Morphological and Ultrastructural Characterization of Antennal Sensilla and the Detection of Floral Scent Volatiles in Eupeodes corollae (Diptera: Syrphidae)

The olfactory sensing system of the syrphid fly Eupeodes corollae is essential in pollination and prey localization, but little is known about the ultrastructural organization of their olfactory organs. In this study, the morphology, distribution, and ultrastructural organization of antennal sensilla of E. corollae in both sexes were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Neuronal responses of a subtype of sensilla basiconica to floral scent compounds were recorded by single sensillum recording (SSR). Ten morphological types, including Böhm bristles, sensilla chaetica, microtrichiae, sensilla trichodea, sensilla basiconica, sensilla clavate, sensilla coeloconica, sensilla styloconica, sensilla placodea, and sensory pit, were identified. Except for Böhm bristles and sensilla chaetica, which were distributed on the scape and pedicel of E. corollae antennae, innervated sensilla were densely distributed on the flagellum, a vital sensory organ. Further, observing ultrastructural organization showed that the sensilla trichodea, basiconica, and clavate are single-walled with multiple nanoscale pores perforating the cuticle. Sensilla coeloconica are double-walled and have no wall pores, but instead, have longitudinal grooves along with the pegs. Sensilla chaetica, Böhm bristles, and microtrichiae did not have wall pores on the cuticle or sensory cells at the base. The SSR results indicated that neuron B housed in the subtype of sensilla basiconica I (SBI) mainly responded to methyl eugenol and other aromatic compounds. Overall, our results provide valuable information to understand the morphology and ultrastructure of antennal sensilla from E. corollae. These findings are beneficial for the studies of the neuronal function map of olfactory sensilla and for determining evolutionary relationships in Diptera.

Morphology of the Antennal Sensilla of Notonectoidea and Comparison of Evolutionary Changes in Sensilla Types and Distribution in Infraorder Nepomorpha (Insecta: Heteroptera)

This article introduces the results of a study of three families of Nepomorpha and is the last part of a series of studies that sums up our work on the morphologies of the antennal sensory structures in this taxon. The morphologies and distribution of the sensilla in the families Notonectidae, Pleidae and Helotrephidae were studied under a scanning electron microscope. Six main types (sensilla trichodea, chaetica, campaniformia, basiconica, ampullacea and coeloconica) and ten subtypes (five subtypes of sensilla trichodea and five subtypes of sensilla basiconica) were described. The results were compared with other studies on the antennal sensilla of Nepomorpha in order to assess evolutionary changes within the infraorder. With the use of cladistics analysis, the monophyly of the families Nepidae, Micronectidae, Corixidae and Gelastocoridae was supported. On the other hand, the occurrence of some clades forming superfamilies was weakly supported by bootstrap analysis. These results, supported by presence of the numerous autapomorphies, suggest that antennal sensilla evolved within inner groups.

Ultramorphological Comparison of Proboscis and Associated Sensilla of Scotogramma trifolii and Protoschinia scutosa (Lepidoptera: Noctuidae)

The proboscis is an important feeding organ for the glossatan moths, mainly adapted to the flower and non-flower visiting habits. The clover cutworm, Scotogramma trifolii Rottemberg, and the spotted clover moth, Protoschinia scutosa (Denis & Schiffermuller), are serious polyphagous pests, attacking numerous vegetables and crops, resulting in huge economic losses. However, the feeding behavior and mechanisms of the adult stage remain unsatisfactorily explored. In this study, the proboscis morphology of S. trifolii and P. scutosa are described in detail using scanning electron microscopy, with the aim of investigating the morphological differences and feeding behavior of these two species. The proboscises of S. trifolii and P. scutosa are similar in morphology and structure and are divided into three zones (Zone 1–3) based on the morphological changes of the dorsal legulae. Three sensillum types are located on the proboscises of both species, sensilla chaetica, sensilla basiconica, and sensilla styloconica. Significant differences were observed in the length of the proboscis and each zone between these two species, as well as in sensilla size and number. Based on the morphology of the proboscis and associated sensilla, S. trifolii and P. scutosa are potential flower visitors, which was also reinforced by the pollen observed at the proboscis tip. These results will strengthen our understanding of the structure of the proboscis related to the feeding behavior of Noctuidae.

The variability of antennal sensilla in Naucoridae (Heteroptera: Nepomorpha)

The morphology and distribution of sensilla on the surface of the antennae of the naucorids’ species were studied via scanning electron microscopy. Eleven types of sensilla were identified regarding specific sensory modalities, based on their cuticular morphology. Cuticle morphology identifies five types of sensilla trichodea, four types of sensilla basiconica, one type of sensillum coeloconicum and sensillum ampullaceum. Three new types of mechanosensitive sensilla were found. Moreover, the morphological diversity between the antennae allowed the distinction of ten different antennal types that correspond to different sensillar sets. The sensilla found in Naucoridae share similarities with the sensilla of other nepomorphan taxa, as well as of terrestrial insects. However, no sensillar synapomorphy was found between Naucoridae and Aphelocheiridae.

Morphology and Distribution of Antennal Sensilla in Three Species of Thripidae (Thysanoptera) Infesting Alfalfa Medicago sativa

Thrips are important pests to alfalfa Medicago sativa. Similar as many other plant-feeding insects, thrips rely on the antennae to receive chemical signals in the environment to locate their hosts. Previous studies indicated that sensilla of different shapes on the surface of insect antenna play an important role in signal recognition. However, morphological analysis of the antennal sensilla in Thysanoptera has been limited to only a few species. To expand the understanding of how antennal sensilla are related to semiochemical detection in thrips, here we compared the morphology and distribution of antennal sensilla in three thrip species, Odontothrips loti, Megalurothrips distalis, and Sericothrips kaszabi, by scanning electron microscope (SEM). The antennae of these three species are all composed of eight segments and share similar types of sensilla which distribute similarly in each segment, despite that their numbers show sexual dimorphism. Specifically, nine major types of sensilla in total were found, including three types of sensilla basiconica (SBI, SBII, and SBIII), two types of sensilla chaetica (SChI and SChII), and one type for each of sensilla coeloconica (SCo), sensilla trichodea (ST), sensilla campaniformia (SCa), and sensilla cavity (SCav). The potential functions of sensilla were discussed according to the previous research results and will lay a morphological foundation for the study of the olfactory mechanism of three species of thrips.

Antennal Sensilla of Chrysis shanghalensis (Hymenoptera: Chrysididae), a Larval Ectoparasite of Monema flavescens (Lepidoptera: Limacodidae)

Chrysis shanghalensis Smith (Hymenoptera: Chrysididae) is an ectoparasitoid and important natural enemy of Monema flavescens Walker (Lepidoptera: Limacodidae), a serious defoliator of a number of tree species. The external morphology of the antennal sensilla of this parasitoid was examined by scanning electron microscopy, and types and distribution of sensilla were recorded. The antennae of C. shanghalensis were geniculate in shape and composed of a scape with radicula, a pedicel, and a flagellum divided into 11 flagellomeres in males and females. Cuticular pore and 14 types of sensilla were identified on the antennae of both sexes. These included aporous Böhm's bristles, sensilla trichodea 1, 2, and 3 (nonporous), sensilla trichodea 4 (multiporous), two types of sensilla chaetica (nonporous), four morphological types of sensilla basiconica (multiporous), two morphological types of sensilla coeloconica (multiporous), and one type of sensillum campaniforme (nonporous).

Sensation of the tiniest kind: the antennal sensilla of the smallest free-living insect Scydosella musawasensis (Coleoptera: Ptiliidae)

Miniaturization is a major evolutionary trend prominent in insects, which has resulted in the existence of insects comparable in size to some unicellular protists. The adaptation of the complex antennal multisensory systems to extreme miniaturization is a fascinating problem, which remains almost unexplored. We studied the antennal sensilla of Scydosella musawasensis Hall, 1999 (Coleoptera: Ptiliidae), the smallest free-living insect, using scanning electron microscopy. The antenna of S. musawasensis bears 131 sensilla; no intraspecific variation in the number or position of the sensilla has been revealed. Nine different morphological types of sensilla are described according to their external morphological features and distribution: four types of sensilla trichodea, one type of sensilla chaetica, two types of sensilla styloconica, and two types of sensilla basiconica. Morphometric analysis of the sensilla of S. musawasensis, based on measurements of the lengths and diameters of sensilla and their location and number, showed the absence of significant differences between females and males. Comparative allometric analysis of S. musawasensis and larger Coleoptera showed that the number of sensilla and the size of sensilla chaetica decrease with decreasing body size. However, the number of the types of sensilla and the length and diameter of the multiporous sensilla basiconica revealed no correlation with the body size. Comparison of the acquired data with the results of our earlier study of the antennal sensilla of some of the smallest parasitic wasps is used to put forward hypotheses on the common principles of miniaturization of the antennal sensory systems of insects.

Comparative Morphology of the Mouthparts in Three Predatory Stink Bugs (Heteroptera: Asopinae) Reveals Feeding Specialization of Stylets and Sensilla

Mouthpart structures were observed in three species of Asopinae using scanning electron microscopy to investigate their morphological disparity. The examined species attack mainly slow-moving, soft-bodied insects, primarily larval forms of the Lepidoptera, and are the natural enemies of many pests. This is the first detailed description of their external mouthparts. The triangular and elongated labrum and four-segmented tube-like labium are longer in Picromerus species (Picromerus bidens (Linnaeus, 1758) and Picromerus lewisi Scott, 1874 than in Cazira bhoutanica Schouteden, 1907. The labrum of P. lewisi and C. bhoutanica appear to be equipped with olfactory sensilla basiconica Sb3, a special type of sensilla with nanopores. The labium surface in all studied species bears 14 types of sensilla (St1–St4, Sb1–7, Sst, Sca1–2). A new characteristic of sensilla trichodea is represented in sensillum St1; in both Picromerus species, it is classified as an olfactory sensillum with nanopores. The tripartite apex of the labium consists of two lateral lobes and a central membranous lobe having microtrichial extensions. Each lobe has one sensory field, including sensilla basiconica (Sb7), sensilla styloconica (Sst), and sensilla trichodea (St4). In the three studied predatory stink bugs, each mandibular stylet tip has five irregular teeth and three long, pointed hooks. The two opposing maxillae, which are held together by a tongue-and-groove system, form a food canal and a salivary canal. The apices of the right maxilla have small teeth and few short barbs along the edge of the food canal. In P. bidens and P. lewisi, there are 5 teeth, while in C. bhoutanica there are 2. Based on structural differences, we inferred that the hook-shaped mandibular teeth, right maxilla with small teeth, and few short barbs along edge of the food canal are more adapted for a predatory lifestyle. Predatory stink bugs use sharp recurved hooks and irregular teeth penetrating, tearing, or filing devices that aid in the mechanical disruption of host tissue. Stiff bristles in the food canal may indicate their possible adaptation to feeding on insect larvae. The evolution of mouthpart morphology and the putative functional significance of sensilla are discussed, providing insight into the sensory mechanism.

Scanning Electron Microscopy Reveals the Antennal Micromorphology of Lamprodila (Palmar) festiva (Coleoptera: Buprestidae), an Invasive Pest of Ornamental Cupressaceae in Western Palaearctic

The Cypress jewel beetle, Lamprodila (Palmar) festiva festiva (Linnaeus, 1767), is a serious invasive pest of ornamental Cupressaceae, which has recently expanded its range from the Mediterranean region northwards to central and eastern Europe, and to the Russian Black Sea coast. In this study, we conducted a scanning electron microscopy study of the micromorphology of the male and female antennae of L. festiva to examine the morphology, numbers, distribution, and possible functions of antennal sensilla. Most sensilla are located in the sensory fields within the apical depressions on antennomeres IV–XI. We identified four main types of antennal sensilla in L. festiva: sensilla chaetica (seven subtypes, of which two occur only in females), sensilla basiconica (five subtypes), multiporous grooved pegs (two subtypes), and Böhm sensilla. Females have relatively more sensilla chaetica and multiporous grooved pegs, whereas males have more sensilla basiconica. We discuss possible functions of all examined sensilla and compare them with those in other Buprestidae or other insects. Our study should serve as background information for advanced electrophysiological and behavioral experiments to better understand the functions of different sensilla and mechanisms related to semiochemically based pest control strategies.

How Does the Intricate Mouthpart Apparatus Coordinate for Feeding in the Hemimetabolous Insect Pest Erthesina fullo?

The yellow marmorated stink bug, Erthesina fullo (Thunberg, 1783), is a major pest of certain tree fruits in Northeast Asia. To better understand the feeding mechanism of E. fullo, the fine structure of the mouthparts, including the distribution and abundance of sensilla, are examined with scanning electron microscopy (SEM), and their functions are observed directly under laboratory conditions. The feeding performance is described in detail and illustrated for the first time. The adult feeding process involves several steps, including exploring and puncturing of the host plant epidermis, a probing phase, an engorgement phase, and removal of the mouthparts from the host tissue. Proceeding from labium towards the mandibular stylets, the movement pattern becomes increasingly stereotypical, including the sensilla on the tip of the labium probing, the labium making an elbow-like bend between the first and second segment, the base of the stylet fascicle housing in the groove of the labrum, the mandibular stylets penetrating the site and maxillary stylets feeding. In terms of morphology, the mouthparts are similar to those of other Heteroptera, consisting of a triangular pyramidal labrum, a tube-like and segmented labium with a deep groove on the anterior side, and a stylet fascicle consisting of two mandibular and two maxillary stylets. The four-segmented labium has five types of sensilla basiconica, three types of sensilla trichodea, two types of sensilla campaniformia and 1 type of sensilla coeloconica. Among them, sensilla trichodea one and sensilla basiconica one are most abundant. The tripartite apex of the labium is covered with abundant sensilla trichodea three and a few sensilla basiconica 5. The mandibular stylet tips have two nodules preapically on the dorsal margin of the convex external surface, which may help in penetrating plant tissue and anchoring the mouthparts. The externally smooth maxillary stylets interlock to form a larger food canal and a smaller salivary canal. The structure and function of the mouthparts are adapted for the phytophagous feeding habit in this species. Similarities and differences between the mouthparts of E. fullo and those of other Heteroptera are discussed.