Some snakes are famous for 'seeing' infrared, but did you know that their heat-sensing abilities are rivalled by some beetles that can detect forest fires over considerable distances?

Vision is defined by its access to the visible electromagnetic spectrum. Many animals, notably insects and birds, can see into the ultraviolet, although this comes with the concomitant dangers that these short wavelengths (10-400 nm) are high energy and potentially biologically destructive. At the opposite end of the visible spectrum are orange and red, and there has been considerable speculation whether any optical system could register infrared, drawing upon a suitably adapted opsin. The reason to be sceptical is that the radiation of these long wavelengths (750 nm-1 mm) is of low energy, and indeed to date there is no direct evidence for infrared vision. We can, of course, sense it as radiant heat, but several times animals have independently evolved systems of infrared detection that in a number of respects are closely analogous to the eye. In a variety of organisms, transducers process incoming infrared radiation into a sensory signal that is interpreted by the brain and in at least one case (the snakes) is integrated with visual information. Unsurprisingly, infrared detection is ultimately involved with heat sources, but these can vary from ‘warm-blooded’ prey (snakes, vampire bats and bed bugs) to forest fires (three independent examples in the beetles).

Infrared detection in vertebrates

Snakes

Infrared detection is probably best known from the snakes, where thermosensitive pits on the head have evolved at least twice - once in the pit vipers (Crotalinae) and probably once in the more ancient boas and pythons (which in some taxonomies are united in the family Boidae). The infrared-sensing pit organs of boids and crotalines are similar with regard to their ultrastructure and electrophysiological function, but differ in number, location and overall morphology. While the three or more labial pits of boids are relatively simple, the single loreal pair of crotalines that is located between eye and nostril has a more complex structure. It follows the design of a bolometer, with a thin membrane suspended above a lower air-filled chamber. In both pit types, the heat-sensitive membrane is highly vascular and innervated with sensory dendrites, which are formed from terminal masses of the trigeminal nerve. A recent study has suggested that these nerve fibres are richly endowed with presumed infrared sensors, heat-sensitive ion channels of the TRPA1 type (orthologues of which mediate chemosensation in mammals and other vertebrates). Thus, infrared detection is probably indirect, involving a thermotransduction mechanism, rather than a direct photochemical reaction analogous to the activation of opsins in the vertebrate eye. Still snakes basically ‘see’ infrared, as the thermal signals are combined with visual information in the optic tectum. The localisation of ‘warm-blooded’ prey was long assumed to be the sole function of infrared detection in snakes, but heat pits seem to be more general-purpose organs that might also be used for behavioural thermoregulation and perhaps predator detection or den site selection.

Vampire bats

There is some evidence for an independently evolved capacity to detect infrared radiation in vampire bats (Desmodontinae), which, however, has received only limited attention. These mammals cut up the skin of other vertebrates with their razor-sharp teeth to lick up the blood. They evidently locate their ‘warm-blooded’ prey with the help of three heat-sensitive pits on their nose that are thermally insulated from the surrounding tissue. A specific nucleus in the brain seems to have important similarities in terms of histology and location to the equivalent infrared nucleus found in snakes.

Infrared detection in insects

In terms of convergence, the insects provide a far more striking example than the vertebrates, because the capacity to detect infrared has evolved several times in this group. Analogous to snakes and vampire bats, the blood-sucking bed bugs (e.g. in the genera Cimex and Leptocimex) employ this sensory modality to help locate their ‘prey’, evidently using a cave-like organ situated on the antennae. Amongst the hymenopterans, a parasitoid braconid wasp possesses a peculiar type of antennal sensillum that is inferred to be a wave-guide for infrared perception and might play a role in finding a potential host.

However, the real arthropod masters of sensing infrared radiation are the beetles. Three species (Melanophila acuminata, Merimna atrata and Acanthocnemus nigricans) have been shown to use this capacity for the detection of forest fires, not to facilitate escape but to lay their eggs in newly burnt wood. Despite this shared function, their infrared-detecting organs are very different, with M. atrata showing interesting similarities to the boid snakes and A. nigricans converging on the crotaline snakes. Interestingly, it has recently been shown that the equally pyrophilous Australian flat bug Aradus albicornis has independently evolved infrared receptors that resemble those of Melanophila beetles.