Photinus pyralis

Photinus pyralis, also known by the common names, the common eastern firefly, or big dipper firefly and sometimes called a "lightning bug", is a species of flying beetle. An organ on its abdomen is responsible for its light production. It is the most common species of firefly in North America, and is typically found east of the Rocky Mountains. Photinus fireflies are often confused with fireflies of the similar-sounding genus, Photuris, which are also found in North America. These fireflies are most noticeable around twilight in the spring and summer months. The common name, big dipper firefly, is due to the characteristic flight of the males, whose trajectory appears to follow a J-shape, lighting on the upswing. During flight, this J-pattern is used alongside light flashing to attract females, who rest on vegetation and signal back to males if interested. The firefly flashes are stimulated by light-activated proteins, not rhythmic impulses as originally thought.

Description
Common Eastern fireflies are small soft-bodied beetles that are 9 - 19 mm long. They have flat black or brown bodies with wing coverings, or elytra, that extend the length of their entire thorax and abdomen. The elytra have a yellow border. The head shield, or pronotum, is pale yellow, with a black dot in the center, surrounded by pink or red.

Both males and females of this species have light-emitting organs on the ventral side of their abdomens. While this organ extends the length of the last three segments of the males' abdomens, it appears only in the second to last segment of the females' abdomens.

Like most flying firefly species, Photinus pyralis is nocturnal.

Geographic range
P. pyralisis widespread across the eastern half of North America.

Habitat
Common Eastern fireflies are found in a variety of habitats, ranging from temperate to tropical environments. Some natural habitats of these fireflies include meadows, fields, wetlands, desert canyons, and dense forests, and while they can successfully thrive in all of these areas, they require moisture to survive at all life stages. P. pyralis also tend to form microhabitats using a variety of organic material such as leaf litter and rotting vegetation so they can better signal to other fireflies, establish hunting territories, and secure shelter to lay their eggs and survive through winter months. They are especially drawn to areas with damp soil and long grasses to better conceal females and offspring.

Home range and territoriality
Population studies have been done to investigate whether P. pyralis has always been endemic to the northeast regions of North America, especially because the species continues to occupy a wide variety of habitats across the continent. It has been discovered that ancestrally, the North American population originated in Texas during the Miocene epoch before migrating to the central states of the US and more recently, the Northeast coast. Through the tracking of single nucleotide polymorphisms (SNPs) along with other genetic markers, researchers have concluded that the common Eastern firefly possibly migrated during interglacial periods, which researchers suggest could account for the divergence to other Photinus species.

Life cycle
Like all beetles, the entire life cycle of a common Eastern firefly consists of four unique life stages starting with the eggs, moving to the larval stage, followed by a pupa, before finally completing the cycle as a fully formed adult.

The time it takes for these fireflies to complete all four stages can vary drastically, especially because they inhabit such a wide range of environments that the resources available to them vary significantly. However, all fireflies will spend most of their time in their feeding stage as larvae with most taking approximately two years before they begin to pupate. During this time, they will experience multiple instars, or molting periods. Because all fireflies are very vulnerable on the ground during the larval stage, they have evolved to produce light as an aposematic signaling mechanism to drive away predators, regardless of whether or not they continue to be chemiluminescent in adulthood like the common Eastern firefly.

At the end of the larval stage, P. pyralis  will descend into the microhabitats this species tend to build, whether that be slightly above damp soil, on rotting vegetation, or in the small burrows before shedding their exoskeleton and becoming pupae. A few weeks following the pupa stage, the P. pyralis will grow into a fully formed adult that is capable of flight. Upon emergence as an adult, the common Eastern firefly will live for approximately another three weeks to two months, during which time it will continue its light-signaling so as to attract a mate.

Both adult and larval Phorinua pyralis are carnivorous. They feed on other insects (including other fireflies), earthworms, and snails. When feeding, they inject poison to immobilize and liquefy their prey.

Senescence
As P. pyralis ages, it suffers the effects of a weakened immune system. Age generally changes the ability to fight infections. However, aging uniquely affects different populations of P. pyralis differently, and they seem to age slightly different under various conditions. This is seemingly attributed to pathogens in the habitat, and how the individual populations of P. pyralis have evolved in correlation. The older the common eastern firefly was, the more likely it was to be infected with a pathogen or parasite.

Defense
Although their conservation status is classified as "Least Concern" by the IUCN Red List, these fireflies do face some dangers. Aside from natural predators, the biggest threats to their populations include light pollution, pesticide use, climate change, and human building and development in their habitats.

Photinus pyralis contain steroid compounds called "lucibufagins" which serve as one of their many defense strategies as they can repel spiders and other insects. However, the Photuris fireflies, with whom the common Eastern fireflies are regularly confused, actually use the presence of these lucibufagins to prey on the P. pyralis. Male common Eastern fireflies can be lured by female Photuris fireflies through light signal mimicry, and are eaten when they get close enough. Through this, Photuris can acquire the lucibufagins for themselves, demonstrating that this spider-repellant, while a defense mechanism for the most part, can also serve as an attractant to some predators. Beetles from the family Lampyridae have been known to use certain defenses such as unpleasant odour and the excretion of a sticky substance to avoid predation. Excretion of unpleasant fluids from the areas along the elytra and pronotum is the result of tactile stimulation and has been referred to as reflexive bleeding. This reflex bleeding is a defensive function of P. pyralis, as it can cause certain predators to become entangled in the sticky substance (such as ants) or cause revulsion in others upon predation. The excretion contains lucibufagins, steroids found in P. pyralis that render them distasteful to certain bird predators. Whereas adult flashing is used in mate signaling, pupae glow is thought to be an aposematic display for nocturnal predators.

In relation, males of the Photinus species are the prey for females of a different genus, Photuris. Photuris females actually mimic the effects of the Photinus males light-signaling patterns, and by doing this the females lure in the Photinus males. The males naturally produce the steroid lucibufagin, and the reason that the females prey on these males is to obtain this steroid. Once the females prey on the Photinus males, the females gain the steroid lucibufagin to use to their defense against jumping spiders. A study was performed where the Photuris females were collected from nature and forced to reflex bleed which contains the steroid lucibufagin. It was found that when the females were forced to reflex bleed, the samples taken from each female had different amounts of the steroid in each sample. So after experiments were brought out to see which females the jumping spiders would eat it was decided that the jumping spiders were more likely to eat the females with less lucibufagin inside their bodies and the females with more were constantly rejected by the spiders therefore protecting themselves from predation.

Mate searching behavior
Males are the first to start the series of patrolling flashes needed to locate and mate with a female. Males will actively fly while flashing, whereas females are sedentary. They will flash every 6 seconds and wait for a responding flash from the female, which comes after a 1-2 second delay It has been shown that females only respond to their conspecific males; identifying them by the color of their yellow bioluminescent flash, in combination with the temporal patterning, duration and intensity of the male flash. Females will twist their abdomen towards the males flash, presenting their own flash toward the male. Males can be observed flying in a nearly vertical orientation; their antennae held forward and stiff while their legs are held toward the body during patrolling. They also show an obvious gaze shift towards the last female flash, and continue towards it until the female firefly flashes again. The flashes continue until the male reaches the female. Males congregate in large masses and it is most likely that more than one will find the same female; in this case male P. pyralis display aggression towards one another while not in flight.

Male/male interactions and sexual selection
During the "aggression" stage, males with smaller elytra and smaller lanterns are favored; whereas during the signaling phase, males with longer elytra and bigger lanterns are favoured. Males with larger lanterns are favored in signaling phases of courtship because their broadcasting flashes can be seen by females who are further away, it is also suggested that due to their longer elytra these males may also have an advantage of finding the females faster. Photinus fireflies do not feed as adults and therefore males are better able to attract females by offering nuptial food gifts, in the form of spermatophores which females can use to provide nutrients to their eggs.

Light production
Among fireflies of the genus Photinus, males will first attract a female's  attention with their light flashing patterns, but once they reach the females they will be selected based on the size and quality of the "nuptial gifts" they can offer. These nuptial gifts, also known as spermatophores, are a combination of sperm and high levels of protein, hormones, defensive compounds, and other nutrients which the females will subsequently use to feed the eggs should she choose to mate with the male. During the adult stage of their lives, Photinus fireflies do not eat, so all activity is fueled by energy that is consumed and subsequently stored during the larval stage; this becomes especially important regarding reproductive activity. Consequently, these nuptial gifts are a big factor in determining which male fireflies females choose to mate with; although it ends up being a high cost to males, those with larger spermatophores tend to see increased reproductive success because they are able to provide females with more nutrients to sustain future offspring. The light organ of P. pyralis is composed of two layers; a layer of refractile cells on the dorsal side and a photic layer with light-producing cells on the ventral side. The light organ (specifically the photogenic layer) is supplied with numerous tracheal branches, which are thought to provide the required oxygen for light production. The light-producing enzyme is luciferase, and is found within cells of the lantern. Luciferases require oxygen, luciferin and adenosine triphosphate (ATP) to catalyze a chemical reaction that produces bioluminescence in these insects. It has been shown that the glow is not controlled by the tracheal end cells (which were thought to contain valves) nor by central nerve impulses through studies involving low oxygen conditions. Pupae of these beetles have different light organs than the adult. They do not have the characteristic tracheal end cells of the flashing adults, and whereas the adults emit bright flashes, pupae emit low intensity glowing.

Polyandry
The female common Eastern firefly is polyandrous, meaning it will mate with multiple males over multiple nights albeit it will only mate with a single male in one night. This has been shown to increase female fecundity as well as overall lifespan.

Extensive research has been done regarding female P. pyralis preference of larger spermatophores, especially because females will prioritize spermatophore size to pick a mate out of a large group of males whose light patterns she finds attractive. It was found that females were more likely to mate with virgin males who had never mated because their nuptial gift sizes were larger in comparison to males who had mated the previous night. Because common Eastern fireflies do not eat in adulthood, spermatophore size decreases with each mating because males have less and less resources to put in each one after each successive mating. This is still an active area of research because females do not have an actual way to determine which spermatophores are larger than others because they are created and transferred during copulation.

Biotechnology research
Since discovering that P. pyralis glows through bioluminescence, much research has been done to both figure out the mechanism by which the fireflies can emit light as well as how that tool can be used advantageously in biotechnological contexts. In 2018, when the Photinus pyralis genome was sequenced, the gene that produced luciferase, the light-producing enzyme, was discovered and scientists were able to clone it. Since then, luciferase has become a tool for many different research strategies.

The first use of luciferase was as a reporting marker in many high throughput assays; because it is known that luciferase is activated by oxygen, luciferin, and ATP, the assays were specifically pertaining to reduction-oxidation reactions that occurred in various organisms. It is a highly sensitive marker and is very easy and efficient to use, so it is very widely used among scientists. Recent studies have shown that the luciferase protein has been found specifically in peroxisomes of many eukaryotes, the organelles responsible for carrying out oxidative reactions and producing hydrogen peroxide as a byproduct that is quickly removed. More research is being done about how this information can be used for further advancement in the field of molecular and cell biology.