Whilst infrared detection is probably best known in the snakes (where it has evolved twice), in point of fact in terms of convergence the insects provide by far the most striking example.

Whilst infrared detection is probably best known in the snakes (where it has evolved twice), in point of fact in terms of convergence the insects provide by far the most striking example, because in this group of arthropods such a capacity has evolved at least five times. Broadly this sensory modality is involved either with the detection of forest fires or ‘warm-blooded’ prey. The latter echoes, of course, the infrared detection found in the vampire bats, but the former is not connected to the facilitation of escape, but in fact the exact reverse because the newly burnt wood and other vegetation are a vital opportunity for first mating and then laying eggs in an environment that would otherwise be regarded as hostile. Infrared detection has evolved independently in three groups of beetles, as well as in two other groups of insects.

Melanophila acuminata (beetle)

Probably the best known of infrared-detecting insects is the beetle Melanophila acuminata. These can evidently detect infrared radiation from forest fires for considerable distances, some reports speak of 50 km, but this is evidently augmented also by a capacity to detect smoke, with the antennae evidently highly sensitive to combustion products, including phenols. The reason the beetles fly towards the conflagration whilst everybody else is hurtling in the opposite direction is because the eggs are laid in the still warm wood, and the emerging larvae feed on the wood, which has had its toxins destroyed during combustion. The actual infrared detectors are extraordinarily complex, and consist of an array of detectors located beneath the wings and near to the middle set of wings. The detectors are sensilla, and evidently highly modified mechanoreceptors. Their complex structure includes a more or less globose unit that incorporates a neuron. There is still debate as to how exactly the infrared detection occurs, but evidently the cuticle has chemical bonds that are strongly absorbent of infrared radiation and the local heating deforms the detector and so triggers the nervous signal. Given the location of the infrared organs the beetle evidently must tack as it flies towards the fire, and there has been some speculation that there is a degree of spectral sensitivity. It also seems likely that some sort of infrared image is formed in the brain of the beetle.

Recently it has been suggested that the pyrophilous Australian flat bug Aradus albicornis possesses infrared receptors that are similar to those of Melanophila beetles. This hemipteran has dome-shaped sensilla on its prothorax, which respond to broadband infrared radiation. “The existence of photomechanic IR receptors in both beetles and bugs demonstrates a remarkable convergent evolution towards this particular biophysical transduction mechanism and suggests that it provides selective advantages over other possible solutions” (Schmitz et al. 2008, Naturwissenschaften, vol. 95, p. 455).

Merimna (beetle)

Like Melanophila, the beetle Merimna is also a member of the buprestids, but its infrared detection organ has clearly a completely different origin. It too is attracted to forest fires, and the larvae can only develop if the eggs are laid in freshly burnt wood. The infrared detector is richly endowed with nerve cells and also mitochondria, and has striking similarities to the arrangement found in the equivalent organ of boid snakes. The detector itself is an area of cuticle, with a characteristic honeycomb structure, quite unlike that found in Melanophila. There is some evidence that this detector system has a relatively recent evolutionary origin, not least because the total shows quite wide variation.

Acanthocnemus nigricans (beetle)

The third beetle to evolve infrared detection, Acanthocnemus nigricans, is only distantly related to the two buprestid beetles, and unsurprisingly it has arrived at yet another solution to the detection of infrared radiation, although it too is attracted to forest fires and after a short period of running around having alighted on the hot ash or burnt bark quickly burrows and evidently then lays its eggs. Here the infrared organs are located in front of the first pair of legs and they consist of a sensory plate, richly endowed with nervous tissue and associated mitochondria. The plate is connected to the rest of the body by a narrow stalk, and is located above a cavity. It thus accords to a bolometer, an instrument we employ as infrared detectors, and strikingly is strongly convergent with the arrangement found in pit vipers. The evolution of an insect bolometer is not only striking in terms of the convergences, but also represents a remarkable novelty amongst the arthropods.

Bed bugs (hemipterans)

Hemipterans, specifically bed bugs (species of the family Cimidae) evidently employ infrared detection to help locate their ‘warm-blooded’ prey. Bed bugs from genera such as Cimex and Leptocimex are well adapted to live with and infect humans (via beds or sofas) as well as poultry and bats in some cases. They are attracted to waves of heat (and respiratory gas – carbon dioxide) produced by their hosts, and having reached them, they inject piercing mouthparts through the skin. The mouthparts are divided into two tubes: one tube for injecting saliva containing chemicals to prevent blood clotting, and the second tube for sucking blood from the host.

Braconid and vespid wasps

Among the hymenopterans, a braconid wasp possesses a peculiar type of antennal sensillum that is inferred to be a wave-guide for infrared detection. In addition, infrared detection may also play a possible role in the construction of the comb cells in some vespid wasps.