch
Project K9 | Blinks and Links | Bioluminescence in Fireflies: The luciferase-luciferin reaction in Photinus pyralis | Part I: Application of Torque to Induce Simultaneous Flight Response and Synchrony in Drosophila | The Amateur Naturalist | Firefly Notebooks | Feeding Behavior of Photinid larvae | Firefly Mysteries | Contact the author