Observations on the Ultrastructure of Antennal Sensilla of Adult Glenea
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Journal of Insect Science, (2020) 20(2): 7; 1–9 doi: 10.1093/jisesa/ieaa013 Research Observations on the Ultrastructure of Antennal Sensilla of Adult Glenea cantor (Cerambycidae: Lamiinae) Zishu Dong,1, Yubin Yang,2 Fugen Dou,2 Yujing Zhang,1 Huixin Huang,1 Xialin Zheng,1 Xiaoyun Wang,1, and Wen Lu1,3 1Guangxi Key Laboratory of Agric-Environment and Agric-Products Safety, College of Agriculture, Guangxi University, Nanning 530004, Peoples R China, 2Texas A&M AgriLife Research Center, Beaumont, TX, and 3Corresponding author, e-mail: luwenlwen@163. com Subject Editor: Phyllis Weintraub Received 31 October 2019; Editorial decision 19 February 2020 Abstract The external morphology and distribution of antennal sensilla of Glenea cantor Fabricius were studied with scanning electron microscopy. The antennae of G. cantor were observed to be filiform, consisting of scape, pedicel, and flagellum (nine flagellomeres). Four distinct types of sensory receptors were observed, including sensilla chaetica, sensilla trichodea, sensilla basiconica, and Böhm bristles. Three morphological subtypes of sensilla chaetica were found on the antennae, and sensilla trichodea were also categorized into three morphological subtypes. Sensilla basiconica was grouped into two morphological subtypes that were found on subsegments F2-F9 of the flagellum, and Böhm bristles were only found at the intersegmental joints between the scape and the head and between the scape and the pedicel. The antennae of male and female adults were similar in shape, length, and diameter. However, the length, diameter, distribution, and number of each of the four distinct types of sensilla on the males were significantly different from those on females. The types, lengths, diameters, numbers, and distributions of these sensilla were described, and their possible functions were also discussed. The results indicated that the base and end of an antennal segment have a similar sensillum density, but the middle section sensor density is significantly greater, especially for olfactory and gustatory sensilla, possibly because the joints are more involved in mechanical sensing. The density of sensors is closely related to its sensing function; so, future studies on the biology of olfaction and sexual communication in G. cantor will be facilitated by these observations. Key words: antennal sensilla, scanning electron microscopy, Cerambycidae, Glenea cantor, plant protection Cerambycidae is a cosmopolitan family of beetles with more than antennal sensilla, which have primary functions of chemoreception 36,000 species described around the world (Lu et al. 2007, Wang and mechanoreception. Chemosensation is of crucial importance to 2017). As phytophagous insects, Cerambycidae are economically im- most insects, regulating and triggering various behaviors. Sensilla portant pests of forests, street trees, and fruits (Dong 2017). Glenea are specialized structures of the epidermis, especially in the forms of cantor Fabricius is a longhorn beetle whose larvae bore under the hairs, pegs, and so on (Keil 2012). A mechanoreceptor can allow an bark of living trees of at least seven plant families in Southeast Asia, insect to experience the degrees of various movements and the size entering the wood for pupation (Lu et al. 2013a). Glenea cantor of a host plant (Ma et al. 2019). According to their morphology, the Fabricius is an important pest of Bombax ceiba (Linnacus) (Malvales: sensilla are termed as Böhm bristles, chaetica, trichoid, basiconica, Bombacaceae) (Lu et al. 2011a, Lu et al. 2013b) and Adansonia etc. Sensilla play an important role in the reception of various stimuli digitata (Linnaeus) (Malvales: Bombacaceae). The Bombax ceiba L. (odor, sound, heat, cold, humidity, and tactile information) involved was found to be a new important host plant of the longhorn beetle in finding suitable habitats and locating mates (Barbara et al. 2003, in Nanning, Guangxi Province, China in 2018. The baobab tree has Spathe et al. 2013, Ruschioni et al. 2015). multiple uses and is highly valued in Southern Africa, particularly in The functions of sensilla have been confirmed by electrophysio- rural communities, where people depend on this resource for their logical technology and antennal transcriptome analysis (Zhang livelihoods (Munyebvu 2018). Information on this new potential 2018). Studies of antennal sensilla have mainly been carried out in invasive pest is therefore critical for more effective control on its Diptera, Lepidoptera, and Hymenoptera because of the significant damage to Bombax ceiba L. economic value of insects of these orders (Yao et al. 2005, Mark Insect antennae are segmented appendages that are well- et al. 2017, Wu et al. 2018). In recent years, studies have increasingly equipped. An antenna’s surface is covered by a wide variety of focused on antennal sensilla and their ultrastructure in Coleoptera © The Author(s) 2020. Published by Oxford University Press on behalf of Entomological Society of America. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), 1 which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. 2 Journal of Insect Science, 2020, Vol. 20, No. 2 (Chen et al. 2014, Ali et al. 2017), but no such study in G. cantor the base, middle, and end of each antennal subsegment (Fig. 1). has been conducted. Various options have been explored to control Each sample was magnified 300 times, and the area of each was the G. cantor, including physical control, chemical control, and bio- approximately 4,444 μm2. The number of sensors in each sample logical approaches. For example, several parasites have been found was recorded, and the average value was taken. Then, the number of to be effective (Lu 2007). Compared with other control methods, bio- sensors in the area was extrapolated to the corresponding value of logical control is the most promising way to manage a stem-boring the actual area of the site, and the data were statistically analyzed. pest over large areas. Semiochemicals are natural chemical com- Sensilla that had relatively low numbers (Bb, SChⅠ, SChⅢ, STⅠ, and pounds that play important roles in the behavioral ecology of insects STⅡ) were counted individually. due to their function in modifying insect behavior. Semiochemicals The sensillar distribution patterns were precisely described using play an important role as an alternative to traditional methods of Adobe Photoshop software, Version CS6. The distribution of each pest control, and their development has progressed to reduce the type of sensillum was analyzed between the dorsal and ventral sur- levels of pesticide use. faces of the antennae in both sexes. In all, five individual structures The purpose of this study was to characterize and determine per type were subjected to a quantitative analysis of sensilla. The the antennal morphology and sensillar ultrastructure of the ceiba t-test was applied to determine the possible sexual dimorphism of longhorn beetle G. cantor in both sexes using scanning electron antennal sensilla between males and females using the SPSS stat- microscopy (SEM). This descriptive work will provide a better under- istical software package, version 25.0 (SPSS Inc., Chicago, IL). standing of the behavioral mechanisms in this stem-boring pest. Student’s t-test or analysis of variance were applied to determine the Coupled with sensory physiology, the results of our study may pro- possible differences in of the number, length, and width of antennal vide an effective control measure for this pest with semiochemicals. sensilla, and the values are reported as means ± standard error (SE). The significance level was set at 0.05. Materials and Methods Results Insects In June 2018, we collected three kapok plants from Qingxiu Gross Morphology of Antennae Mountain (22°12′-23°32′N, 107°45′-108°51′E), Nanning, China, The numbers of segments were the same for female and male G. cantor that had withered in the upper stalk due to G. cantor infestation and adults. The antennae of G. cantor were filiform, each consisting of a cut them into 10 sections (10–15 cm in diameter × 40 cm in length). scape, pedicel, and flagellum (nine flagellomeres, F1-F9). The scapes The stalk sections were placed in four bubble containers in a yarn were cylindrically shaped and located in the antennal socket between cages (40 cm × 80 cm × 80 cm) sprayed with water once every 7 d in a the compound eyes. Each pedicel was a trapezoid and clearly shorter controlled environment (25 ± 1°C, 70 ± 5% RH, 14 L: 10 D; Lu et al. than the scape and each flagellomere. The nine flagellomeres of each 2011b). After 1.5 mo, newly emerged adults from the cages were ran- flagellum with blunt tips were similar in shape but different in size. domly collected and transferred to separate glass containers (4 cm There were no significant differences between males and females in the in diameter × 12 cm in length). These adults were fed with kapok length or diameter of the antennae (Table 1). The average lengths of twigs (2–4 cm in diameter × 8 cm in length), which were renewed scapes and flagellomeres in males were not significantly different from every 2 d. We selected five male and five female adults of G. cantor those in females, with an average scape length of 1.75 ± 0.05 mm for and placed them in a freezer at −20°C. After 30 min, the adults were males and 1.67 ± 0.13 mm for females (Table 1). Each flagellomere removed and their antennae cut off under a stereomicroscope PX-1 was a barrel-shaped segment with a length of 13.78 ± 0.31 mm and (Camsonar Technology Co., Ltd., Beijing, China). The antennae were 13.46 ± 0.56 mm for both males and females, respectively (Table 1). stored in a 70% alcohol solution until they were examined. The average length (male = 0.39 ± 0.04 mm, female = 0.48 ± 0.03 mm, t = −4.40, P = 0.02) and diameter (male = 0.33 ± 0.01 mm, fe- Preparation of Specimens male = 0.38 ± 0.01 mm, t = −2.76, P = 0.03) of pedicels in females were The antennae were cleaned three times in an ultrasonic bath JP-010T dramatically and significantly greater than those in males (Table 1).