Nature's Infrared Club

A handful of biological species can detect Infrared radiation. Envious of this evolution-honed sensory superpower, researchers with technological visions are working to emulate it.
01 March 2020
By Ivan Amato
The black fire beetle has sensitive infrared detectors behind its second legs
The black fire beetle has sensitive infrared detectors behind its second legs. Photo Credit: Volker Steger/Science Photo Library

Gang Han had a great 2019. In September, he was promoted to full professor at the University of Massachusetts Medical School in Worcester. And in April, he and a team of colleagues at several Chinese research institutions published a sensational paper in the journal Cell that earned them extensive media attention and even a shoutout from Francis Collins, the longtime director of the National Institutes of Health. Referring to a preprint version of the Cell paper in a March posting of his NIH Director's Blog, Collins wrote, "In a dramatic advance that brings together material science and the mammalian vision system, researchers have just shown that specialized lab-made nanoparticles applied to the retina, the thin tissue lining the back of the eye, can extend natural vision to see in infrared light."

Not human infrared vision. Not yet, anyway. So far, Han said in December in a dark meeting room in Boston at the Materials Research Society meeting, his team has bestowed IR vision on what Han referred to as "supermice." In a subsequent interview, Han acknowledged that the plan is to take necessary steps, through a series of primate studies and then by navigating relevant ethical and regulatory challenges, to develop a safe technology that would modify peoples' eyes to directly see IR without any bulky goggles or other optical gadgetry. It's the sort of superpower that brings soldiers and first-responders to the mind's eye. If the researchers succeed in delivering this human vision-enhancement technology, then people will join what always has been a rarefied and enviable club of the living kingdom that can see infrared (IR) radiation.

• • •

Consider several shrimp species that thrive at pitch-dark depths up to two miles in the Mid-Atlantic Ridge. That's where they feed on sulfur-metabolizing microorganisms that for eons have been making a living near hydrothermal vents that spew sulfur-rich water at searing temperatures of 350 degrees Celcius. In this utter darkness, the arthropods avoid becoming flash-boiled shrimp by virtue of an evolution-honed sensory innovation: a retina-like patch on the shrimps' backs enables the animals effectively to see plumes of lethally hot water from a distance far enough that the animals can steer clear of them.

"They can see the edge of the hot water," says Tom Cronin, a sensory biologist at the University of Maryland, Baltimore County. "The water comes out in a lamellar flow so there is a hard border between the really hot water and the colder surrounding water." To the shrimp, the hot plumes of water might appear like gaudy illuminated fountains in a background of profound blackness.

For most people who have heard about IR vision in animals, it's usually in the context of snakes, most likely pit vipers such as rattlesnakes, copperheads, and bushmasters. It was only in the 1930s that scientists began to get an inkling of the biological roles played by what were then still mysterious apertures on the faces of these and certain other snakes. It was in 1935 that Margarete Ros noticed that her pet African Rock Python paid special attention to warm objects she placed in its terrarium but would lose some interest in the same objects when she clogged her pet's facial pits—later to be called "pit organs"—with petroleum jelly.

Since Ros's pioneering observations, a lineage of pit-organ researchers has teased out much about how these master examples of natural infrared sensing technology work and how these IR sensors equip their serpentine owners with vision specialized for hunting. The pit organs differ in structure among the three groups of snakes known to have them. They range from nearly pitless infrared sensory areas on the faces of boas, to the broad pits with retina-like sensory floors of pythonid snakes like Ros's pet, to the suspended in-pit sensory membranes of the pit vipers. The latter's pit organs, the most sensitive of all among snakes, work like pinhole cameras and probably enable a kind of IR imaging, states University of Geneva herpetologist Andrew Durso in his blog Life is Short, but Snakes are Long.

Pit viper organ SEM. Credit: Bolivar-G. et al./PLOS One 2012

Microscopy of the pit viper infrared receptor organ. Photo Credit: Bolívar-G. et al./PLOS One 2014

Neurophysiological recordings have revealed that the pit organs respond most vehemently at IR wavelengths of about 8–12 microns, the wavelengths emitted by living mice and other potential meals. The IR-triggered neuronal signals originating in pit organs combine downstream in the snake's brain with signals from the reptiles' laterally located eyes into integrated optical/IR visual perception. In this way, even in dark and complex settings snakes with pit organs can track mice and other warm prey.

The pit organs' structural and material details embody the engineering brilliance that evolutionary forces can yield. Among these are arrays of depressions less than a micron deep and few microns across that, as the late Richard Goris, who had been a herpetologist and neuroanatomist at Yokahama City University in Japan, explained it the Journal of Herpetology, "efficiently disperse wavelengths centered at 500 nm, while allowing free passage of longer infrared wavelengths." Tiny dome-like details in some pit organs prevent infrared rays from scattering back out of the pit organs thereby preserving information about the environment that otherwise would literally escape the animal. "For maximum efficiency," Goris noted, "the pit organs must be able to fend off, weaken, or dissipate all extraneous wavelengths."

Joining shrimp and snakes in biology's IR sensing world are butterfly species, including Mormon Metalmarks, which use their near-IR vision to find plants they prefer for hosting eggs; ticks that presumably rely on IR sensing to home in on hosts and particular locations for their blood-tapping ways; the bloodsucking bug, Rhodnius prolixus, the principal vector of Chagas Disease, which relies on two kinds of antennae-born sensory cells—so-called "peg-in-pit" cells and "tapered hair" cells—to locate warm-blooded hosts; and vampire bats that deploy "leaf pits" surrounding their nose to collect IR energy for an overall sensory purview that helps the bats find warm, blood-bearing bodies in the darkness of the night.

• • •

The members of biology's IR club that perhaps have inspired the most technological ambition are collectively known as fire-loving or pyrophilous insects, mostly beetles. These insects show up by the thousands at forest fires as the conflagrations burn themselves out. These insects also have been assumed to follow the heat to fires at chemical plants and oil refineries, a less biologically profitable behavior that nonetheless provided some of the evidence of these insects' IR sensitivity.

Scientists have identified at least 17 pyrophilous beetle species among the roughly million known insect species. For pyrophilous beetles, trees freshly killed by fire serve as safe havens that further the cause of procreation. "The freshly burnt area serves as a meeting place for both sexes and, after copulation, the females start to deposit their eggs under the bark of burnt trees," Helmut Schmitz, a longtime researcher of pyrophilous beetles at the Institute of Zoology at the University of Bonn and his colleagues, state in one of their papers. Subsequently, the larvae feed on the wood and emerge in a year or two as the next generation of fire beetles. For their part, the adult beetles rely on their exquisitely sensitive IR sensors to locate forest fires many kilometers away.

Found in the various but related anatomies of the IR-sensing beetle species are at least three different types of receptors. Little ash beetles (Acanthocnemus nigricans), for one, sense IR with a pair of sensilla-lined discs in their prothoracic segment, which is adjacent to the head. Australian fire beetles (Merimna atrata) sport their trough-shaped IR organs in the cuticles of their abdominal regions. In a 2018 paper in PLOS One, Schmitz and coworkers reported experiments that revealed these beetles apparently use their IR organs to avoid depositing the eggs in what would be lethally hot spots in freshly burned wood. For 11 species collectively known as black fire beetles (Melanophila genus), a single IR organ comprises about 70 tiny dome-shaped sensilla (each one with an accompanying pore-riddled wax gland) in a pit structure in the metathorax (a shoulder-like location). This is the type of sensor that Schmitz and coworkers have been aiming to emulate. Each individual sensillum has a complex, fluid-filled interior pressure chamber that includes the dendritic end of a single sensory neuron feeding neurodata about IR in the environment into the insect's central nervous system.

The current view on how a sensillum works is that it behaves like mechanoreceptor. When infrared radiation impinges on a sensillum's stiff dome, it heats it up. This has the effect of raising the interior pressure of each liquid-filled sensillum's microanatomy and consequently puts a squeeze on the more compliant neuronal dendrite that innervates each structure. This, in turn, activates receptors on each neuron that act as gateways for ions. The changing ionic traffic leads to the firing of these neurons and thereby a relay of signals about the IR environment into the insect's brain. Data from years of research, much of it by Schmitz and various colleagues, portray this mechanoreceptive system as one of astonishing—though still undetermined—IR sensitivity, including in the infrared wavelength range of 2.8 to 3.5 microns.

Melanophila acuminata sensilla organ

The infrared organ of Melanophila acuminata. At the bottom of a small pit about 70 IR sensilla can be found. Eeach of the dome-shaped sensilla is associated by small wax gland. Photo Credit: Schmitz/PLOS One 2012

• • •

The wavelengths the sensilla respond to map onto the IR emissions of forest fires and that has inspired Schmitz and others with technological visions. A combination of experimental measurements, modeling, and calculations indicate a sensitivity that could be 500 microwatts per square centimeter or possibly an even much lower threshold. The more conservative sensitivity is good enough, Schmitz and colleagues calculate, to detect a 20-hectare forest fire-with areal dimensions comparable to the height of the Empire State Building (about 450 meters)—from 12 kilometers away. It's possible the beetles can detect fires from ten times that distance.

This detection range would be an improvement over commercially available uncooled bolometer infrared sensors currently used for fire detection. In a 2015 paper published in the journal Micromachines, Schmitz and his colleagues outline their vision for building wide-area fire detectors using what they call "beetlecopters,"—drone-mounted sensors that mimic the IR-sensing ways of Melanophila beetles.

In a warming world ever more wracked by forest fires, the disaster prevention and response communities surely could benefit from better and widely deployable fire-detection systems, but Han has his sights on human beings with what he calls "the superpower of IR vision." At the MRS meeting, he wowed his audience with videos of the IR-seeing "supermice" his team has created. At the heart of the achievements are so-called "photoreceptor-binding upconversion nanoparticles" (pbUCNPs). These nanoantennae particles are made with rare-earth ingredients that absorb invisible infrared light and convert it into visible green light. To the same nanoparticles, the researchers also attach organic components that effectively glue the nanoparticles to the outer segments of retinal photoreceptor cells. When the researchers injected these tiny IR nanoantennae into the eyes of mice (using standard clinical optical injection techniques, Han assured his audience), the nanoantennae bound like Christmas-tree decorations to the rods and cones of the animals' retinas.

Physiological and behavioral tests comparing the eye-modified mice with unmodified cohorts were telling. The pupils of the "supermice" dilated when the researchers shined IR light into them, while the pupils of unmodified mice showed no response in the same test. Moreover, given a choice to occupy either a dark compartment or one "lit" up with infrared light, the supermice consistently opted for the dark compartment. Mice prefer dark places. Mice with untreated retinas randomly explored both compartments, which suggests both compartments appeared equally dark to them. These results indicate that the nanoparticles enable the mice to see IR as light.

In an interview, Han said he is working on organic dye molecules that he thinks will smooth the way to tests with people, who he expects will one day join biology's infrared-seeing club. "These should be safer and brighter" than the nanoparticles, Han said, noting that the technology could open the way to soldiers with IR-sensing retinas and IR-seeing first responders who might be able to spot survivors of collapsed buildings. More uplifting, however, was Han's musing about looking up at the night sky with eyes modified to see infrared radiation. "This could let us see a more beautiful view of the universe for us," he said.

Ivan Amato is a writer, editor, podcaster, and science cafe organizer in Hyattsville, Maryland.

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