Research Article |
Corresponding author: Leif Moritz ( moritz.leif@gmail.com ) Academic editor: Gregory Edgecombe
© 2018 Leif Moritz, Thomas Wesener, Markus Koch.
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:
Moritz L, Wesener T, Koch M (2018) An apparently non-swinging tentorium in the Diplopoda (Myriapoda): comparative morphology of the tentorial complex in giant pill-millipedes (Sphaerotheriida). In: Stoev P, Edgecombe GD (Eds) Proceedings of the 17th International Congress of Myriapodology, Krabi, Thailand. ZooKeys 741: 77-91. https://doi.org/10.3897/zookeys.741.21909
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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.
Arthrosphaeridae , micro-CT, 3D-reconstruction, swinging tentorium, volvation
Recent molecular studies (
Vouchers are stored in natural history collections of the Zoological Research Museum A. Koenig (
A connection of the tentorium to the head capsule by a transverse bar (sensu
Zoosphaerium sp., light micrographs of peristomatic structures. A Epipharynx, showing absence of the tentorial transverse bar B Preoral chamber, frontal view on hypopharynx (mandibles removed) C Hypopharynx and endochilarium, dorsal view (hypo- and epipharyngeal bar of right tentorium broken off). Scale bars: 500 µm. Abbreviations:eb = epipharyngeal bar of left tentorium; ed = endochilarium; ep = epipharynx; h1 = projection of hypopharyngeal bar; hb = hypopharyngeal bar of tentorium; hy = hypopharynx; il = incisura lateralis; lb = labrum; ll = lamella lingualis; lm = lamella-mentum; nt = nebententorium; ss = suspensorial sclerite; stg = stipes of gnathochilarium.
(1) The epipharyngeal bar:
The plate-like epipharyngeal bar (eb) is in connection with the wall of the epipharynx (Fig.
The tentorial complex of the Sphaerotheriida, 3D visualization. GREY = Head capsule; BROWN = mandible; ORANGE = tentorium; YELLOW = nebententorium; OLIVE = außententorium. A–D Arthrosphaera brandtii (Humbert, 1865),
(2) The hypopharyngeal bar:
In the three analysed species, the epipharyngeal bar (eb) of the tentorium (Fig.
(3) The posterior process:
The posterior process (pp) is a large triangular plate projecting posteriorly into the head capsule parallel to the mandibular gnathal lobe sclerite (sensu
(4) The nebententorium:
The nebententorium (nt) is a short, flat sclerite parallel to the distal portion of the hypopharyngeal bar (hb) of the tentorium (Fig.
The strong condylus (co) of the mandibular gnathal lobe (Fig.
The tentorial complex of the Arthrosphaeridae is associated with a set of seven muscles (Fig.
Head musculature of Sphaeromimus kalambatritra. A, B, D–G micro-CT images C 3D segmentation. A Arrangement of mandible, tentorium and head capsule, frontal section B Incisura lateralis in detail, frontal section C 3D segmentation of the isolated tentorium and its muscles, medial view D, E Muscles of the epipharyngeal bar, fronto-medial view F Muscles of the posterior process, frontal section G Muscles of the nebententorium, frontal section. Top is frontal, left is lateral. Scale bars: A, D 1000 µm B, E–G 500 µm C not to scale. Abbreviations: a1 = lateral antennal muscle (m.); a2 = anterior antennal m.; at1 = first antennomere; co = condyle of mandibular gnathal lobe; eb = epipharyngeal bar; et = external tooth of mandible, g1 = lamella lingualis m.; g2 = lamello-mentum m.; aut = mandibular gnathal lobe sclerite (außententorium); hb = hypophayrangeal bar; hc = head capsule; il = incisura lateralis; ilp = projection arising from incisura lateralis; it = internal tooth of mandible; lab = labrum; ll = lamella lingualis of gnathochilarium; lm = lamello-mentum; mdb = mandibular base; mdg = mandibular gnathal lobe; mp = molar plate; nt = nebententorium; p1 = pharyngeal dilator m.; ph = pharynx; pl = pectinate lamellae of mandible; pp = posterior process; st = stipes of gnathochilarium; t1 = anterior tentorial m.; t2 = dorsal tentorial m.; t3 = posterior tentorial m.
The tentorium of the three studied representatives of Sphaerotheriida shows the same basic structure (Fig.
The most striking character of the giant pill-millipede tentorium is the absence of the transverse bar (Fig.
The general function of the transverse bar is the connection of the tentorial complex to the head capsule at the incisura lateralis (= clypeal notch), around which the tentorium is deemed to perform its swinging movements (
Despite these differences, the main muscles considered essential for movements of the tentorial complex are present.
The differences in the composition of the tentorium and in its muscular equipment might not only be correlated with the absence of the tentorial transverse bar, but also with the presence of a strong condylus on the mandibular gnathal lobes of Sphaerotheriida that unquestionably impacts on the mandibular mechanism. In Juliformia, the connection of the transverse bar to the incisura lateralis is deemed to fix a swing of the tentorium, causing the mandibular gnathal lobe to abduct (
The tentorium of Sphaerotheriida contributes more characters to the list of head modifications that likely correlate with adaptations to volvation (see, e.g.,
Although the general appearance of the tentorium is conserved within Arthrosphaeridae there are some differences in details. These mainly concern the epipharyngeal bar, with its projections varying in their shape and length (Fig.
The reduction of the transverse bar of the tentorial complex as well as the presence of the mandible condyles in Sphaerotheriida must have an enormous impact on the mandibular abduction, resulting in a probably non-swinging tentorium. The reduction of the transverse bar in Sphaerotheriida is probably correlated to the volvation and suggests a convergent evolution of volvation in the pentazonian orders Sphaerotheriida and Glomerida. In Glomerida the posterior process of the tentorial complex is reduced as an adaptation to volvation. This could furthermore support a previously suggested (
We thank Thorsten Klug and Hans-Joachim Krammer (both