Fireflies, or lightning bugs, undergo complete metamorphosis, and their diet changes dramatically depending on the life stage. The juvenile (larval) stage is dedicated to voracious eating, while many adults focus solely on reproduction during their brief lifespan.
The Diet of Adult Fireflies
Many species of adult fireflies, known as the imago stage, have a very short existence focused primarily on mating, and some do not eat at all. These non-feeding adults subsist entirely on energy reserves stored up as larvae. For species that do feed, their diet is typically liquid, consisting of nectar and pollen gathered from flowers, or simply water and dew.
Some adult fireflies are carnivorous, exhibiting a predatory exception. Females of the genus Photuris are known as “femmes fatales.” These predators mimic the flash patterns of females from other firefly species, such as Photinus, to lure in unsuspecting males. When the attracted male arrives expecting to mate, the Photuris female captures and eats him.
This predation allows the Photuris female to acquire defensive chemicals called lucibufagins from her prey. The consumed male fireflies contain these toxins, which she then uses for her own defense against predators.
The Predatory Life of Firefly Larvae
The firefly’s larval stage, which can last from several weeks to two or more years, is the primary period for growth and feeding. These larvae, sometimes called glowworms, are true carnivores and voracious predators. Their diet is composed of soft-bodied invertebrates found on or in the soil, such as snails, slugs, earthworms, and other insect larvae.
To consume prey, firefly larvae employ a specialized and highly effective method. The larva first injects its victim with a paralyzing chemical, which quickly disables the soft-bodied creature. Following paralysis, the larva injects digestive enzymes that liquefy the prey’s internal tissues. This process of external digestion allows the larva to simply suck up the resulting nutrient-rich liquid. Some species of larvae have specialized grooved mandibles to assist in delivering these digestive fluids directly to their prey.
Providing Sustenance for Temporary Observation
When people capture fireflies for temporary observation, they are dealing with the short-lived adult stage. To keep these adults healthy for a day or two, the focus should be on moisture and simple liquid sustenance. A damp paper towel or a moist, unbleached coffee filter placed in the container helps maintain the humidity these soft-bodied beetles need to avoid drying out.
For a food source, a small amount of sugar water or a slice of fresh fruit, such as an apple, can provide a nectar substitute. The fireflies will drink the juice from the fruit or the sugar solution, which mimics the nectar they might seek in the wild. Given the naturally short lifespan of adult fireflies, they should be released back into their environment within 24 to 48 hours to allow them to complete their mating cycle.