Glowworm

Quick Take

• A glowworm isn't actually a worm, and for most species, it isn't even what you think it is. Unpack the naming confusion →
• Some glowworms use their light not to find a mate but to set a deadly trap, and the mechanism behind it is disturbingly clever. See the deadly trap mechanism →
• The female European glowworm's glow signals something specific to males, and brighter is not simply prettier. Discover what the glow signals →
• Most glowworm species barely eat as adults, though what they do as larvae is surprisingly gruesome. Explore their gruesome diet →

Many people appreciate the glowworm’s strange ethereal beauty, but there are many misconceptions and much confusion about what this insect actually is. For a start, the name itself is a significant misnomer. It is not really a worm at all. Instead, “glowworm” is a catch-all term that describes a larva (which resembles a worm) with a glowing body. Adding to the confusion, several species of insects, including both larval and adult stages, are called glowworms as well.

The ability to produce light through a chemical reaction within a living organism is called bioluminescence. The glow is typically used to attract a mate, capture prey, or warn off predators. If you’ve spotted a glowing insect, then it’s almost certainly a species of beetle (though some gnats can produce light as well). This article will discuss many different types of glowworms, including both the larval and adult forms of some species.

3 Incredible Glowworm Facts

• In some species, only the female is capable of producing a bioluminescent light. This usually indicates it is used for reproductive purposes only.
• One species of fungus gnats that lives in the wet caves of New Zealand have the remarkable ability to produce glowing tubes of mucus in their larval form. These mucous tubes, measuring up to about 16 inches long, hang from the ceiling of the caves and contain tiny silk threads, only about an inch long, produced from the larva’s mouth. When other insects are attracted to the glowing light of the tube, they become trapped in the silk threads and become a tasty meal for the larvae. Adult fungus gnats, which survive for only a few days, also produce light, but it’s not clear what purpose this serves. It might simply be a leftover feature of the larval stage.
• The organs responsible for producing light vary so much in size, location, and structure that they likely had separate evolutionary origins.

Scientific Name and Evolution

As mentioned previously, “glowworm” is a somewhat confusing catch-all term. While several species of insects and/or larvae are called glowworms, it’s not really a scientific term and doesn’t neatly correspond to any taxonomic classification. “Glowworm” can refer to many different species, some of them only loosely or not at all related to each other. The only commonality between them is their bioluminescent ability.

Types and Evolution

Bioluminescence in animals, including marine animals, evolved in different ways and for different purposes. Some illumination functions serve to attract mates, while others help capture prey.

Glowworms generally fall into two broad categories: they are either beetles or they’re fungus gnats. The glowing insects most people are familiar with, including the fireflies, true glowworms, and lightning bugs, are part of the beetle family called Lampyridae (though in fireflies, it’s mostly the larvae that are called glowworms). Another closely related family called Phengodidae also contains many species called glowworm beetles. The fungus gnats, by contrast, are completely different from the beetles. They’re more closely related to flies within the order of Diptera. From an evolutionary perspective, their bioluminescent adaptation evolved separately from the beetles.

Glowworm Beetles

• Click Beetles: The Family Elateridae beetles are able to produce a click from a spine on their body, which actually launches them into the air. There are about 10,000 different species of these beetles.
• Glowworm Beetles: The Family Phengodidae beetles are usually known as glowworm beetles. males are much smaller than the females, are believed not to feed, and are shorter-lived. The females, however, prey upon other insects found in soil.
• True Fireflies: The Family Lampyridae beetles include a couple of thousand different species, including fireflies, lightning bugs, and other glowworms that illuminate usually near twilight.
• Glowworms in the Family Rhagophthalmidae: The Family Rhagophthalmidae beetles are still being debated as far as their classification is concerned. It was previously assumed to be under the Family Phengodidae.

Glowworm Fungus Gnats

These glowworms have a entirely different approach to using illumination to capture food. They are able to produce sticky webs and lower them from cave roofs. The fungus gnats’ illumination capability is believed to have evolved separately in each of the three groups.

• Keroplatus: This genus is found in Eurasia. They feed solely on fungus spores.
• Arachnocampa: This genus comprises nine species and is located in Australia and New Zealand.
• Orfelia: This cosmopolitan genus also feeds on fungus spores and comprises approximately 46 described species found across multiple continents, including one bioluminescent species, Orfelia fultoni, found in North America.

Appearance: How to Identify

These insects vary dramatically in size and appearance. Each type has its own unusual quirks and distinctive traits. The bioluminescent glow is the common feature among them. This light can be emitted as a series of flashes or as a constant glow, and it most commonly ranges in color between blue, green, and yellow. Bioluminescence is produced by a chemical reaction involving a light-emitting pigment. The nature of this chemical reaction varies from one group to another.

As mentioned previously, glowworm usually refers to the larval stage of a glowing insect. But there are entire species that have acquired the name of glowworm as well. The common European glowworm (a species of long, narrow beetles within the firefly family of Lampyridae) is one of the more well-known examples.

Unlike the closely related fireflies, only the female of this species is capable of producing light, at least after reaching her adult form. She is larviform, meaning she has retained many larval characteristics into adulthood and therefore lacks wings. When the mating season arrives around June and July, she will climb up on a plant and produce a bright glow continuously to attract one of the winged males. His large, photosensitive eyes can see the light from up to 50 yards away. The brightness is itself a sign of the number of eggs she’s carrying. Therefore, the male glowworms, which only measure about half her size, will try to mate with the brightest females.

Habitat: Where to Find Them

These insects are found in many different environments all over the world. Depending on the species, their habitat includes forests, grasslands, meadows, and even gardens. By contrast, the larval glowworms of the fungus gnats are mostly found in caves or rock cavities.

Diet

Most of these insects are actually carnivorous. Even the species that primarily use their bioluminescence for reproductive purposes tend to eat meat.

What eats glowworms?

These insects provide a stable source of food for many different birds, mammals, reptiles, amphibians, spiders, and other insects. There are almost too many predators to even name.

What do glowworms eat?

Many of these insects tend to be predatory, and their means of feeding can be quite gruesome. Fungus gnats, as mentioned previously, use their bioluminescence to trap their prey in silky threads. Some also live up to their names by consuming fungi.

The larva of the common European glowworm, by contrast, has the ability to inject a toxic digestive fluid into its prey (usually slugs and snails). This fluid will gradually paralyze and liquefy its victim, so the larva can easily lap up the remains without a struggle. Because the prey is often many times larger than the larva itself, it provides quite a substantial meal.

The larval beetle needs to be a voracious eater because once it reaches adulthood, it lives on fat reserves and barely eats at all. Once it reproduces, it has reached the end of its life cycle.