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Proc. Arthropod. Embryol. Soc. Jpn. 48, 43–45 (2017)Ⓒ 2017 Arthropodan Embryological Society of Japan
ISSN 1341–1527[SHORT COMMUNICATION]

Tentorial Invaginations of the Collembolan Tomocerus cuspidatus
Börner, 1909 (Hexapoda: Collembola, Tomoceridae)*
Shigekazu TOMIZUKA1, 2) and Ryuichiro MACHIDA1, 3)
Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305–8572, Japan
Current address: Echigo-Matsunoyama Museum of Natural Science, Matsunoyama Matsuguchi, Tokamachi, Niigata 942–1411,Japan3)
Current address: Sugadaira Research Station, Mountain Science Center, University of Tsukuba, Sugadaira Kogen, Ueda, Nagano
386–2204, Japan
E-mail: tomizuka@matsunoyama.com (ST)1)2)

Collembola, Protura, and Diplura belong to theEntognatha, on the basis of entognathy as the most reliable
autapomorphy. Recent paleontological and morphologicalresearches have provided evidence discounting the homologyof entognathy among these three entognathan orders(Kukalová-Peck, 1987; Koch, 1997). Irrespective of whether
they can be elucidated as homologous, undoubtedly
entognathy and the mouth fold, which is a most staple
constituent structure of entognathy, are novel structures.
Although there is no doubt that entognathy is derived fromectognathy, which has been confirmed to be the groundplan of
the mouth parts of hexapods, the evolutionary process, thatis, evolutionary transformation from an ectognathous toentognathous conformation, has not been well examined anddiscussed so far, with only Folsom (1900) addressing the issue.
In the embryological study of the collembolan Annurida
maritima, he correlated the mouth fold of Entognatha withthe subgena of Ectognatha, comparing the relative positionsof the mouth fold and subgena with the gnathal appendages. A
crucial weakness in Folsomʼs argument may be the lack of alandmark defining the extension of the mouth fold. The dorsal
limit of subgena is defined by the subgenal suture, which is alateral submarginal groove on the cranium of Ectognatha
roughly connecting the anterior and posterior tentorial pits
(cf. Snodgrass, 1935). However, the tentorial pits have notbeen identified in Entognatha, including Collembola (cf.
Matsuda, 1965). As the result of our embryological study ofTomocerus cuspidatus Börner, 1909, we succeeded inidentifying the posterior tentorial pits and designating theapproximate position of the anterior tentorial pits for the firsttime in Entognatha. The details of the tentoria will be given in
the full description of the entognathy formation of this species(Tomizuka and Machida, in prep.), and in the present paper wegive a preliminary description.

Figures 1 and 2 are the cross sections of the head of afirst instar larva of Tomocerus cuspidatus, in which theinvaginations of tentoria are clearly demonstrated. Figures 1and 2A are through the antennal base and posterior tentorialpit, respectively, and Figure 2B is through the maxillary basein the plane slightly anterior to Figure 2A. Figure 1 shows theanterior tentorial pit (arrowhead) in the region of the cranium
ventrolateral to the antennal base. The anterior tentorium
extends internally from its external pit and divides into twobranches, one of which directs along the gnathal pouch
enfolding the mandible (white arrow), and another of which
extends in the direction toward the stomodeum (black arrow).
Figure 2A and B show the posteior tentorium. The posteriortentorial pit is in the lateral region of the cranium, insidewhich the proximal end of the maxilla is located (arrowhead inFig. 2A). The posterior tentorium extends internally along theblind end of the gnathal pouch, which enfolds the maxilla (Fig.
2B). In the later stage of embryonic development, in which
the formation of entognathy is almost completed, theposterior tentorial pits are observed externally as a pair ofinvaginations (black arrowhead) between the maxillary andlabial terga around the level of the proximal end of the labium(arrow) (Fig. 3). The anterior tentorial pit is located in theregion of the cranium ventrolateral to the antennal base as in
the first instar larva (shown by white arrowhead in Fig. 3),
although the pit is concealed by the base of the antenna.
The mouth fold of Tomocerus cuspidatus is the ventral
extension of the fused terga of the intercalary, mandibular andmaxillary segments (Tomizuka and Machida, 2012; in prep.).
The mouth fold can be defined as the structure extending
ventrally, surpassing the appendicular bases and covering themandibular and maxillary sides, and the dorsal limit of themouth fold is should be designated by the appendicular bases.
However, the mandibles and maxillae should be excluded* This article, which was received in 2013 and should have been published in 2014, was printed in 2017 being much delayed due to various circumstances.

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S. TOMIZUKA AND R. MACHIDA

Figs. 1, 2 Cross sections of the head of a first instar larva of Tomocerus cuspidatus.
Fig. 1 A section through the antennal base, showing the anterior tentorium. The arrowhead and black and white arrows indicate theanterior tentorial pit and two branches of the anterior tentorium, respectively.
Fig. 2 Sections through the plane including the posterior tentorial pit (A) and that including the maxillary base, slightly anterior to A (B),showing the posterior tentorium. The arrowhead and asterisks indicate the posterior tentorial pit and ducts of the salivary gland,respectively.
An: antenna, AT: anterior tentorium, BGP: blind end of gnathal pouch, GP: gnathal pouch, LbT: labial tergum, Md: mandible, MF:
mouth fold, MMx: musculature of maxilla, MxT: maxillary tergum, PT: posterior tentorium, Sd: stomodeum, SG: salivary gland.
Scales = 20 µm.

from defining the dorsal limit of the mouth fould because,
during entognathy formation, their bases are largely shifteddorsally from their original positions. Therefore, we use theline connecting the antennal and labial bases as theapproximate dorsal limit of the mouth fold (broken line in Fig.
3). Figure 3 shows that the approximate dorsal limit of themouth fold (broken line) can be approximated as the line
connecting the anterior (white arrowhead) and posterior(black arrowhead) tentorial pits. As addressed above, thesubgena in Ectognatha is the area below the subgenal suture,
which is roughly the connection line between the anterior andposterior tentorial pits. Hence, the mouth fold in Collembola

TENTORIAL INVAGINATIONS OF TOMOCERUS CUSPIDATUS

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and subgena in Ectognatha are approximately homologous,
and Folsomʼs (1900) correlation of the mouth fold and subgena
is supported.
Acknowledgments: We thank our friend Peter Rühr ofRheinische Friedrich-Wilhelms-Universität Bonn, Dr. MakikoFukui of Ehime University, and Dr. Kaoru Sekiya of SugadairaMontane Research Center, University of Tsukuba for theirvaluable suggestions. This study was supported by Grants-inAid from the JSPS (Scientific Research C: 21570089 &2544020; Scientific Research B: JP16H04825) to RM. This iscontribution by the Sugadaira Montane Research Center,University of Tsukuba.

References
Folsom, J. W. (1900) The development of the mouth-parts of Anurida
maritima Guér. Bulletin of Museum of Comparative Zoology, 36,
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Fig. 3 An SEM of the cephalic region of an embryo ofTomocerus cuspidatus, lateral view. The left antenna
was removed. Black and white arrowheads indicate
the posterior tentorial pit and the approximate
position of the anterior tentorial pit, respectively.
Arrow indicates the labial base. Broken line indicatesthe dorsal limit of the mouth fold, approximated by
the line connecting the antennal and labial bases. An:
antenna, Cl: clypeus, E: eye, LbT: labial tergum, Lr:labrum, MF: mouth fold, MxP: maxillary palp, MxT:maxillary tergum, Prm: prementum. Scale = 50 µm.

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