An apparently non-swinging tentorium in the Diplopoda (Myriapoda): comparative morphology of the tentorial complex in giant pill-millipedes (Sphaerotheriida).

Leif Moritz,Markus Koch,Thomas Wesener

doi:10.3897/zookeys.741.21909

Abstract

The presence of a swinging tentorium is a key apomorphy of Myriapoda, but this character has been studied in detail in only few species. Here the tentorium, i.e., the peristomatic skeleton of the preoral chamber, is comparatively studied in three species of the millipede order Sphaerotheriida Brandt, 1833. Since dissections of the fragile tentorial components proved to be difficult, despite the large head size, they were analysed mainly in situ via micro-computed tomography. Our results confirm previous observations of large differences in the tentorial construction in the giant pill-millipedes compared to chilognathan diplopods. The tentorium of Sphaerotheriida consists of a curved, plate-like epipharyngeal bar with distal projections, an elongate and thin hypopharyngeal bar, and a plate-like triangular posterior process; a transverse bar is absent. Only seven muscles attach at the tentorium in giant pill-millipedes, including two antennal muscles and two muscles of the gnathochilarium. Within the order Sphaerotheriida, the composition of the tentorium and its muscular equipment seems to be conserved, except for some variability in the shape of the epipharyngeal bar. As the transverse bar has been considered essential for the mobility of the tentorium in myriapods, its absence in Sphaerotheriida may indicate that their tentorium is not capable of performing a swing. Loss of tentorial mobility may also pertain to the order Glomerida Brandt, 1833, inferred here from the absence of a posterior process. An apparently immobile tentorium in Glomerida and Sphaerotheriida can straightforwardly be correlated with transformations of the head related to their ability of volvation. The different transformations of the tentorium, here hypothesised to cause immobility, may support current assumptions that the ability of volvation evolved convergently in Glomerida and Sphaerotheriida. This conclusion, however, still requires more detailed studies of the head anatomy in Glomerida and Glomeridesmida Cook, 1895.

Highlights

Recent molecular studies (Gai et al 2006, Regier et al 2010, Miyazawa et al 2014, Fernández et al 2016) as well as studies combining molecular and morphological data (Lee et al 2013, Giribet et al 2001) consistently retrieve the Myriapoda as a monophylum
The paired tentorial complex consists of only four major parts: the epipharyngeal bar (Fig. 1C, eb), the hypopharyngeal bar (Fig. 1B, hb), the posterior process (Fig. 2, pp), which forms a single tripartite sclerite along the mouth opening, and the separate nebententorium located on the hypopharynx (Fig. 1B, hy)
The proximal part of the epipharyngeal bar is a rectangular plate, which is wider in A. brandtii (Fig. 2D) and S. kalambatritra (Fig. 2F), while it is more slender in Z. bemanevika (Fig. 2H)

Summary

Introduction

Recent molecular studies (Gai et al 2006, Regier et al 2010, Miyazawa et al 2014, Fernández et al 2016) as well as studies combining molecular and morphological data (Lee et al 2013, Giribet et al 2001) consistently retrieve the Myriapoda as a monophylum. The tentorial complex consists of sclerotised exoskeletal bars and endoskeletal processes (Koch 2003, Koch 2015). This complex provides stability to the largely membranous epi- and hypopharynx and serves as muscle attachment sites. Associated with the tentorial complex is an additional sclerite, the nebententorium (sensu Attems 1926, Verhoeff 1928, Koch 2015) or hypopharyngeal lateral sclerite (sensu Wilson 2002), serving as an attachment site for musculature (Verhoeff 1928). The tentorium of Zoosphaerium bemanevika Sagorny & Wesener, 2017 from Madagascar was visualised in 3D using volume renderings of a μCT scan

Methods

Results

Discussion

Conclusion