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Infrared sensing

It is not a second pair of eyes. It is a thermometer behind a pinhole, and it is fast enough to aim a strike in total darkness.

A pit viper has a membrane suspended in a cavity on each side of its face, thick with nerve endings that respond to a rise in temperature. It measures radiant heat rather than detecting light, and the resolution is poor — but it is enough to strike accurately at a warm animal in complete darkness.

The pit organ is one of the few places in sensory biology where the physics and the anatomy can be laid alongside each other completely. Warm objects radiate, the radiation arrives at a thin membrane stretched across a cavity, the membrane warms very slightly, and ion channels in the nerve endings there open in response to the temperature change. That last step is the part that was pinned down most recently, and it is what settles a long argument about whether the organ was detecting photons directly: it is not. It is a thermometer, extremely sensitive and extremely fast, behind a hole. Because it is a hole rather than a lens, the image is very coarse — the pit is a pinhole camera with a wide aperture, so a point of heat is smeared across a large part of the membrane. What makes the animal accurate is not the optics but the processing: the smeared pattern is sharpened neurally, and the result is combined with visual information in the optic tectum, where individual cells respond to both. That last finding is the strongest evidence available for how heat is used, and it is also the origin of the phrase this page spends most of its time qualifying. Cells that answer to heat and light in one map is a recording. That a snake "sees heat" is an interpretation of it.

Developed coverage · 44% complete · reviewed 2026-09-02

What this page covers

Pit organs are found in pit vipers and, in a structurally different form, in some boas and pythons. Heat-seeking of a simpler kind occurs in some blood-feeding insects, including certain bugs and beetles that locate fires.

Often confused with: Night vision, which is about collecting scarce light rather than about heat; Thermal cameras, which detect the same radiation but produce an image in a way the pit organ does not

Quick facts

What the organ measures
A temperature change in a suspended membrane — not light
Optics
A pinhole with no lens, so the raw image is very coarse
Where it joins vision
The optic tectum, in cells that answer to both
Evolved separately
Twice — pit vipers, and boas and pythons, with different anatomy

Where this appears

Every organism below has been linked to this page because the evidence links them. Each one carries its own evidence, and its own limits.

A thermometer, not an eye

Diagram

How a hole in the face measures radiant heat

Schematic. The membrane is far thinner relative to the pit than drawn.

Not a second eye. A thermometer with a pinhole in front of it.a membrane a few cells thick,suspended in air so it warms fastradiant heatThe membrane warms.A heat-gated channel inthe nerve reports it.That is the whole mechanism.The information reaches the same midbrain map as vision, and some cells answer toboth. That is why “sees heat” is half right — and only half.Most snakes have no pit organs at all.
The same explanation in words

A cross-section through a pit organ. A wide opening on the left admits radiation from a warm object, drawn as arrows converging on a thin membrane suspended across the middle of the cavity, with air on both sides of it. Nerve endings are marked densely across the membrane. Labels set out the sequence: radiation warms the membrane by a very small amount; temperature-sensitive ion channels in the nerve endings respond to the change; the signal travels to the brain. A note on the right explains that because the opening is a pinhole with no lens, the pattern falling on the membrane is very coarse, and that the sharpening happens in the nervous system rather than in the optics.

A pit viper does not detect infrared light the way an eye detects visible light. It suspends a very thin membrane in air, lets radiant heat warm it, and measures the warming with a temperature-sensitive channel.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

The pit organ comprises a thin suspended membrane densely innervated by trigeminal fibres expressing TRPA1, a heat-activated ion channel. Transduction is thermal rather than photochemical: incident infrared radiation warms the membrane and the resulting temperature change gates the channel.

Who this applies to
The three lineages with pit organs; most snakes have none and cannot do this at all.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Crotalinae, Boidae, Pythonidae
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

The transducer was identified by comparing gene expression between pit and non-pit nerve tissue across three lineages and confirmed by direct measurement of the channel’s temperature response.

How far it can be extended

Pit organs evolved separately in pit vipers and in boas and pythons and are absent from the great majority of snakes.

Caveats

  • The organ is a thermometer of extraordinary resolution rather than a second eye.
  • Spatial resolution is coarse: a pit is a pinhole with no lens, and the image is blurry.
  • Pit anatomy differs substantially between vipers and boids, and the boid organs are far less studied.

Still unanswered

  • How the nervous system sharpens such a blurred thermal image into usable spatial information.

Last reviewed 2026-09-02

The evidence (2 studies)

Two consequences follow from it being a thermometer. The first is that anything which changes the membrane’s temperature is a signal, so the organ has to be insulated from the animal’s own body — which is exactly what the suspended, air-backed arrangement achieves, and it is the anatomical detail that gives the mechanism away. The second is that a pit organ reports temperature differences rather than temperatures: a warm mouse against warm ground in the middle of a desert afternoon is a much harder target than the same mouse at night, and the sense is correspondingly more useful after dark.

Words used here
Pit organ
A cavity on the face of certain snakes containing a thin membrane densely supplied with heat-sensitive nerve endings. Pit vipers have one on each side, between eye and nostril.
Radiant heat
Infrared radiation emitted by anything warmer than its surroundings. It travels without needing air or contact, which is why a pit organ works at a distance.

One map, two senses

The recording that shows heat and light arriving at the same cells — and the sentence it does not license.

How we know

Finding the cells that answer to both heat and light

A pit viper has eyes and it has heat-sensing pits. Are those two separate senses, or one?

Electrodes were placed in the optic tectum, the midbrain structure that in a snake carries a map of the space around the animal. Visual stimuli were then presented at known positions, and infrared stimuli — warm objects — at known positions, and the responses of individual cells recorded against both. The question is whether any single cell answers to both, and whether the two maps line up in space.

What happened

The tectum contains cells responding only to light, cells responding only to heat, and cells responding to both — with the visual and infrared maps aligned, so a given cell answers to a warm object and a visible object in the same direction. Some bimodal cells responded more strongly to both together than to either alone.

What it shows

That the two are combined rather than kept apart. A warm mouse in the dark and a visible mouse in daylight excite the same population of cells in the same place on the map, which is the strongest available reason for describing infrared information as spatial rather than as a temperature reading.

What it does not show

It does not show that a snake experiences heat as a kind of seeing, and no recording can. Cells combining two inputs is integration, not a unified percept — the phrase "sees heat" is licensed by this result only in the narrow sense that the information is spatial and shares a map. It also covers pit vipers; boas and pythons have differently arranged pits that are far less studied.

The controls — what makes this evidence rather than a story
  • Visual and infrared stimuli presented separately as well as together, so a bimodal response can be distinguished from two unimodal ones.
  • Stimulus position varied systematically, testing whether the two maps are in register.
  • Cells classified by which stimuli they respond to rather than assumed to be one kind.

From Integration of visual and infrared information in bimodal neurons of the rattlesnake optic tectum

In the snake’s midbrain there are cells that answer to light, cells that answer to heat, and cells that answer to both — with the two maps of space lined up.

Well supported

Good evidence backs this, though some details remain open.

Recordings from the optic tectum of pit vipers identify unimodal visual, unimodal infrared and bimodal neurons, with the infrared and visual receptive-field maps in spatial register and some bimodal cells responding supralinearly to combined stimulation.

Who this applies to
Pit vipers; boid pit organs project differently and are much less studied.
Studied in
Crotalinae
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The recordings are clear and have not been seriously disputed, but the primary literature is decades old and the accessible account is a popular article, which is a weaker citation than this claim deserves.

Caveats

  • Convergence shows the signals meet; it does not establish a unified percept, and no recording could.
  • The accessible source is a popular-magazine account, cited as such.
  • Whether the integration improves strike accuracy in the dark is inferred rather than measured.

Still unanswered

  • What a snake with both channels does when they disagree — a warm object where nothing is visible, or the reverse.

Last reviewed 2026-09-02

The evidence (2 studies)

The short answer

Do snakes see heat?

Heat and light information converge on the same cells in the same brain region, arranged in the same spatial map — which is a real and striking finding. Whether the animal experiences that as seeing is not something a recording electrode can answer, and no experiment has tried.

The convergence is genuinely the interesting part, and it is worth being precise about what it shows. In the optic tectum some cells respond only to light, some only to infrared, and some to either — and those cells are arranged so that a given position in space is represented at a given place in the map regardless of which sense reported it. Functionally, that is integration: the animal has one spatial account of the world assembled from two very different inputs. Calling it seeing imports a claim about what that assembly is like from the inside, which the same recording would look identical either way. The more careful description is also more useful: a snake locates warm objects in the same coordinate frame it locates visible ones, which is why it can strike accurately at something it cannot see.

Check it for yourself

Where this sits in the animal

How the pit organ was worked out

  1. 1683

    First observation

    A hole in the face, and no idea what for

    Edward Tyson, dissecting a rattlesnake at the Royal Society, gave the first description of the facial pits — and took them for an organ of hearing. Proposals over the following two and a half centuries included a second nostril and a gland; a heat sense was not among them.

  2. 1937

    Landmark experiment

    The pits respond to warmth

    Behavioural and physiological work established that the organ responds to a warm object at a distance, with the animal striking accurately at warmed targets in the dark. What the receptors were doing remained open.

  3. 1981

    Landmark experiment

    Heat and vision meet in one map

    Recordings from the optic tectum found cells responding to infrared, to light, and to both, arranged in a single spatial map. The two senses were not running in parallel; they were being combined.

    Integration of visual and infrared information in bimodal neurons of the rattlesnake optic tectum

  4. 2010

    Reinterpretation

    The molecule that does it, and the end of the photon argument

    The receptor was identified as a temperature-sensitive ion channel, TRPA1, expressed at very high levels in the pit membrane. The organ detects a temperature change in its own tissue rather than radiation directly.

    Changes how the 1937 result reads

    Warmth-sensitivity had been compatible with the organ detecting infrared radiation directly, in the way a retina detects light. Identifying a thermal ion channel settled it the other way: the pit is a thermometer, and everything it can and cannot do follows from that.

    Molecular basis of infrared detection by snakes

What is still unsettled

  • How much detail does the snake actually recover from such a coarse image?

    Why it matters: The optics are poor and the strikes are accurate, so a great deal is being done neurally. How much, and by what computation, is not settled.

    What would settle it: Behavioural discrimination thresholds measured against the physical resolution of the pit, in the same animals.

  • What is the organ mainly for?

    Why it matters: Prey detection is the usual answer, and there is evidence for thermoregulatory use and for predator detection as well. Which dominates in the wild is largely assumed.

    What would settle it: Field observation of pit use across contexts, which is difficult in animals that spend most of their time motionless.

  • Why do boas and pythons have a structurally different organ doing a similar job?

    Why it matters: Their pits are shallower, more numerous, and lack the suspended membrane, which should make them less sensitive — and the behavioural differences that would predict have not been mapped.

    What would settle it: Directly comparable sensitivity and behavioural measurements across the two groups, which have mostly been studied separately.

Claims about this, checked

Things people have heard, and what the evidence actually supports.

The research behind this page

2 studies, newest first. Each one has a page explaining what it found and what it could not show.

2010Nature

Molecular basis of infrared detection by snakes

TRPA1, a channel used elsewhere in vertebrates as an irritant receptor, is massively enriched in the pit-bearing trigeminal ganglion and functions in these snakes as a heat-activated channel.

1981Science

Integration of visual and infrared information in bimodal neurons of the rattlesnake optic tectum

Cells responding only to light, only to infrared, and to both were found in the same structure, with the infrared and visual maps in spatial register and some bimodal cells responding more strongly to the two together than to either alone.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 44% completeness against what we would call a finished subject, and was last reviewed on 2026-09-02. It carries 2 claims and answers 14 mapped search questions.

  • 2 high-priority search intent(s) not yet covered
  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • Infrared-seeking insects — fire beetles and blood-feeding bugs — are named and not treated, and they use a different mechanism again.
  • The neural sharpening that turns a blurred thermal pattern into an accurate strike is described in outline rather than explained.
  • Nothing here covers how pit organs develop or their evolutionary origin, which is an active area.