Electrophysiological characterisation of the infrared organ of the Australian "Little Ash Beetle" Acanthocnemus nigricans (Coleoptera, Acanthocnemidae)

Hunting with heat: thermosensory-driven foraging in mosquitoes, snakes and beetles

Animals commonly use thermosensation, the detection of temperature and its variation, for defensive purposes: to maintain appropriate body temperature and to avoid tissue damage. However, some animals also use thermosensation to go on the offensive: to hunt for food. The emergence of heat-dependent foraging behavior has been accompanied by the evolution of diverse thermosensory organs of often exquisite thermosensitivity. These organs detect the heat energy emitted from food sources that range from nearby humans to trees burning in a forest kilometers away. Here, we examine the biophysical considerations, anatomical specializations and molecular mechanisms that underlie heat-driven foraging. We focus on three groups of animals that each meet the challenge of detecting heat from potential food sources in different ways: (1) disease-spreading vector mosquitoes, which seek blood meals from warm-bodied hosts at close range, using warming-inhibited thermosensory neurons responsive to conductive and convective heat flow; (2) snakes (vipers, pythons and boas), which seek warm-blooded prey from ten or more centimeters away, using warmth-activated thermosensory neurons housed in an organ specialized to harvest infrared radiation; and (3) fire beetles, which maximize their offspring's feeding opportunities by seeking forest fires from kilometers away, using mechanosensory neurons housed in an organ specialized to convert infrared radiation into mechanosensory stimuli. These examples highlight the diverse ways in which animals exploit the heat emanating from potential food sources, whether this heat reflects ongoing metabolic activity or a recent lightning strike, to secure a nutritious meal for themselves or for their offspring.

Sensory equipment and adaptations to the fire habitat of the antennae of the Australian ´firebeetle´ Merimna atrata (Coleoptera; Buprestidae)

The ‘Australian firebeetle’ Merimna atrata approaches fires in Eucalyptus forests for reproduction. Beetles stay on a postfire area as long as burning wood or hot ashes emit heat and smoke. Abdominal infrared receptors protect the beetles from landing on hot spots; however, until now fire-specific adaptations of the antennae have not been investigated in more detail. This affects the localization of olfactory sensilla used for the perception of smoke and in addition mechanisms to protect delicate sensilla against desiccation and pollution. Moreover, nothing was known about antennal thermo-/hygroreceptors in Merimna atrata. We found strong evidence for a functional grouping of the sensilla into receptors used on the ground or in flight, respectively. A first group comprises the outer visible sensilla, i.e. mechanosensory bristles, short gustatory sensilla and a small field of very short olfactory sensilla. They are used when the beetle is running around on the fireground on burnt bark or ashes. A second group of sensilla is hidden in closeable cavities on antennomeres 4–11. If the cavities are closed, the sensilla inside are fully protected. If the cavities are opened in flight, the beetles can make use of many multiporous basiconic sensilla and multiporous basiconic grooved peg sensilla for smoke detection. Minute modified sensilla coelocapitula occurring in small numbers in the cavities too, most probably serve as thermoreceptors. As a result the placing of sensilla deserving protection in closeable cavities and the reduction in number and length of the external sensilla can be interpreted as adaptations to the fire habitat.

Status of and Future Research on Thermosensory Processing

Thermosensation is critically important for survival of all animals. In the cockroach Periplaneta americana, thermoreceptor neurons on antennae and thermosensory interneurons in the antennal lobe have been characterized electrophysiologically, and recent studies using advanced transgenic technologies in the fruit fly Drosophila melanogaster have added much to the knowledge of these neurons, enabling us to discuss common principles of thermosensory processing systems in insects. Cockroaches and many other insects possess only one type of thermoreceptor neurons on antennae that are excited by cooling and inhibited by warming. In contrast, the antennae of fruit flies and other dipterans possess oppositely responding warm and cold receptor neurons. Despite differences in their thermoreceptive equipment, central processing of temperature information is much the same in flies and cockroaches. Axons of thermoreceptor neurons project to the margin of the antennal lobe and form glomeruli, from which cold, warm and cold-warm projection neurons originate, the last neurons being excited by both cooling and warming. Axons of antennal lobe thermosensory projection neurons of the antennal lobe terminate in three distinct areas of the protocerebrum, the mushroom body, lateral horn and posterior lateral protocerebrum, the last area also receiving termination of hygrosensory projection neurons. Such multiple thermosensory pathways may serve to control multiple forms of thermosensory behavior. Electrophysiological studies on cockroaches and transgenic approaches in flies are encouraged to complement each other for further elucidating general principles of thermosensory processing in the insect brain.

Detection of Minute Temperature Transients by Thermosensitive Neurons in Ants

The antennae of leaf-cutting ants are equipped with sensilla coeloconica that house three receptor neurons, one of which is thermosensitive. Using convective heat (air at different temperatures), we investigated the physiological characteristics of the thermosensitive neuron associated with the sensilla coeloconica in the leaf-cutting ant Atta vollenweideri. The thermosensitive neuron very quickly responds to a drop in temperature with a brief phasic increase (50 ms) in spike rate and thus classifies as cold receptor (ambient temperature = 24°C). The short latency and the brief phasic response enable the thermosensitive neuron to follow temperature transients up to an estimated frequency of around 5 Hz. Although the neuron responds as a cold receptor, it is extremely sensitive to warm stimuli. A temperature increase of only 0.005°C already leads to a pronounced decrease in the resting activity of the thermosensitive neuron. Through sensory adaptation, the sensitivity to temperature transients is maintained over a wide range of ambient temperatures (18–30°C). We conclude that the thermosensitive neuron of the sensilla coeloconica is adapted to detect minute temperature transients, providing the ants with thermal information of their microenvironment, which they may use for orientation.

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.

Is there a fatty acid taste?

Taste is a chemical sense that aids in the detection of nutrients and guides food choice. A limited number of primary qualities comprise taste. Accumulating evidence has raised a question about whether fat should be among them. Most evidence indicates triacylglycerol is not an effective taste stimulus, though it clearly contributes sensory properties to foods by carrying flavor compounds and altering texture. However, there is increasing anatomical, electrophysiological, animal behavior, imaging, metabolic, and psychophysical evidence that free fatty acids are detectable when non-taste cues are minimized. Free fatty acids varying in saturation and chain length are detectable, suggesting the presence of multiple transduction mechanisms and/or a nonspecific mechanism in the oral cavity. However, confirmation of "fatty" as a taste primary will require additional studies that verify these observations are taste specific. Oral exposure to free fatty acids likely serves as a warning signal to discourage intake and influences lipid metabolism.