It is the stuff of fairytales: larvae that flicker blue lodged between fallen branches surrounded by a cushion of their own secreted mucus – okay, perhaps it’s more Brothers Grimm than Disney. 

The scientists weren’t actually on the hunt for a new species. Instead, wearing red light headlamps, they were scouring the Atlantic Forest of Brazil for bioluminescent mushrooms. The light-emitting gnat larvae, then, were quite the surprise, especially as it’s the first discovery of a blue bioluminescent species in South America.

Insects and fungi do brighten the forest night with little displays of light, but usually these glowing emissions come in one of three colors: green, yellow, or red.

"It’s the first record of blue emission from a living being on land in South America. It’s also the first report of a bioluminescent Diptera in the same location," study author Cassius Stevani, from the University of São Paulo's Institute of Chemistry, told IFLScience.

"We've been conducting scientific work on bioluminescent fungi in the same region for almost 20 years. My group is responsible for almost 20 percent of all species of bioluminescent fungi of the world. We found 15 out of 102 species in this location."

The translucent larvae of the fungus gnats, officially named Neoceroplatus betaryiensis, were collected from fallen trees during a particularly hot and rainy period, with a relative humidity of 90 percent. When touched, the larvae cease to luminesce from their tail and two spots near their eyes, only glowing again when they no longer feel agitated from the presence of a possible predator. The adult fungus gnats (Keroplatidae) do not emit light.

"The larvae are very active, especially at night and can move constantly whilst completely covered by mucus. When disturbed, they quickly move under their mucus," write the researchers in the study published in Scientific Reports.

There was, however, something off-kilter about one of their specimens, collected from the underside of a fallen leaf. This larvae glowed all over its body and exhibited what the team called “bizarre behavior”, moving slower and hiding less than its peers. The team scooped it up and took it to their laboratory’s terrarium to watch it pupate, essentially when the larva emerges and becomes a full-grown gnat. Weeks later, a fungus gnat did not emerge from the pupa. Instead, a parasitic wasp did. 

The diffuse light found in this critter "could be the result of either a defensive reaction against the parasite or the consequence of internal organ damage spreading the photogenic material along the body of the larva. However, it may also belong to another new species that can emit light along the whole body as observed in Keroplatus nipponicus."

Insects that glow blue are rare in nature, with such bioluminescence usually reserved for other colors or creatures like algae, sea stars, and fish. The discovery opens up a possible new biological avenue for bioluminescence, which itself could have far-reaching applications in biotechnology and gene markers.

The team found that the larvae contain a luciferin-storing protein known as SBF. In the future, the team will isolate the luciferin, clone luciferase from Neoceroplatus, and use imaging techniques to determine its structure.

Such discoveries are dependent on a healthy ecosystem, note the team, so they recommend staying alert of environmental pressures in the region.

"The Atlantic Forest in the South region of São Paulo State is carefully monitored and preserved," said Stevani. "Neoceroplatus betaryiensis was found in the borders of a State Park. However, the region has some issues with economic and social development as farms and industries are not allowed in the region for logical reasons. Locals must learn how to 'use' the forest in a sustainable way to live there. There is some pressure to burn some parts of the forest for farming and cattle breeding. Education, scientific activity, and discoveries like the one we did can help the population to see the forest and its preservation with care."