Fire loving (pyrophilous) insects depend on forest fires for their reproduction. Such insects approach ongoing fires and invade the burnt area immediately after a fire. For the long-range navigation toward a fire as well as for the short-range orientation on a freshly burnt area these insects have special sensors for smoke and infrared (IR) radiation. Whereas the olfactory receptors for smoke are located on the antennae, the IR receptors are housed in extra-antennal sensory organs, which can be found on the thorax or on the abdomen. In the pyrophilous beetle Melanophila acuminata
infrared receptors and their associated sensory neurons are derived from mechanoreceptors [1
]. Unlike other mechanosensory neurons, IR sensitive neurons directly send their information to be processed centrally (e.g., by the brain) rather than locally in their respective ganglia of origin [2
]. It is suggested that smoke-derived odours and IR information converge on descending brain neurons which, in turn, control and direct flight toward the forest fire.
Two genera of jewel beetles (family Buprestidae) can be classified as pyrophilous: About a dozen species of the genus Melanophila,
which are distributed nearly all over the world except for Australia, and the ”fire-beetle” Merimna atrata,
which is endemic to Australia [3
]. Despite the fact that Melanophila
show almost the same behaviour and belong to the same family of jewel beetles, their IR receptors are very different from each other.
On a freshly burnt area, the males of both genera often stay on the stems of trees close to burning or glowing wood or hot ashes. As soon as they become aware of a conspecific female, they try to copulate vigorously. After mating, the females deposit their eggs under the bark of burnt trees. The main reason for the pyrophilous behaviour is that the wood-boring larvae of Melanophila
can only develop in the wood of burnt trees [3
]. As a morphological speciality, both pyrophilous buprestid genera are equipped with antennal smoke receptors and thoracic or abdominal IR organs [6
Another pyrophilous beetle can also be found in Australia, i.e., the “little ash beetle” Acanthocnemus nigricans
(family Acanthocnemidae). This inconspicuous beetle is only 4 mm long and highly attracted by hot ashes: Little is known about its biology. Similarly, Acanthocnemus
also depends on fires for its reproduction and is equipped with a pair of sophisticated prothoracic IR receptors [10
]. Recently, IR receptors have also been discovered in a few pyrophilous members of the flat bug genus Aradus
(Heteroptera, Aradidae) [12
]. With respect to morphology and function, the IR receptors of Aradus
bugs are very similar to those described for Melanophila
beetles. Fire detection is obviously an important requirement for the survival of all of the pyrophilous insect species noted above. However, the outbreak of a forest fire is highly unpredictable. Therefore, pyrophilous beetles and bugs must be able to detect fires from distances as large as possible. Furthermore, when flying over a burnt area in search for a place to land, the small insects have to avoid “hot spots” with dangerous surface temperatures above about 60 °C.
Melanophila beetles and Aradus bugs are equipped with sensory structures that allow both the detection of hot fires at considerable distances as well as being able to locate moderately hot spots close by. These insects feature so-called photomechanic IR receptors, which might serve well as models for the technical design of un-cooled IR receptors. In the present paper we focus on the structure and function of the biological model as well as on the design and theoretical evaluation of a technical photomechanic IR sensor.