Helmut Schmitz

Insect antenna as a smoke detector

The larvae of jewel beetles of the genus Melanophila (Buprestidae) can develop only in the wood of trees freshly killed by fire. To arrange this, the beetles need to approach forest fires from as far as 50 kilometres away, . They are the only buprestid beetles known to have paired thoracic pit organs, which behavioural, ultrastructural and physiological experiments have shown to be highly sensitive infrared receptors, useful for detecting forest fires. It has been suggested that Melanophila can sense the smoke from fires, but behavioural experiments failed to show that crawling beetles approach smoke sources. We find that the antennae of jewel beetles can detect substances emitted in smoke from burning wood.

Water as a major modulator of the mechanical properties of insect cuticle.

The mechanical properties of the sternal cuticle of the locust were investigated by nanoindentation. Modulus and hardness of the exo-, meso-, and endocuticular layers were locally measured under dry and fully wetted conditions in the normal (i.e. perpendicular to the outer surface) as well as in the transverse direction (i.e. parallel to the alignment of the respective layers). The results show that water has a major impact on the mechanical properties of all layers. After drying the endocuticle, in particular, became harder by a factor of up to 9 and stiffer by a factor of up to 7.4. Additionally the gradual decrease in hardness and Youngs modulus from the outer exo- to the inner endocuticle, characteristic of native cuticle, was eliminated or even reversed in dried cuticle. A pronounced anisotropy was revealed in all layers when comparing data obtained by probing in the normal (lower values) vs. probing in the transverse direction (higher values). Cyclic drying and rewetting of the endocuticle showed that the mechanical properties can be reproducibly changed by altering the water content. Based on our results we propose a new role of the epicuticle: fine-tuning of the mechanical properties of the different cuticular layers can be accomplished by setting the local cuticular transpiration.

Fine structure and physiology of the infrared receptor of beetles of the genus Melanophila (Coleoptera: Buprestidae)

Abstract Buprestid beetles of the genus Melanophila (Coleoptera: Buprestidae) possess paired thoracic pit organs. Each pit houses about 70 tightly packed infrared sensilla that enable the beetles to detect forest fires at long range. The cuticular apparatus of a single infrared sensillum consists of an endocuticular sphere with an average diameter of 15 μm. Each sphere is innervated from below by the dendritic outer segment of a single sensory neuron that shows the same ultrastructure as a typical hair mechanoreceptor (e.g., sensillum trichodeum). Several transitional stages between cuticular mechanoreceptors and infrared sensilla can be found in the neighbourhood of the infrared sensillum field (intermediate sensilla and so-called suppressed systems). First electrophysiological recordings show that the receptors at the bottom of the pit are true infrared receptors. The sensilla respond in a fast phasic manner to infrared radiation and even an exposition time of 2 ms is sufficient to release a single spike. All morphological and physiological findings suggest that the infrared sensilla have evolved from ordinary cuticular mechanoreceptors and that mechanical events are still part of the transduction process. A model about the possible function of the infrared receptors of Melanophila is presented.

A new type of infrared organ in the Australian "fire-beetle" Merimna atrata (Coleoptera: Buprestidae)

Abstract The Australian buprestid beetle Merimna atrata (Coleoptera: Buprestidae) approaches forest fires because its larvae develop in freshly burnt wood. So far nothing is known about possible sensory systems enabling the beetles to detect fires and to cope with the thermal environment close to the flames. We found that M. atrata has two pairs of infrared (IR) organs on the ventrolateral sides of the abdomen. Each IR organ consists of a specialized IR-absorbing area which is innervated by one thermosensitive multipolar neuron. The primary dendritic branches ramify into more than 800 closely packed terminal endings which contain a large number of mitochondria. We called the special morphology of the dendritic region a terminal dendritic mass. The type of IR receptor found in M. atrata is unique in insects and can best be compared with the IR organs of boid snakes.

Morphology of a thermosensitive multipolar neuron in the infrared organ of Merimna atrata (Coleoptera, Buprestidae).

Two pairs of infrared (IR) organs are situated ventrolaterally on the second and third abdominal sternites of the Australian fire beetle Merimna atrata (Buprestidae). In ventral view, each IR organ has a round IR absorbing area under which a sensory complex is attached to the epidermis. The main component of the complex is a single large multipolar neuron and its mass of highly branched dendrites. All parts of this neuron are enveloped in glial cells. The proximal primary dendrites, which arise from the soma, finally branch into several hundred tightly packed terminal dendrites, which contain many mitochondria. We term this unusual morphology of the dendritic region a terminal dendritic mass (TDM). Additionally, two chordotonal organs were found in each sensory complex. Their somata are integrated in the complex and the dendrites extend to the periphery of the absorbing area. The bauplan of the dendritic region is reminiscent of the thermosensitive trigeminal nerve fibers innervating the absorbing structures in the IR receptors in boid and crotalid snakes. Because this multipolar neuron also functions as a thermoreceptor, another example of a functional analogy between insect and vertebrate sensory systems could be demonstrated.

Sensitivity threshold and response characteristics of infrared detection in the beetle Melanophila acuminata (Coleoptera: Buprestidae)

The minimum detection threshold of the infrared sensitive beetle, Melanophila acuminata, was measured with a helium-neon laser that emitted light at a wavelength of 3.39 microm. Extracellular recordings were taken both at the pit organ responsible for detection and at the interganglionic connectives in the thorax of the beetle. At the pit organ, generator and action potentials from single neurons were measured with a sharpened tungsten electrode. At the connectives that linked the fused second meso-/metathoracic and prothoracic ganglia, compound action potentials were measured with a tungsten hook electrode that encircled the connective. The latter recordings confirmed conveyance of infrared information through specific pathways to rostrally-situated sites in the nervous system of the beetle. The 50% probability irradiance threshold at which action potentials were elicited from the receptor and connectives occurred at 17.3 and 14.6 mW/cm(2), respectively. In addition to sensitivity threshold, several other characteristics of the response were quantified including dependence of generator potential latency, generator potential duration, spike frequency, and spike latency on irradiance, dependence of response strength (spike count) on exposure time, and flicker fusion frequency. The ability to detect infrared radiation is rare in nature, and these results provide valuable information necessary to understand this unique sensitivity.

Modelling a Historic Oil-Tank Fire Allows an Estimation of the Sensitivity of the Infrared Receptors in Pyrophilous Melanophila Beetles

Pyrophilous jewel beetles of the genus Melanophila approach forest fires and there is considerable evidence that these beetles can detect fires from great distances of more than 60 km. Because Melanophila beetles are equipped with infrared receptors and are also attracted by hot surfaces it can be concluded that these infrared receptors are used for fire detection. The sensitivity of the IR receptors is still unknown. The lowest threshold published so far is 0.6 W/m2 which, however, cannot explain the detection of forest fires by IR radiation from distances larger than approximately 10 km. To investigate the possible sensitivity of the IR receptors we assumed that beetles use IR radiation for remote fire detection and we made use of a historic report about a big oil-tank fire in Coalinga, California, in 1924. IR emission of an oil-tank fire can be calculated by “pool fire” simulations which now are used for fire safety and risk analysis. Assuming that beetles were lured to the fire from the nearest forests 25 and 130 km away, our results show that detection from a distance of 25 km requires a threshold of the IR receptors of at least 3×10−2 W/m2. According to our investigations most beetles became aware of the fire from a distance of 130 km. In this case the threshold has to be 1.3×10−4 W/m2. Because such low IR intensities are buried in thermal noise we suggest that the infrared sensory system of Melanophila beetles utilizes stochastic resonance for the detection of weak IR radiation. Our simulations also suggest that the biological IR receptors might be even more sensitive than uncooled technical IR sensors. Thus a closer look into the mode of operation of the Melanophila IR receptors seems promising for the development of novel IR sensors.

Responses of the infrared sensilla of Melanophila acuminata (Coleoptera: Buprestidae) to monochromatic infrared stimulation

Abstract The pit organs of the beetle Melanophilaacuminata were stimulated with monochromatic infrared radiation using a continuous wave CO overtone infrared laser. Best sensitivity was in the wavelength range 2.8–3.5 μm. In this range a stimulus intensity of 14.7 mW cm−2 was sufficient to generate single action potentials. At a wavelength of 5 μm receptor performance significantly decreased. An increase in stimulus intensity caused a decrease in response latency and an increase in the number of action potentials elicited. At a given wavelength (3.4 μm) the dynamic amplitude range of action potential responses covered 12 dB. At high stimulus intensities (94.2 mW cm−2) a stimulus duration of 4 ms was sufficient to generate one to two action potentials and a stimulus duration of 60 ms already caused response saturation (with up to nine action potentials). In a repetitive stimulus regime distinct receptor potentials were visible up to a frequency of 600 Hz.

Distribution and functional morphology of photomechanic infrared sensilla in flat bugs of the genus Aradus (Heteroptera, Aradidae)

Globally the flat bug genus Aradus comprises about 200 species. About half a dozen Aradus species can be primarily found on burnt areas and, therefore, have been called pyrophilous. Bugs and their offspring feed on fungi growing on burnt wood. Recently, prothoracic infrared (IR) receptors have been described in the pyrophilous Australian species Aradus albicornis. In our study we investigated 10 Aradus species, once again including A. albicornis, and found prothoracic as well as hitherto unknown mesothoracic IR sensilla in A. albicornis, Aradus lugubris and Aradus fuscicornis. In Aradus flavicornis only prothoracic IR receptors were found. Currently the latter two species are not known as pyrophilous. However, there is considerable evidence that these flat bugs also approach forest fires. In all four species where IR receptors were identified, the dome-shaped IR sensilla look very similar. An IR sensillum consists of an internal exocuticular sphere reinforced by consecutive layers of chitin fibres. In the center of the sphere, a microfluidic core is located which consists of a cup-shaped plug of cuticle and an underlying fluid filled annular channel surrounding the tip of the dendrite of a mechanosensitive neuron. Like the IR receptors of buprestid beetles of the genus Melanophila, the IR sensilla found in Aradus species can be classified as photomechanic IR receptors.

Infrared receptors in pyrophilous (“fire loving”) insects as model for new un-cooled infrared sensors

Summary Beetles of the genus Melanophila and certain flat bugs of the genus Aradus actually approach forest fires. For the detection of fires and of hot surfaces the pyrophilous species of both genera have developed infrared (IR) receptors, which have developed from common hair mechanoreceptors. Thus, this type of insect IR receptor has been termed photomechanic and shows the following two special features: (i) The formation of a complex cuticular sphere consisting of an outer exocuticular shell as well as of a cavernous microfluidic core and (ii) the enclosure of the dendritic tip of the mechanosensitive neuron inside the core in a liquid-filled chamber. Most probably a photomechanic IR sensillum acts as a microfluidic converter of infrared radiation which leads to an increase in internal pressure inside the sphere, which is measured by a mechanosensitive neuron. A simple model for this biological IR sensor is a modified Golay sensor in which the gas has been replaced by a liquid. Here, the absorbed IR radiation results in a pressure increase of the liquid and the deflection of a thin membrane. For the evaluation of this model analytical formulas are presented, which permits the calculation of the pressure increase in the cavity, the deformation of the membrane and the time constant of an artificial leak to compensate ambient temperature changes. Some organic liquids with high thermal expansion coefficients may improve the deflection of the membrane compared to water.