Description of the third instar larva of the stag beetle Prismognathus dauricus Motschulsky, 1860 (Coleoptera: Scarabaeoidea: Lucanidae) using electron microscopy
Introduction
Lucanidae is a basal lineage of the superfamily Scarabaeoidea, containing approximately 1300 species, representatives of which are distributed across all continents except Antarctica (Holloway, 2007; Kim and Farrell, 2015). The adults of Lucanidae, commonly called stag beetles, are named as the males possess enlarged mandibles (Wan and Yang, 2006; Fujita, 2010). Lucanidae have long received great attention from taxonomists and amateur collectors, due to their ornament mandibles, coloration, and sometimes dramatically impressive mating behavior (Eldred et al., 2016; Goyens et al., 2016; Chen et al., 2017). Many stag beetles spend the majority of their life cycle inside or under decaying logs in the forests (Hangay and De Keyzer, 2017). While the beetles are regularly captured for the pet market; less frequently the larvae are collected and reared though to the adult (Lai and Ko, 2008). Although many aspects of adult Lucanidae are well studied (Hosoya and Araya, 2005), detailed larval morphological descriptions of most species are yet to be published.
In many countries the larvae of Lucanidae are referred to as white grubs and are superficially similar to those of the other Scarabaeoidea (Ritcher, 1966). The larvae of most lucanid species are saproxylic (woody debris dependent) (Meggs and Munks, 2003; Hangay and De Keyzer, 2017), while others are edaphic (soil dependent) or utilized the soil-wood interface (Richards and Spencer, 2014). These habitus differ from those phytophagous grubs in Melolonthinae and Rutelinae (Fleming, 1972; Pathania and Chandel, 2016), and dung-feeding larvae such as in Geotrupidae, Scarabaeinae and Aphodiinae (Jerath, 1958; Verdú et al., 2004; Scholtz et al., 2009). The larvae of northern hemisphere Lucanidae were previously well described by Ritcher (1966) for a number of genera, including Lucanus, Dorcus, Platycerus, Sinodendron, and Ceruchus, and more recently some from Tasmania, Australia belonging to Hoplogonus and Lissotes (Richards and Spencer, 2014). Larval morphological characters of Lucanidae have also been utilized in the phylogenetic analysis of Scarabaeoidea (Grebennikov and Scholtz, 2004). However, the previous larval morphological studies remain insufficient to reconstruct the phylogenetic relationships on generic level.
Prismognathus Motschulsky, 1860 is a minor genus containing 23 species, mainly distributed in the northeastern part of Asia (Bartolozzi and Wan, 2006; Wan et al., 2007; Schoolmeesters, 2018). One previous study considered Prismognathus to be closely related to Cladophyllus, Cyclorasis, Eligmodontus and Gonometopus, sharing a number of morphological characters (Huang and Chen, 2012). Based on molecular data, however, the genus Prismognathus is argued to be the sister group of Lucanus (Hosoya and Araya, 2005; Kim and Farrell, 2015). Clearly further research is required to resolve the relationship between these groups. The larvae Prismognathus dauricus were previously described in their phylogenetic study of Scarabaeoidea by Grebennikov and Scholtz (2004). Following the taxonomic standard, larval characters were illustrated, using the classical line drawing methods of Hayes (1929) and Ritcher (1966). However, despite the level of detail, some of the morphological features remain unresolved, in particular those at the ultramorphological level.
In this study, we present the first description of the larval morphology of Prismognathus dauricus Motschulsky, 1860, employing light and scanning electron microscopy to provide the description.
Section snippets
Study organisms
Adults of Prismognathus dauricus were captured under light trap at Mount Huabo (41°06′N, 125°02′E, elev. 650 m asl.), in Kuandian County, Liaoning Province of northeastern China at the mid August in 2016. The paired adults (one male and one female) were maintained in plastic boxes containing moist fermented sawdust (Beetle-Password, Shenyang, China). An off-cut of decayed wood log was provided for the potential bore and oviposition. Third instar larvae were collected in the sawdust the
Results
The third instar larva of Prismognathus dauricus is of typically scarabaeiform, bearing three pairs of thoracic legs on the C-shaped trunk (Fig. 1A). The thoracic segments are more slender than the abdominal segments. The anal opening is Y-shaped, with the stem of Y much longer than arms (Fig. 1B). The sternite of the distal abdominal segment possesses two rows of spines arranged in V-shape, accompanied by two groups of setae pointing inward (Fig. 1B).
The head is heavily sclerotized, bearing a
Discussion
In this study, the larvae of Prismognathus dauricus were morphologically described using compound and scanning electron microscopy. We found that the larva of P. dauricus is atypical for the following characters: i) the epipharynx possesses anteriorly eleven protophoba arranged in a semi-round line; ii) the par stridens consists of 31 ± 0.7 (N = 10) subconical teeth arranged in a slightly curved, longitudinal row; iii) the plectrum is composed of 60 ± 1.4 (N = 10) carinae; iv) the claws of
Acknowledgements
We are grateful to the anonymous reviewers for their valuable comments and kindly polishing the language. We also thank Yong-Xu Han, Chun-Feng Li, Jian Hao, and Yun-Long Ma for their helpful comments during the collecting and rearing periods. This research was financially supported by the National Natural Science Foundation of China (grant no. 31702036) and Scientific Foundation for the Introduced Talent of Shenyang Agricultural University (grant 880417008).
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These authors contribute equally to this study.