Physiology and central projections of putative thermosensitive multipolar neurons in the 'Australian Firebeetle' Merimna atrata: solving the processing of peripheral temperature information in the insect brain
Sensory and Behavioural Biology
Final Report Abstract
The project focussed on peripheral sensory neurons in insects that project with their primary afferents directly into the protocerebrum (PC) of the brain. It was known that in the legs of the locust Locusta migratoria 2 - 3 multipolar neurones (MN) are localised, whose numerous dendrites form a compact, so-called terminal dendrite mass (TDM), which remains close to the soma. These MN project with their primary afferents into a special neuropil in the PC, which has been termed the superficial ventral inferior protocerebrum (SVIP). However, the function of such MN was unknown. Some older neuroanatomical studies have also described fibres in the antennae and labial palps of various orthopterans, which also ramify in the SVIP; the associated neurons have not yet been identified. In the pyrophilious "Australian fire beetle" Merimna atrata, MNs with a TDM were also described, which function as thermoreceptors in the abdominal infrared organs. Here, however, nothing was known about the central projections. Due to the structural similarities of the MNs in the legs of Locusta with those in the IR organ of Merimna, a thermoreceptive function was also postulated for the MNs in the legs of Locusta. Therefore, it was assumed that the previously unknown neurones in the antennae and palps were also MNs with a TDM. Within the framework of the project, the hypothesis was formulated that peripherally arranged MN with a TDM in the body appendages such as legs, antennae and palps, but also in the IR organ of Merimna, form a system of peripheral thermoreceptors, whereby the neurones always project into the protocerebrum. The results obtained in the project show that fibres that obviously originate from the MNs in the IR organ of Merimna also ramify in a ventrally located neuropil in the PC of the beetle. Due to the anatomical differences between the brains of Merimna and Locusta, it is postulated that this could be a neuropil that is functionally analogous to the SVIP. Furthermore, it could be shown that the fibres in the nerves of the antennae and labial palps ramifying in the SVIP of Locusta belong to distally arranged scolopidia. An electron microscopic examination of the antennal sensilla of Merimna also revealed no evidence of MNs with a TDM in distal antennal region. Therefore, the hypothesis of a peripheral system of thermoreceptive MNs with TDM in the body appendages cannot be upheld. For the internal scolopidia, a possible function for measuring haemolymph pressure in the antennal and palp tips was postulated. Extracellular electrophysiological recordings from the leg nerves of Locusta showed that relatively sensitive cold receptors are present here. It can be assumed with high probability that the responsible receptors are the MNs with a TDM.
Publications
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Unusual discovery of the ‘Australian Firebeetle’ Merimna atrata on an older postfire area. Records of the Western Australian Museum, 35(1), 50.
Schmitz, Anke & Schmitz, Helmut
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Sensory equipment and adaptations to the fire habitat of the antennae of the Australian ´firebeetle´ Merimna atrata (Coleoptera; Buprestidae). Zoomorphology, 140(4), 487-504.
Schmitz, Anke & Schmitz, Helmut
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Neuroanatomy and functional morphology of peripheral receptor neurones with direct projections into the protocerebrum of the brains of the locust and a jewel beetle. Zoomorphology, 142(3), 341-356.
Hinz, Marcel; Schmitz, Anke & Schmitz, Helmut
