Combining simplicity and functionality: mouthpart sensilla in the miniature parasitoid wasp Megaphragma viggianii (Hymenoptera, Trichogrammatidae)

Combining simplicity and functionality: mouthpart sensilla in the miniature parasitoid wasp Megaphragma viggianii (Hymenoptera, Trichogrammatidae) Kseniia T. Abu Diiak1, Alexey A. Polilov1 1Department of Entomology, Faculty of Biology, Lomonosov Moscow State University, Leninskie Gory 1, Moscow, Russia Corresponding author: Kseniia T. Abu Diiak ([email protected]) Academic editor: Petr Janšta|Received 4 August 2025|Accepted 19 October 2025|Published 17 November 2025 https://zoobank.org/6F2178EC-669C-4986-9478-7DAF4EBB9463 Citation: Abu Diiak KT, Polilov AA (2025) Combining simplicity and functionality: mouthpart sensilla in the miniature parasitoid wasp Megaphragma viggianii (Hymenoptera, Trichogrammatidae). Journal of Hymenoptera Research 98: 1067–1085. https://doi.org/10.3897/jhr.98.167764 Abstract Insects depend on complex sensory systems to detect food sources, locate mates, and navigate in the environment. In the smallest insects, miniaturization imposes major constraints on anatomy and physiology, but it remains poorly understood how this affects the structure and function of sensory systems, such as the sensilla on the mouthparts. The specific problem addressed in this study is how extreme miniaturization influences the diversity, distribution, and structural complexity of mouthpart sensilla in the minute parasitoid wasp Megaphragma viggianii Fusu, Polaszek & Polilov. Using scanning and volumetric electron microscopy, we show that despite its minute body length of 0.2–0.3 mm, M. viggianii possesses a set of 70 sensilla on its mouthparts, including four major types (chaetoid, basiconic, coeloconic, and campaniform). These wasps display no noticeable sexual dimorphism in the structure and distribution of their mouthpart sensilla, and the set of sensilla is generally identical in different individuals. Compared to larger Hymenoptera, M. viggianii exhibits a reduced number of sensilla, but maintains the same types of sensilla and their similar position, suggesting stable developmental and functional traits. These findings confirm that miniaturization does not reduce the diversity of the sensilla, but instead drives simplification at the structural level, while preserving their functional roles. These insights not only advance our understanding of insect sensory biology but also contribute to elucidating broader questions of evolutionary morphology and functional scaling of biological systems. Keywords Megaphragma, miniaturization, mouthparts, sensilla, Trichogrammatidae JHR 98: 1067–1085 (2025) doi: 10.3897/jhr.98.167764 https://jhr.pensoft.net Copyright Kseniia T. Abu Diiak & Alexey A. Polilov. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. RESEARCH ARTICLE K. T. Abu Diiak & A. A. Polilov / Journal of Hymenoptera Research 98: 1067–1085 (2025) 1068 Introduction The sensory system plays an important role in the lives of insects, being a source of information about their surroundings and the state of their own bodies. It is used to search for food and for sexual partners and as to perform spatial orientation and movement coordination (Kamikouchi et al. 2009). The sensory systems of insects possess a variety of sensory organs including sensilla. These are modified cuticular hairs that have at least one bipolar neuron and, usually, several auxiliary cells. Sensilla are located over the entire surface of the insect body, but are most numerous on the antenna, mouthparts, legs, wings, and genitalia (Chapman 1969). Insect sensilla have various modalities and can be mechano-, chemo-, thermoand hygroreceptors (Ivanov 2000). Various classifications of sensilla have been proposed depending on their structure. Snodgrass (1926) classified sensilla based on the morphology of their cuticular part: hair, plate, campaniform, and scolopophorous organs. Hair organs are further divided into sensilla trichodea, sensilla chaetica, sensilla basiconica, sensilla coeloconica, sensilla ampullacea, and sensilla squamiformia. According to Altner and Prillinger (1980), sensilla are divided into aporous, uniporous and multiporous (singleor double-walled). Aporous sensilla can perform a mechano-, hygro-, or thermoreceptor function; uniporous sensilla can be mechanoreceptors or contact chemoreceptors; multiporous sensilla can contain chemo-, thermo-, and hygroreceptor cells. It was also proposed to divide chemoreceptor sensilla into thin-walled (olfactory chemoreceptors) and thick-walled (contact chemoreceptors) (Slifer 1970). Miniaturization is one of the key trends in insect evolution, which in some cases leads to an extreme decrease in body size, making it comparable to that of unicellular organisms. Such a change in body size inevitably leads to a significant restructuring of all systems and organs of insects, including modifications at the cellular level (Polilov 2015, 2016; Minelli and Fusco 2019). Changes also affect the sensory system of miniature insects, making it interesting in terms of the peculiar optimization of the structure of sensory organs and their distribution on the body surface. Although sensory organs of many insects have been previously studied in detail (Mazokhin-Porshnyakov et al. 1983; Zacharuk and Shields 1991; Hallberg and Hansson 1999; Ivanov 2000; Missbach et al. 2014; van der Kooi et al. 2021), studies on the scaling of insect sensory system began to appear relatively recently, and there are few publications on this subject. The available studies mostly focus on the antennae and eyes of some insects. Research in this area shows that the body size of insects significantly affects the structure and quantitative characteristics of compound eye, as well as antenna and antennal sensilla both on the interspecific (Symonds and Elgar 2013; Ramirez Esquivel 2017; Godfrey et al. 2021) and on the intraspecific level (Smallegange et al. 2008; Ramirez Esquivel et al. 2014; van der Woude and Smid 2016). Some review articles on the scaling of insect eyes (Makarova et al. 2022a) and sensilla (Makarova et al. 2022b) have been published recently. Both antennae and eyes have a tendency to decrease the number of functional units (sensilla or ommatidia) during miniaturization. Mouthpart sensilla in Megaphragma viggianii 1069 For example, in the miniature wasp Trichogramma evanescens Westwood, the length of the ommatidium and rhabdom decreases, the processes of the pigment cells lose contact with the basement membrane, and average pigment granule volume is reduced (Fischer et al. 2011, 2019). The eyes of the smaller wasp M. viggianii Fusu, Polaszek & Polilov, 2022 contain several times fewer ommatidia than in T. evanescens, a difference probably functionally compensated by the larger lens diameter of the ommatidia and by their wider and shorter rhabdoms (Makarova et al. 2025). In general, smaller insects tend to have apposition eyes instead of superposition ones (Gokan and Meyer-Rochow 2000; Fischer et al. 2010; Makarova et al. 2015, 2019). This may be related to the fact that superposition is limited by the minimum size of the clear zone, the zone between the lens and retina (Gokan and Meyer-Rochow 2000; Meyer-Rochow and Gál 2004; Honkanen and MeyerRochow 2009). The number of ommatidia also correlates with body and head size, being higher in larger insects (Gokan and Meyer-Rochow 2000; Makarova et al. 2019). Similarly, the number of antennal sensilla within the same species is often higher in larger insects, which was shown for the housefly Musca domestica L. (Smallegange et al. 2008) and for the T. evanescens (van der Woude and Smid 2016). Studies of the antennae of the Megaphragma viggianii (Diakova et al. 2018; Diakova and Polilov 2021) and the featherwing beetle Scydosella musawasensis Hall (Diakova and Polilov 2020) have shown that the number of sensilla on the antenna diminishes with decreasing body size, but the size of the sensilla themselves and the number of their types remain unchanged. The number of sensilla can vary considerably among wasps of different body sizes: depending on the species and sex, representatives of the genus Megaphragma Timberlake have 39–49 sensilla on their antennae, while the antennae of large parasitoid wasps can bear up to 9000 sensilla (Das et al. 2011). Miniature insects are also characterized by simplification of the structure of their sensilla: e.g., placoid sensilla without auxiliary cells have been found in M. viggianii (Diakova and Polilov 2021). Megaphragma includes some of the smallest parasitoid wasps that parasitize the eggs of the thrips Heliothrips haemorrhoidalis (Bouché) (Bernardo and Viggiani 2002). Their body length is approximately 0.2 mm, and they have strongly reduced heart, tracheal system, and muscular system (Polilov 2017). Their extremely small size affects even the internal structure of neurons: they have anucleate neurons, not known in other organisms (Polilov 2012), except for miniature wasps of another genus, Camptoptera Foerster (Polilov et al. 2023). Despite the greatly simplified structure, Megaphragma females have a fairly complex behavior that allows them to detect host eggs, which indicates a fully functional sensory system. Despite the growing interest in the miniaturization of insect sense organs, data on the structure of sense organs of the smallest insects and on their modifications related to miniaturization remain incomplete. In order to build a more complete picture, reflecting the principles of and limits to the miniaturization of insect sensory organs, data on their structure and location on mouthparts are required, which are provided in this study. The aim of this study is to continue the studies of the sensory organs of miniature insects and to reveal the patterns of miniaturization of mouthpart sensilla in M. viggianii. K. T. Abu Diiak & A. A. Polilov / Journal of Hymenoptera Research 98: 1067–1085 (2025) 1070 Material and methods This study is based on 41 adult males and 35 adult females of Megaphragma viggianii, (Hymenoptera: Trichogrammatidae) reared in the laboratory from eggs of Heliothrips haemorrhoidalis (Bouché, 1833). The material was fixed in 2% glutaraldehyde + 2% paraformaldehyde + 0.1% Triton + 0.1M phosphate-buffered saline solution for 20–24 hours and transferred to 70% ethanol. Then the material was dehydrated in 96% ethanol for 1 hour, in 100% ethanol for 30 minutes (two iterations) and in acetone for 15 minutes (two iterations). After dehydration it was dried in a Hitachi HCP-2 Critical Point Dryer. The dried specimens were mounted on scanning electron microscopy holders by using conductive tape and coated with a 20 nm layer of gold using a Cressington-208HR Ion Coater. Photographs of the mouthparts and individual sensilla were obtained using Jeol JSM-6380 and FEI Quattro S scanning electron microscopes. To obtain photographs of hidden medial sides of mouthparts, the heads of multiple insects were split into parts with entomological pins, then parts of the heads in water drops were put on the surface of glass pieces attached to scanning electron microscopy holders. After drying, the same coating method was applied to these specimens. To obtain information on the ultrastructure of the sensilla types and to study the sensilla that are not visible from the surface, three-dimensional electron microscopy (vEM) was used. In this study we used images obtained earlier (Chua et al. 2023; Desyatirkina et al. 2023) by using the protocol described previously (Polilov et al. 2021). The length and base width of the sensilla were measured in series of images with ImageJ 1.52a program. The obtained data were used to calculate the mean values of sensilla sizes and standard errors of the mean, as well as the minimum and maximum sizes of these structures. The measured lengths of the sensilla were used to carry out one-way Kruskal–Wallis ANOVA test with sex as the grouping variable and lengths of sensilla of every type as the dependent variables. The goal of this analysis was to determine whether M. viggianii has sexual dimorphism in sensillum length. Results Four types of sensilla were found on the mouthparts of males and females of Megaphragma viggianii: Chaetoid sensilla (chs) are the most numerous type of sensilla on the mouthparts of M. viggianii. These are elongated, slightly curved or straight structures, with slight striation or without it (Fig. 3A, B). They also have sockets with edges slightly elevated above the surface of the cuticle. The width of chaetoid sensilla gradually decreases from the base to the tip; the tips are usually rounded. The length of these sensilla varies depending on their position on the mouthparts (Table 1). Most chaetoid sensilla are innervated by a single neuron and have no auxiliary cells (Fig. 4A, D). The dendrite of the neuron ends near the base of these sensilla and has a prominent tubular body. Mouthpart sensilla in Megaphragma viggianii 1071 On the distal parts of the maxilla, there is a non-innervated subtype of chaetoid sensilla, which lacks either dendrites or auxiliary cells (Figs 3C, 4B). These structures are thicker than innervated chaetoid sensilla and resemble spikes. Coeloconic sensilla (cos; Fig. 3D) are partially submerged into a deep cuticular pocket. Their external part is slightly flattened, while their submerged part has a round cross-section. The surface of these sensilla is smooth and has no visible pores or grooves; their tips are rounded. They are innervated by one dendrite without a tubular body and have no auxiliary cells (Fig. 4E). The length of the visible part of these sensilla is 1.00 ± 0.05 µm. Basiconic sensilla (bcs) are peg-like structures that are usually shorter than chaetoid sensilla. Two subtypes of these sensilla were found on the mouthparts of M. viggianii. Porous basiconic sensilla (Fig. 3E) can be observed on the maxillary palp. Their shape resembles a stick with a rounded tip. The surface of the sensilla is covered with visible pores that are numerous on the tip of the sensilla, sometimes occur on their lateral side and are almost absent near their base. The bases of the sensilla have sockets with a smooth cuticular ridge. The sensilla have six dendrites entering the sensillar processes (Fig. 4C). Some of the dendrites reach the tip of the sensillum, others end in the middle. The length of the sensilla is 5.0 ± 0.15 µm. Aporous basiconic sensilla (Fig. 3F) are positioned on the tip of the labium. These sensilla have smooth sockets and a conical shape: their width gradually decreases from the base to the tip. Their surface has no visible pores. The sensilla are innervated by three dendrites that reach at least the middle of the processes of the sensilla (Fig. 4D). One of the dendrites has a tubular body. Campaniform sensilla (cfs) are completely submerged below the cuticle and not visible from the surface (Fig. 4F). These sensilla are comprised of a single dendrite with a tubular body and a socket septum which attaches the dendrite to the cuticle. Table 1. Average sizes of sensilla with different positions on M. viggianii mouthparts, µm. Position Length Width md-chs-1 3.9 ± 0.17 0.66 ± 0.014 md-chs-2 7.8 ± 0.33 0.64 ± 0.013 md-chs-3 4.3 ± 0.15 0.59 ± 0.008 md-chs-4 10.6 ± 0.25 0.72 ± 0.012 md-cos-1 1.0 ± 0.05 0.34 ± 0.014 md-cos-2 0.9 ± 0.03 0.34 ± 0.013 mx-chs-1 11.9 ± 0.41 0.70 ± 0.011 mx-chs-2 5.0 ± 0.2 0.53 ± 0.013 mx-chs-3 4.4 ± 0.12 0.57 ± 0.019 mxp-chs 13.4 ± 0.28 0.85 ± 0.011 mxp-bcs 5.0 ± 0.1 0.92 ± 0.014 lab-chs-1 12.1 ± 0.43 0.67 ± 0.011 lbp-chs-1 11.0 ± 0.25 0.81 ± 0.009 lbp-chs-2 2.8 ± 0.09 0.64 ± 0.014 K. T. Abu Diiak & A. A. Polilov / Journal of Hymenoptera Research 98: 1067–1085 (2025) 1072 Distribution of the sensilla Distribution of the sensilla is identical in males and females. The sensilla set is generally stable, but shows very rare aberrations in some individuals. The normal set of sensilla is shown in Figs 1, 2. The numbers of different sensilla on the mouthparts are given in Table 2. A mandible (Figs 2A, B, 5A, B) has three chaetoid and two coeloconic sensilla on the lateral surface and four chaetoid sensilla on the medial surface. Chaetoid sensilla on the lateral surface are arranged in a row extending from the proximal to the distal part of the mandible. The tips of the chaetoid sensilla are pointing in the medial and distal directions. One of the chaetoid sensilla on the medial surface is close to the ventral margin of the mandible and is visible from the outside, its tip pointing distally. The Table 2. Number of sensilla of each type on M. viggianii mouthparts. Mouthparts chs cfs bcs cos Mandibles (both) 14 20 0 4 Maxillae (both) 20 0 2 0 Labium 8 0 2 0 Figure 1. Map of mouthpart sensilla distribution in M. viggianii, with sensilla codes. Image of male mouthparts is given as an example; females show the same distribution of sensilla. Abbreviations: md – mandible; mx – maxilla; mxp – maxillary palp; lab – labia; lbp – labial palp; chs – chaetoid sensilla; chsn – non-innervated chaetoid sensilla; cos – coeloconic sensilla; bcs – basiconic sensilla; ant – anterior direction; dor – dorsal direction; lat – lateral direction. Mouthpart sensilla in Megaphragma viggianii 1073 Figure 2. Schemes showing the position of sensilla on M. viggianii mouthparts. A lateral surface of mandible B medial surface of mandible C lateral surface of maxilla D medial surface of maxilla E ventral surface of labium. Abbreviations: md – mandible; mx – maxilla; mxp – maxillary palp; lab – labia; lbp – labial palp; chs – chaetoid sensilla; chsn – non-innervated chaetoid sensilla; cos – coeloconic sensilla; bcs – basiconic sensilla; cfs – campaniform sensilla; ant – anterior direction; dor – dorsal direction; lat – lateral direction. Hidden part of the mandible, not visible from the outside, is shown in gray. K. T. Abu Diiak & A. A. Polilov / Journal of Hymenoptera Research 98: 1067–1085 (2025) 1074 Figure 3. Types of sensilla on M. viggianii mouthparts. A chaetoid sensillum on lateral surface of mandible B chaetoid sensillum on labial palp C non-innervated chaetoid sensillum on the distal part of maxilla D coeloconic sensillum on lateral surface of mandible E porous basiconic sensillum on maxillary palp F aporous basiconic and chaetoid sensilla on distal part of labium. Abbreviations: chs – chaetoid sensilla; chsn – non-innervated chaetoid sensilla; cos – coeloconic sensilla; bcs – basiconic sensilla; po – pores. other three chaetoid sensilla on the medial surface are located close to each other near the distal part of the mandible and not are visible from the outside. Coeloconic sensilla are positioned on the lateral surface of the mandible. There is one coeloconic sensillum Mouthpart sensilla in Megaphragma viggianii 1075 Figure 4. Ultrastructure of sensilla on M. viggianii mouthparts. A chaetoid sensillum on mandible B non-innervated chaetoid sensillum on maxilla C basiconic sensillum on maxillary palp, cross-section D basiconic and chaetoid sensilla on tip of labium, cross-section of the base E coeloconic sensillum on mandible F campaniform sensillum on mandible. Abbreviations: chs – chaetoid sensilla; chsn – noninnervated chaetoid sensilla; bcs – basiconic sensilla; cos – coeloconic sensilla; cfs – campaniform sensilla; so – socket; tb – tubular body; de – dendrite; ss – socket septum. in the proximal part and one on the distal part of the mandible. Each mandible also bears 10 campaniform sensilla distributed from the base to the tip. The sensilla occur on both the medial and lateral surfaces. K. T. Abu Diiak & A. A. Polilov / Journal of Hymenoptera Research 98: 1067–1085 (2025) 1082 Boudinot BE, Beutel RG, Gorb SN, Polilov AA (2021b) Functional diversity of attachment and grooming leg structures is retained in all but the smallest insects. 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The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited. Link: https://doi.org/10.3897/jhr.98.167764.suppl1