Short Communication |
Corresponding author: Anna V. Diakova ( anndiakova@yandex.ru ) Academic editor: Maksim Proshchalykin
© 2021 Anna V. Diakova, 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.
Citation:
Diakova AV, Polilov AA (2021) Porous or non-porous? The challenge of studying unusual placoid sensilla of Megaphragma wasps (Hymenoptera, Trichogrammatidae) with electron microscopy. In: Proshchalykin MYu, Gokhman VE (Eds) Hymenoptera studies through space and time: A collection of papers dedicated to the 75th anniversary of Arkady S. Lelej. Journal of Hymenoptera Research 84: 69-73. https://doi.org/10.3897/jhr.84.68707
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Extreme miniaturization implies a high degree of optimization, rendering the retention of non-functional organs almost impossible. Two unique non-porous placoid sensilla on the antennae of females of Megaphragma were described in the literature. Placoid sensilla in Hymenoptera have an olfactory function and always bear pores; the apparent absence of pores therefore raises the questions whether such sensilla are functional in Megaphragma and whether their surface sculpture had been sufficiently well examined. We examined in detail the external microsculpture and internal ultrastructure of the placoid sensilla using Focused Ion Beam Scanning Electron Microscopy and Scanning Electron Microscopy with various types of sputtering and show that these sensilla actually have a porous cuticle and are innervated by 11 or 12 neurons with branched cilia, which is typical of olfactory sensilla. Comparison of various methods of electron microscopy allows us to conclude that for an accurate determination of the morphofunctional types of sensilla, especially in miniature insects, it is necessary to study both the internal ultrastructure of the sensilla and their external morphology using carefully selected scanning electron microscopy methods.
antenna, SEM, sensilla, sputtering, TEM, Trichogrammatidae
Placoid sensilla are common in Hymenoptera and are always multiporous (
The third flagellomere of the female antenna of the parasitic wasp Megaphragma (Hymenoptera: Trichogrammatidae) bears two peculiar placoid sensilla (PS), described as having no pores (
The purpose of this study was to perform a comprehensive analysis of the external and internal ultrastructure of the PS in Megaphragma amalphitanum using FIB-SEM and various SEM methods to determine if the placoid sensilla really do not have pores.
Adult females of Megaphragma amalphitanum Viggiani, 1997 were reared from eggs of Heliothrips haemorrhoidalis (Bouché, 1833) (Thysanoptera: Thripidae).
The study of the external morphology and microsculpture of the placoid sensilla (PS) was performed by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and Scanning Electron Microscopy (SEM). The SEM used was a Jeol JSM-6380 (accelerating voltage (AV) 20–30 kV, working distance (WD) 10–20 mm) and FEI Inspect F50 (AV 10 kV, WD 5–10 mm), following fixation, dehydration, critical-point drying, and gold sputtering of the specimens (Giko IB-3, sputtering thickness 20–25 nm; for more details see
The analysis of internal ultrastructure was based on the three-dimensional electron microscopy data obtained from a custom FIB-SEM (Zeiss Merlin scanning electron microscope with a Zeiss Capella focused ion beam), following fixation of the samples, en block staining, and embedding in EPON (for more details, see
The three-dimensional reconstruction was performed using the Bitplane Imaris software. All structures were manually segmented using the “Surfaces” function. The resulting reconstructions were processed in the Blender program (reduction of the number of polygons, smoothing of artifacts, rendering).
The earlier examination (
Study of the internal structure of the PS showed the presence of the poorly discernible pores with a slight relief (less than 0.01 μm), 0.12 ± 0.02 μm deep 0.10 ± 0.02 μm in diameter (mean ± SD) (Fig.
Structure and ultrastructure of unusual placoid sensella (PS) in female Megaphragma amalphitanum A–G SEM H 3D-reconstruction I, J FIB-SEM A third flagellomere bearing PS B, C PS wall sputtered with gold, sputtering thickness 20–25 nm D, E PS wall sputtered with chromium in ESEM mode F, G PS wall sputtered with gold, sputtering thickness 15 nm, orbital rotation with an inclination of up to 45° I cross section through the middle of PS J cross section through the outgrowth of PS H three-dimensional reconstruction of PS and one of the sensory neurons. Abbreviations: cu – cuticle, de – dendrite, le – ledge, nb – neuron body. Arrows designate the locations of the pores.
The re-examination of PS surface after sputtering with chromium made it possible to detect pores externally (Fig.
Thus, the small size of the pores and their slight relief, together with the unevenness of the sputtering or its excessive thickness, can make the pores indistinguishable.
SEM studies were performed using the Shared Research Facility “Electron Microscopy in Life Sciences” at Lomonosov Moscow State University (Unique Equipment “Three-dimensional Electron Microscopy and Spectroscopy”) and we are very grateful to the center staff for their help. We are also grateful to Harald Hess, Song Pang, and C. Shan Xu (Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, United States) for their assistance in obtaining the FIB-SEM data. We thank reviewers John Huber and Margarita Yavorskaya, who provided helpful comments on our manuscript. We thank Egor Zhirkov for his work on the three-dimensional reconstruction of sensilla. This study was supported by the Russian Foundation for Basic Research (project no. 19-34-90162).