Morphological Disparity of the Mouthparts in Polyphagous Species of Largidae (Heteroptera: Pentatomomorpha: Pyrrhocoroidea) Reveals Feeding Specialization

Microstructural Adaptation for Prey Manipulation in the Millipede Assassin Bugs (Hemiptera: Reduviidae: Ectrichodiinae)

Species in Ectrichodiinae are known for their prey specialization on millipedes. However, knowledge of the morphological adaptations to this unique feeding habit was limited. In the current study, we examined the microstructures of the antennae, mouthparts, and legs of four millipede feeding ectrichodiines, Ectrychotes andreae (Thunberg, 1888), Haematoloecha limbata Miller, 1953, Labidocoris pectoralis (Stål, 1863), and Neozirta eidmanni (Taueber, 1930), and compared them with those of three species of tribelocephalines, a group closely related to Ectrichodiinae. On the antennae, we found four types of antennal sensilla. On the mouthparts, we recognized four types of labial sensilla. Sampled ectrichodiines have distinctly more and denser slightly transverse ridges on the external side of mandibles than tribelocephalines. E. andreae and H. limbata possess numerous small papillae fringed with densely arranged finger-print-like grains on the trochanter and femur; these probably facilitate the immobilization of prey. Overall, our study illustrates, at a microstructural level, the remarkable morphological adaption of prey manipulation in ectrichodiine, and has enhanced our understanding about stenophagy in the family Reduviidae.

Morphological comparison of the sensilla on the maxillary and labial palps between male and female adults of Moechotypa diphysis (Coleoptera: Cerambycidae)

Moechotypa diphysis (Pascoe) is one of the most important pests in agriculture and forestry. However, there are few studies focusing on the external morphology of M. diphysis adults. In this study, the mouthparts of M. diphysis adults were observed by the scanning electron microscope to compare the quantity and distribution of the sensilla on the maxillary and labial palps. The results showed that there are four segments on the maxillary palps and three segments on the labial palps. The length of each segment of the maxillary and labial palps is larger in the female than that in the male. Six types of sensilla are found on the maxillary and labial palps of M. diphysis adults: sensilla basiconca (SB1, 2, 3, and 4), sensilla trichodea (ST1, 2 and 3), sensilla chaetica (SC), sensilla placodea (SP), hair plate (HP) and sensilla coeloconica (SCo). There is no significant difference in the number of most types of sensilla between females and males in the same position. However, the number of ST1 on the maxillary and labial palps is significantly higher in the female than in the male. In addition, the number of most types of sensilla (SB2, ST1, SC, SP, HP, and SCo) on the maxillary palps is significantly higher than on the labial palps both for females and males. Maxillary palps may play a more important role in the activities of M. diphysis adults than the labial palps. Based on this study, we discussed the functions of the sensilla on the maxillary and labial palps of M. diphysis adults to provide the theoretical basis and statistical data for further research on the behavior and the electrophysiology of this devastating forest pest.

Labial Sensory Organs of Two Leptoglossus Species (Hemiptera: Coreidae): Their Morphology and Supposed Function

A detailed description of the labial sensory organs of Leptoglossus occidentalis Heidemann, 1910 and L. zonatus (Dallas, 1852) (Hemiptera: Heteroptera: Coreidae) is presented. The detailed morphology, location, and distribution of different sensilla types on mouthparts were investigated and shown in micrographs taken with a scanning electron microscope. Nine types of aporous sensilla, and three uniporous sensilla were found. The possible functions of these sensilla as well as similarities and differences between the mouthparts of Leptoglossus and those of other terrestrial Heteroptera are discussed. The tip of the labium constitutes a functional "touch and taste area", combining the chemosensitivity of uniporous sensilla P1-P3 and the mechanoreceptivity of A8 and A9 hair-like sensilla. A set of two cone-like chemosensilla types (9 + 2) was found on each lateral lobe of the labial tip. Literature analysis showed that such a set of cone-like sensory organs on the labial tip may be common in terrestrial Heteroptera. This observation confirms that the number and arrangement of sensilla is conservative and can be important in diagnosing taxa at various levels and in phylogenetic studies based on morphology.

Fine-Structural Morphology of the Mouthparts of the Polyphagous Invasive Planthopper, Ricania speculum (Walker) (Hemiptera: Fulgoromorpha: Ricaniidae)

Mouthparts are the crucial sensory and feeding organs associated with food detection and feeding in insects. The Asian ricaniid planthopper Ricania speculum (Walker), recently introduced into Europe, can cause severe economic damage by sucking the phloem sap of tea, camphor, citrus, black locust and other plants using piercing-sucking mouthparts. To facilitate comprehensive understanding of feeding mechanisms in the Ricaniidae, the fine structure of the mouthparts of Ricania speculum was observed by scanning electron microscopy for the first time. The mouthparts are tubular, consist of a cone-shaped labrum, with a wrinkled epidermis and without sensilla; the tubular labium is divided into three segments: a slender stylet fascicle consisting of two mandibular stylets with four ridged processes and a row of longitudinal striations on the distal part of the outer surface; and two maxillary stylets with a smooth and sharp distal part, interlocked to form a larger food canal and a smaller salivary canal. On the labium, 15 kinds of sensilla of different functions were recognized. Two rows of short sensilla basiconica (SB I) are symmetrically distributed along the labial groove on the first segment. Two pairs of long sensilla basiconica (SB II) (proprioceptors) are on both sides of the labial groove at the junction of the second and third segments. A placoid, flattened sensillum (SPF) is symmetrically located laterally on the proximal end of the last segment and several flattened sensilla campaniformia (SFC) were visible on the ventral side on the second and third segments. The distribution of four types (I-IV) of sensilla cheatica of different lengths on the dorsal surface of the labium is significantly denser than on the lateral and ventral surfaces. The labial apex is divided into dorsal and ventral sensory fields, mainly including uniporous long peg sensilla (I), as well as smaller peg sensilla (II) and nonporous peg sensilla (PGSN) on each dorsal field. These nonporous sensilla basiconica (BSN I and III) occur on the ventral sensory fields and are constant in number and distribution. The nonporous sensilla basiconica (BSN II) are symmetrically arranged near the opening of the stylet fascicle similarly to two oval multiporous plate sensilla (OPSM). The sensilla arrangement is slightly different from that observed in previously studied Fulgoromorpha using scanning electron micrographs, which may reflect differences in feeding preference or behavior.

Diversification of the phytophagous lineages of true bugs (Insecta: Hemiptera: Heteroptera) shortly after that of the flowering plants

More than 95% of phytophagous true bug (Hemiptera: Heteroptera) species belong to four superfamilies: Miroidea (Cimicomorpha), Pentatomoidea, Coreoidea, and Lygaeoidea (all Pentatomomorpha). These iconic groups of highly diverse, overwhelmingly phytophagous insects include several economically prominent agricultural and silvicultural pest species, though their evolutionary history has not yet been well resolved. In particular, superfamily- and family-level phylogenetic relationships of these four lineages have remained controversial, and the divergence times of some crucial nodes for phytophagous true bugs have hitherto been little known, which hampers a better understanding of the evolutionary processes and patterns of phytophagous insects. In the present study, we used 150 species and concatenated nuclear and mitochondrial protein-coding genes and rRNA genes to infer the phylogenetic relationships within the Terheteroptera (Cimicomorpha + Pentatomomorpha) and estimated their divergence times. Our results support the monophyly of Cimicomorpha, Pentatomomorpha, Miroidea, Pentatomoidea, Pyrrhocoroidea, Coreoidea, and Lygaeoidea. The phylogenetic relationships across phytophagous lineages are largely congruent at deep nodes across the analyses based on different datasets and tree-reconstructing methods with just a few exceptions. Estimated divergence times and ancestral state reconstructions for feeding habit indicate that phytophagous true bugs explosively radiated in the Early Cretaceous-shortly after the angiosperm radiation-with the subsequent diversification of the most speciose clades (Mirinae, Pentatomidae, Coreinae, and Rhyparochromidae) in the Late Cretaceous.

Ultrastructural Variations of Antennae and Labia Are Associated with Feeding Habit Shifts in Stink Bugs (Heteroptera: Pentatomidae)

The family Pentatomidae (stink bugs) is one of the largest groups in Heteroptera, containing many important pests and natural enemies. They exhibit highly diversified feeding habits and related structural modifications, but the key morphological characteristics associated with feeding habit radiation remain unclear. In the current study, we address this question by analyzing morphological variations of feeding related organs. We compare the ultrastructures of antennae and mouthparts across the chosen 17 species in Pentatomidae, representing both plant feeders and predators from four subfamilies. A strong association between ultrastructural adaptation and feeding habit transition has been revealed. The long, sharp, and hook-like mandibular teeth and maxillary barbs are exclusively present in predatory Pentatomidae, suggesting their tight association with the shift of feeding habit from phytophagy to predation. Significant differences between phytophagous and predatory species are also found in antennal and labial sensilla types and arrangements, implying their important function in food selection. Our data identify a series of key morphological structures associated with feeding habit variations among stink bugs, which will facilitate future studies on adaptive evolution of feeding habits, utilization, and population control of economic species in Pentatomidae as well as in other heteropteran lineages.

External structures of the metathoracic scent gland efferent system in the true bug superfamily Pyrrhocoroidea (Hemiptera: Heteroptera: Pentatomomorpha)

Pyrrhocoroidea represents an important group of true bugs (Insecta: Hemiptera: Heteroptera) which includes fire bugs, cotton stainers and other taxa widely used in experimental studies or known as pests. However, the morphology and phylogeny of Pyrrhocoroidea have been only poorly studied so far. Here, structures of the external scent efferent system of the metathoracic scent glands are examined in 64 out of 71 currently valid genera of Pyrrhocoroidea and scanning electron micrographs are provided for most taxa. Several characters are revealed which define each of the three higher taxa within Pyrrhocoroidea: Larginae (small auriculate peritreme lacking manubrium and median furrow; metathoracic spiracle never surrounded by evaporatorium), Physopeltinae (large, widely open ostiole; large peritremal disc with manubrium [new term], lacking median furrow; mace-like mycoid filter processes of equal shape and size on both anterior and posterior margins of metathoracic spiracle), and Pyrrhocoridae (elongate auriculate peritreme with deep median furrow). Within Pyrrhocoridae, three main types (A, B and C) of the external scent efferent system are distinguished, differring in the amount of reductions. The findings are interpreted in the context of phylogenetic hypotheses available for Pyrrhocoroidea and their close relatives, Coreoidea and Lygaeoidea. An updated identification key to the families and subfamilies of Pyrrhocoroidea applicable for both sexes is provided.

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.

First description and comparison of the morphological and ultramicro characteristics of the antennal sensilla of two fir longhorn beetles

Allotraeus asiaticus Schwarzer and Callidiellum villosulum Fairmaire are repeatedly intercepted in wood and wood products all over the world. As two common stem borers of Cunninghamia lanceolata (Lambert) Hooker, to further understanding of the differences in their living habits, behaviors and the mechanism of insect-host chemical communication, we observed the external morphology, number and distribution of antennal sensilla of A. asiaticus and C. villosulum with scanning electron microscopy (SEM), respectively. The results showed that 1st-5th subsegments of the flagellum are spined endoapically in A. asiaticus which is different from the previous report (1st-3rd of the flagellomere). Meanwhile, there were five subsegments on the flagellum of C. villosulum that were clearly specialized as serrated shapes on the 4th-8th flagellomeres. Four types (ten subtypes) of sensilla were both found on the antennae of these two fir longhorn beetles, named Böhm bristle (Bb), sensilla trichodea (ST I and II), sensilla basiconica (SB I, II and III), sensilla chaetica (SCh I, II, III and IV). There is one additional kind of morphological type of sensilla found on the antennae of C. villosulum compared to A. asiaticus which was related to their habit of laying eggs only on dry and injured fir branches, named sensilla campaniformia (SCa). These differences may vary according to their own biological habits. For research purposes, the observed difference in the sensillum distribution and function between the two fir longhorn beetles will greatly facilitate the design of better semiochemical control methods of these insect pests.

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

Simple Summary

To better understand the feeding mechanism of Erthesina fullo, the fine structure of the mouthparts is examined with scanning electron microscopy, and feeding performance are observed directly under laboratory conditions 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. The morphology of mouthparts is similar to those of other Heteroptera. The four-segmented labium has eleven types of sensilla. The mandibular stylet tips have two nodules preapically on the convex external surface. The structure and function of the mouthparts are adapted for the phytophagous feeding habit in this species. This study increases the available detailed morphological and behavioral data for Hemiptera and will potentially contribute to improving our understanding of this pest’s feeding behavior and sensory mechanisms.

Abstract

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.

Unique Fine Morphology of Mouthparts in Haematoloecha nigrorufa (Stål) (Hemiptera: Reduviidae) Adapted to Millipede Feeding

Millipede assassin bugs are a diverse group of specialized millipede predators. However, the feeding behavior of Ectrichodiinae remains poorly known, especially how the mouthpart structures relate to various functions in feeding. In this study, fine morphology of the mouthparts and feeding performance of Haematoloecha nigrorufa (Stål, 1867) was observed and described in detail for the first time. The triangular labrum is divided by a conspicuous transverse membrane into a strongly sclerotized basilabrum and a less sclerotized distilabrum. Fifteen types of sensilla are distributed on the mouthparts. Each mandibular stylet has an expanded spatulate apex and about 150 approximately transverse ridges on the external middle side; these help in penetrating the ventral trunk area and the intersegmental membranes of millipede prey. The right maxilla is tapered. On the internal surface are a row dorsal short bristles near the apex and a row of ventral bristles preapically. A longitudinal row of long lamellate structures extend proximate for a considerable distance, lie entirely within the food canal, and bear several short spines and short bristles. There is no obvious difference between males and females in the distribution, number, and types of sensilla on mouthparts. The adult feeding process involves several steps, including searching and capturing prey, paralyzing prey, a resting phase, and a feeding phase. The evolution of the mouthpart morphology and the putative functional significance of their sensilla are discussed, providing insight into the structure and function of the mouthparts adapted for millipede feeding.