Sensing of Substrate Vibrations in the Adult Cicada Okanagana rimosa (Hemiptera: Cicadidae)

Vibration receptor organs in the insect leg: neuroanatomical diversity and functional principles

Detecting substrate vibrations is essential for insects in different behavioural contexts. These vibrational behaviours are mediated by mechanoreceptor organs detecting and processing vibrational stimuli transmitted in the environment. We discuss recently gained insights about the functional principles of insect vibration receptors, mainly leg chordotonal organs highly sensitive to vibrational stimuli, and the mechanisms of their diversification in neuroanatomy and functional morphology, in relation to the attachment structures and mechanical coupling. The two main input pathways for vibration stimuli transferred by the insect legs to vibrosensory organs via the cuticle and via the hemolymph are fundamental for explaining sensory specialisations. The vibroreceptor organs can diversify in their neuroanatomy and morphology in several key aspects. This provides the structural basis for complex adaptations in sensory evolution.

Phylogeny of North Americas largest cicada radiation redefines Tibicinoides and Okanagana (Hemiptera: Auchenorrhyncha: Cicadidae: Tibicininae)

Tibicinoides, with three small endemic California cicada species, has a confusing, intertwined systematic history with Okanagana that we unravel here. An ingroup including all species of Tibicinoides and the majority (84.7%) of Okanagana species were sampled for six gene regions, polarized with Clidophleps, Okanagodes, Subpsaltria, and Tibicina outgroups, and subjected to Bayesian phylogenetic analysis. Although the ingroup was monophyletic from all outgroups including Tibicina, Tibicinoides rendered Okanagana paraphyletic among two major ingroup clades. To bring classification into agreement with phylogeny, we redescribe and redefine Tibicinoides to include all Okanagana species with a hooked uncus in the male genitalia, all of which grouped with the type T. cupreosparsa (Uhler, 1889) in the first of these clades: T. boweni (Chatfield-Taylor & Cole, 2020) comb. n., T. catalina (Davis, 1936) comb. n., T. hesperia (Uhler, 1876) comb. n., T. mercedita (Davis, 1915), T. minuta (Davis, 1915), T. pallidula (Davis, 1917a) comb. n., T. pernix (Bliven, 1964) comb. n., T. rubrovenosa (Davis, 1915) comb. n., T. simulata (Davis, 1921) comb. n., T. striatipes (Haldeman, 1852) comb. n., T. uncinata (Van Duzee, 1915) comb. n., T. utahensis (Davis, 1919) comb. n., and T. vanduzeei (Distant, 1914) comb. n. Okanagana is redescribed and restricted to the species of the second major clade which contained the type O. rimosa (Say, 1830). We describe two new genera for morphologically distinct orphan lineages: Chlorocanta gen. nov. for C. viridis (Davis, 1918) comb. n. and Hewlettia gen. nov. for H. nigriviridis (Davis, 1921) comb. n. We recognize O. rubrobasalis Davis, 1926 stat. rev. as a species and relegate two former species to junior subjective synonyms: O. noveboracensis (Emmons, 1854) = O. canadensis (Provancher, 1889) and O. occidentalis (Walker in Lord, 1866) = O. lurida Davis, 1919. Tibicinoides and Okanagana together represent a rapid radiation that presents challenges to phylogenetic analysis including suboptimal outgroups and short internodes.

Ambush predation and the origin of euprimates

Primates of modern aspect (euprimates) are characterized by a suite of characteristics (e.g., convergent orbits, grasping hands and feet, reduced claws, and leaping), but the selective pressures responsible for the evolution of these euprimate characteristics have long remained controversial. Here, we used a molecular phyloecological approach to determine the diet of the common ancestor of living primates (CALP), and the results showed that the CALP had increased carnivory. Given the carnivory of the CALP, along with the general observation that orbital convergence is largely restricted to ambush predators, our study suggests that the euprimate characteristics could have been more specifically adapted for ambush predation. In particular, our behavior experiment further shows that nonclaw climbing can significantly reduce noises, which could benefit the ancestral euprimates' stalking to ambush their prey in trees. Therefore, our study suggests that the distinctive euprimate characteristics may have evolved as their specialized adaptation for ambush predation in arboreal environments.

Vibration detection in arthropods: Signal transfer, biomechanics and sensory adaptations

In arthropods, the detection of vibrational signals and stimuli is essential in several behaviours, including mate recognition and pair formation, prey detection, and predator evasion. These behaviours have been studied in several species of insects, arachnids, and crustaceans for vibration production and propagation in the environment. Vibration stimuli are transferred over the animals' appendages and the body to vibrosensory organs. Ultimately, the stimuli are transferred to act on the dendrites of the mechanosensitive sensilla. We refer to these two different levels of transfer as macromechanics and micromechanics, respectively. These biomechanical processes have important roles in filtering and pre-processing of stimuli, which are not carried out by neuronal components of sensory organs. Also, the macromechanical transfer is posture-dependent and enables behavioural control of vibration detection. Diverse sensory organs respond to vibrations, including cuticular sensilla (slit sensilla, campaniform sensilla) and internal chordotonal organs. These organs provide various adaptations, as they occur at diverse body positions with different mechanical couplings as input pathways. Macromechanics likely facilitated evolution of vibrosensory organs at specific body locations. Thus, vibration detection is a highly complex sensory capacity, which employs body and sensory mechanics for signal filtering, amplification, and analysis of frequency, intensity and directionality.

Large abdominal mechanoreceptive sense organs in small plant-dwelling insects

The Hemiptera, with approximately 98 000 species, is one of the largest insect orders. Most species feed by sucking sap from plant tissues and are thus often vectors for economically important phytopathogens. Well known within this group are the large cicadas (Cicadomorpha: Cicadoidea: Cicadidae) because they produce extremely loud airborne sounds. Less well known are their mostly tiny relatives, the leafhoppers, spittlebugs, treehoppers and planthoppers that communicate by silent vibrational signals. While the generation of these signals has been extensively investigated, the mechanisms of their perception are poorly understood. This study provides a complete description and three-dimensional reconstruction of a large and complex array of mechanoreceptors in the first abdominal segments of the Rhododendron leafhopper Graphocephala fennahi (Cicadomorpha: Membracoidea: Cicadellidae). Further, we identify homologous organs in the spittlebug Philaenus spumarius (Cicadomorpha: Cercopoidea: Aphrophoridae) and the planthopper Issus coleoptratus (Fulgoromorpha: Fulgoroidea: Issidae). Such large abdominal sensory arrays have not been found in any other insect orders studied so far. This indicates that these sense organs, together with the signal-producing tymbal organ, constitute a synapomorphy of the Tymbalia (Hemiptera excl. Sternorrhyncha). Our results contribute to the understanding of the evolution from substrate-borne to airborne communication in insects.