Fireflies Light up Caves and Termite Mounds in Amazon Rainforest to Attract Prey

Researchers conduct world's first scientific investigation of light-emitting beetle larvae in clay caves in Pará State, Brazil, and report occurrence of luminous termite mounds in the Amazon (photo: Wikimedia Commons)

In parts of the Cerrado biome, such as Parque Nacional das Emas in Goiás State, Brazil, warm wet spring evenings afford an opportunity to observe termite mounds shining bright green.

The ghostly light is emitted by larvae of the click beetle Pyrearinus termitilluminans as they expose their luminescent thoraxes while on the surface of the mound to attract the flying insects on which they feed.

Researchers from the Sorocaba campus of the Federal University of São Carlos (UFSCar) in São Paulo State have observed the same phenomenon of luminous termite mounds deep inside the Amazon Rainforest.

They have also discovered luminescent click beetle larvae on the walls of clay caves in the Amazon region. In this case the species is P. pumilus, but the luminescence is similar and also serves to attract prey.

The discoveries were made as parts of projects supported by FAPESP and are described in an article published by Annals of the Entomological Society of America.

“Bioluminescence, the production and emission of light by living organisms, has hitherto been reported in caves only in New Zealand and Australia, where the larvae of a group of fungus gnat species spin threads of silk from the roof to catch prey,” said Vadim Viviani, a professor at UFSCar and principal investigator for the study, in an interview with Agência FAPESP. “This is the first time the occurrence of luminescent larvae of Pyrearinus sp. living inside caves has been scientifically investigated anywhere in the world.”

Viviani was elected president of the International Society for Bioluminescence & Chemiluminescence (ISBC) at its 2016 annual meeting, held on May 29-June 2 in Tsukuba, Japan.

There have been occasional anecdotal reports of luminous termite mounds in the Amazon and other parts of South America, Viviani said, but none has previously been investigated.

Since 2009, Viviani and his research group have made several expeditions to the Amazon region with the aim of determining whether these accounts could be substantiated. They began in a transition zone between savanna and rainforest along the Araguaia River as far as the north of Tocantins State.

Next, they explored the Juruena River in northwestern Mato Grosso before finally traveling to Canaã dos Carajás in Pará State to verify information supplied by Cleide Costa, a researcher at the University of São Paulo’s Zoology Museum (MZ-USP), indicating that geologists had reported seeing luminescent larvae in clay caves in Carajás National Forest.

These expeditions brought the researchers into contact with luminous termite mounds inhabited by larvae of P. fragilis and P. termitilluminans in three different parts of the Amazon Rainforest (Caseara, Tocantins; Canaã dos Carajás, Pará; and Juruena, Mato Grosso) and in the Cerrado (Novo Santo Antônio, Mato Grosso).

They also confirmed the presence of luminescent larvae of the species P. pumilus in Carajás National Forest in laterite clay caves embedded in an iron-rich surface crust known to Brazilian scientists as canga.

“We believe these larvae very probably emit light to attract the flying insects on which they prey, as do the luminescent larvae found on termite mounds in the Cerrado and flies of the species Arachnocampa luminosa that inhabit caves in New Zealand,” Viviani said.

Food sources

There is no definitive explanation for the evolutionary origin of these insects’ colonization of termite mounds and caves, according to Viviani. All researchers know for sure is that hundreds of eggs laid at the bases of the mounds and on cave floors and walls hatch into luminescent larvae.

The most likely explanation is that, as exemplified by the larvae of other carnivorous species of the genus Pyrearinus that are normally found on decomposing logs or in soil, this association with termite mounds was an advantageous adaptation because these sites are rich in termites and the other small insects on which they feed.

“Our hypothesis is that these click beetle larvae were associated early on with sites containing decaying logs and hence termites. Later on, they adapted to life inside the forest, where they found termite mounds to be a suitable habitat,” Viviani said.

“The replacement of forest by savanna-like Cerrado due to climate change may have resulted in their adaptation to open environments, where they developed a more intense bioluminescence capacity to attract flying insects from more distant locations.”

As for adaptation to caves, he said this may have happened because, like termite mounds, flies and other small insects either inhabit or enter accidentally these areas, serving as food for the larvae.

“At some point, these larvae must have adapted to the tunnels found in the canga caves in the Carajás area, possibly burrowed by giant armadillos that have since become extinct,” Viviani said.

The researchers performed a preliminary molecular phylogenetic analysis to gain information on the evolutionary relationships of the luminescent larvae found on termite mounds and in canga clay caves in the Amazon forest. The results of the analysis, in combination with ecological data, showed that in addition to the species P. fragilis, which inhabits only forest environments, two distinct lineages of the species P. termitilluminans have adapted to termite mounds in savanna and forest areas.

P. termitilluminans was found on termite mounds in Parque Nacional das Emas, Goiás, and in the transition zone between the Cerrado and the Amazon rainforest, while P. fragilis was found on termite mounds in the Amazon rainforest, and P. pumilus was found in clay caves in Carajás, Pará.

All these species of Pyrearinus belong to the pumilus group. “These species of Pyrearinus in the pumilus group share a common ancestor that was associated with habitats rich in organic matter, preferably termites. Some species eventually adapted to termite mounds, others to caves,” Viviani said.

Insect banquet

The light emitted by the larvae found in caves is apparently less intense than the light emitted from their close relatives found on the surfaces of termite mounds, Viviani said. These mounds can reach 1.7 m in height.

Unlike fireflies of the Lampyridae family, which are usually found in open areas and emit flashes of light, the luminescent larvae ofPyrearinus sp., which are click beetles belonging to the family Elateridae, emit very intense light continuously. “Besides greater intermediate intensity, the light emitted by luminescent larvae of Elateridae on termite mounds and in caves is maintained for a very long time,” Viviani said.

The larvae that inhabit termite mounds start emitting light, which is greener than the light emitted by other beetles, in the late afternoon around sunset and continue to do so throughout the evening, Viviani explained.

In addition to the advantage to themselves produced by emitting light to attract flying insects, these bioluminescent larvae also benefit the termite mound’s entire microecosystem and its surroundings, he added, as previously observed on termite mounds in the Cerrado in the 1980s by Etelvino Bechara, a researcher at the University of São Paulo’s Chemistry Institute (IQ-USP), and Cleide Costa, another researcher at MZ-USP.

The light they emit attracts spiders, centipedes, frogs and birds, which benefit from the banquet of termites and other small insects attracted by the tiny bright spots of light.

The excrement from the animals that enjoy this banquet then helps fertilizes the soil near the termite mound, which typically displays richer flora than the surrounding Cerrado, according to Viviani.

“Inside the forest, termite mounds are themselves part of an environment that’s rich in organic matter, including many fallen leaves,” he said. “But the prey attracted by these luminescent larvae certainly enrich their vicinity even more, creating a microecosystem.”

Biotechnology applications

In addition to creating this microecosystem, bioluminescent click beetle larvae that inhabit termite mounds and caves can also benefit human society by providing new luminescent enzymes (luciferases) and substrates (luciferins) that can be used as bioanalytical reagents and cellular markers in pollution biosensors, as well as in research and development for cancer drugs and antibiotics, aside from many other applications, according to Viviani.

The research group he leads at UFSCar specializes in biochemical studies of the molecular structures and functions of luciferases (light-emitting enzymes) from fireflies and beetles and is a world leader in the field.

Luciferase from click beetle larvae collected on termite mounds in the Cerrado has been cloned and modified by the group, and it has the bluest bioluminescence and highest efficiency of all beetle luciferases. Furthermore, this luciferase resulted in the development of a mammalian cell marker by a group of researchers in Japan, with which Viviani’s lab collaborates.

Several other beetle luciferases cloned by his lab at UFSCar are also being tested with the aim of developing more biosensors and cellular markers.

At ISBC 2016, Gabriele Verônica Gabriel, a PhD student supervised by Viviani who is a research intern at the National Institute of Advanced Industrial Science & Technology in Japan, won an honorable mention for her research on luminescent cell biosensors for pH and heavy metals that change color in response to these factors.

“It’s very important to study the biodiversity of luminescent insects,” Viviani said. “They’re bioindicators of environmental quality in areas such as marshlands, watercourses and forests. Furthermore, each species exhibits luminescence with different characteristics, such as color and type of light emission, all of which can also be used in different biotechnological, biomedical and environmental applications.”

The article “First Report of Pyrearinus Larvae (Coleoptera: Elateridae) in Clayish Canga Caves and Luminous Termite Mounds in the Amazon Forest with a Preliminary Molecular-Based Phylogenetic Analysis of the P. pumilus Group” (doi: 10.1093/aesa/saw002) by V. R. Viviani and D. T. Amaral, published in Annals of the Entomological Society of America, can be read by subscribers to the journal