Ultrastructure and electrophysiology of thermosensitive sensilla coeloconica in a tropical katydid of the genus Mecopoda (Orthoptera, Tettigoniidae)

Abstract

In many acoustic insects, mate finding and mate choice are primarily based on acoustic signals. In several species with high-intensity calling songs, such as the studied katydid Mecopoda sp., males exhibit an increase in their thoracic temperature during singing, which is linearly correlated with the amount of energy invested in song production. If this increased body temperature is used by females as an additional cue to assess the male's quality during mate choice, as has been recently hypothesized ("hot-male" hypothesis), thermosensory structures would be required to evaluate this cue. In the present study, therefore, we investigated the ultrastructure and physiology of thermosensitive sensilla coeloconica on the antennal flagella of Mecopoda sp. using a combination of electron microscopy and electrophysiological recording techniques. We could identify three distinct types of sensilla coeloconica based on differences in the number and branching pattern of their dendrites. Physiological recordings revealed the innervation by antagonistically responding thermoreceptors (cold and warm) and bimodal hygro-/thermoreceptors (moist or dry) in various combinations. Our findings indicate that Mecopoda sp. females are capable of detecting a singing male from distances of at least several centimetres solely by assessing thermal cues.

Neural Code for Ambient Heat Detection in Elaterid Beetles

“…Most insects need a sensitive thermo-sensory system to reliably sense ambient heat so as to avoid thermal stress. Besides antennal DSSs of carabids and elaterids, the classical sensory triad of thermo-and hygroreceptor neurons resides in various morphological types of antennal sensory organs, for example, in coeloconic sensilla widespread on the antennae in many insect taxa (Altner and Prillinger, 1980;Chapman, 1998;Ruchty et al, 2009;Schneider et al, 2018). The neurons of the sensory triad in coeloconic sensilla probably also use various spike burst patterns to encode ambient heat, but so far, this is not verified.…”

“…Adequate information on ambient temperature is of vital importance to exhibit a proper behavioral response but very little is known of the neural pathways and coding of noxious heat by peripheral thermoreceptor neurons in these arthropods (Dhaka et al, 2006;Tang et al, 2013;Must et al, 2017;Nurme et al, 2018). The sensory cells responsible for detection of external temperature are located in various morphological types of cuticular structures on the insect antennae classified as sensilla coeloconica, basiconica, trichodea, styloconica, capitula, coelocapitula, dome shape sensilla, et cetera (Altner and Prillinger, 1980;Altner and Loftus, 1985;Ruchty et al, 2009;Di Giulio et al, 2012;Nagel and Kleineidam, 2015;Zauli et al, 2016;Must et al, 2017;Nurme et al, 2018;Schneider et al, 2018). In a sensillum, a single thermoreceptor (cold) neuron usually combines with two antagonistically responding hygroreceptor neurons, the moist air and the dry air neuron, respectively, forming a sensory triad.…”

Insect evolution toward aquatic habitats; reassessment of antennal sensilla in the water bug families Ochteridae, Gelastocoridae and Aphelocheiridae (Hemiptera: Heteroptera: Nepomorpha)

“…The thermo-hygroreceptive sensilla found in the studied families (ampulacea or aporous sensilla coeloconica) are mostly the same types and functions, although their size and how they are embedded is different. Aporous sensilla coeloconica on the flagellum can also be regarded as playing a thermo-hygroreceptive role, in other insects/heteropteran species: Nezara viridula (Pentatomidae), Odontopus nigricornis (Pyrrhocoridae), Cyclopelta siccifolia (Dinidoridae),Chrysocoris purpureus (Scutelleridae), Piezodorus guildinii (Pentatomidae), Leptoglossus zonatus (Coreidae) as well as in some gerromorphan species (Usha Rani & Madhavendra, 1995Brézot et al, 1997;Chapman, 1998;Silvaet al, 2010Ruchty, 2009Gonzaga-Segura et al, 2013Nowińska & Brożek, 2017Schneider, 2018). For terrestrial heteropteran insects, another common type of mechanoreceptive sensilla is sensilla trichodea, but usually in different lengths and even distributions on all antennomeres, similar to mechanoreceptive sensilla basiconica.…”

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

“…However, sensilla coeloconica or s. coelocapitula innervated by only two hygroreceptors have not been described so far (Tichy and Loftus 1996;Tichy et al 2017;Enjin 2017). Therefore, we postulate that the sensillum coelocapitulum of Merimna is innervated by two antagonistically acting thermoreceptors as described for sensilla coeloconica in some other insects (Davis and Sokolove 1975;McIver and Siemicki 1985;Zopf et al 2014;Schneider et al 2018).…”

“…It can be speculated that it consists of the same material as the ds and, therefore, has been sequestered by the thecogen cell. Electron-dense material and especially a thick ds seems to be characteristic for many thermo-/hygroreceptors (Altner et al 1981(Altner et al , 1983Yokohari 1983;Kreiss et al 2005;Schneider et al 2018).…”