Sensing the Underground – Ultrastructure and Function of Sensory Organs in Root-Feeding Melolontha melolontha (Coleoptera: Scarabaeinae) Larvae

Abstract

IntroductionBelow ground orientation in insects relies mainly on olfaction and taste. The economic impact of plant root feeding scarab beetle larvae gave rise to numerous phylogenetic and ecological studies. Detailed knowledge of the sensory capacities of these larvae is nevertheless lacking. Here, we present an atlas of the sensory organs on larval head appendages of Melolontha melolontha. Our ultrastructural and electrophysiological investigations allow annotation of functions to various sensory structures.ResultsThree out of 17 ascertained sensillum types have olfactory, and 7 gustatory function. These sensillum types are unevenly distributed between antennae and palps. The most prominent chemosensory organs are antennal pore plates that in total are innervated by approximately one thousand olfactory sensory neurons grouped into functional units of three-to-four. In contrast, only two olfactory sensory neurons innervate one sensillum basiconicum on each of the palps. Gustatory sensilla chaetica dominate the apices of all head appendages, while only the palps bear thermo-/hygroreceptors. Electrophysiological responses to CO2, an attractant for many root feeders, are exclusively observed in the antennae. Out of 54 relevant volatile compounds, various alcohols, acids, amines, esters, aldehydes, ketones and monoterpenes elicit responses in antennae and palps. All head appendages are characterized by distinct olfactory response profiles that are even enantiomer specific for some compounds.ConclusionsChemosensory capacities in M. melolontha larvae are as highly developed as in many adult insects. We interpret the functional sensory units underneath the antennal pore plates as cryptic sensilla placodea and suggest that these perceive a broad range of secondary plant metabolites together with CO2. Responses to olfactory stimulation of the labial and maxillary palps indicate that typical contact chemo-sensilla have a dual gustatory and olfactory function.

Development of mouthparts in the cicada Meimuna mongolica (Distant): successive morphological patterning and sensilla differentiation from nymph to adult

“…Studies of Coleoptera1617, Diptera18, Dermaptera19, Lepidoptera2021 and Mecoptera22, have highlighted dramatic differences in larval mouthpart morphology related to different feeding habits22. Unfortunately, mouthpart morphology of hemipteran nymphs remains little studied.…”

Comparative morphology of sensilla on antenna, maxillary palp and labial palp of larvae of Eucryptorrhynchus scrobiculatus (Olivier) and E. brandti (Harold) (Coleoptera: Curculionidae)

“…The neuronal structure of digitiform sensilla was similar to that of the neurons for hygro-/thermo-receptors (Keil 1996;Eilers et al 2012;Ortloff et al 2014), and, based on electrophysiological experiments, the strongest reactions of the maxillary palp and labial palp were clearly to CO 2 (Honomichl and Guse 1981). The neuronal structure of digitiform sensilla was similar to that of the neurons for hygro-/thermo-receptors (Keil 1996;Eilers et al 2012;Ortloff et al 2014), and, based on electrophysiological experiments, the strongest reactions of the maxillary palp and labial palp were clearly to CO 2 (Honomichl and Guse 1981).…”

“…Sensilla basiconica are also found on Anoplophora glabripennis Motschulsky (Coleoptera: Cerambycidae), Dendroctonus valens LeConte (Coleoptera: Curculionidae, Seolytinae) and (Chen et al 2010;Eilers et al 2012;Xu et al 2017). From Coleoptera, we described the sensilla on the antenna, maxillary palp and labial palp of E. scrobiculatus and E. brandti larvae for the first time.…”

“…On the antennae of E. scrobiculatus and E. brandti larvae, compared with Anoplophora glabripennis larvae, S.tb.1 were similar to 'S.t.b.4', S.tb.2 to 'S.t.b.1', S.tb.3 to 'S.t.b.2' and S.tb.5 to 'S.t.b.6'. Eilers et al (2012) showed that sensilla twig basiconica have a tubular body and also function in mechanoreception (Eilers et al 2012). Based on the apical pore of S.tb.1 and S.tb.2, these sensilla might function as gustatory receptors (Sen 1988), and the apical and radial channels of S.tb.3 and S.tb.4 were very similar to sensilla coeloconicum, indicating these sensilla might function in the reception of olfactory signals.…”

Aboveground endophyte affects root volatile emission and host plant selection of a belowground insect

“…Conclusively, this correlated with the enhanced performance of D. virgifera on infested roots (Robert et al 2012). Investigations into the olfactory apparatus and chemosensory capabilities of Melolontha hippocastani and Melolontha melolontha larvae suggested that these are as highly developed as in many adult insects (Eilers et al 2012;Weissteiner et al 2012).…”