SOIL ORGANISMS
Volume 81 (3) 2009
pp. 327–335
ISSN: 1864 - 6417
First ultrastructural investigation of the pharynx apparatus of
Scutigera coleoptrata (Chilopoda: Notostigmophora)
Gero Hilken & Jörg Rosenberg
Central
Animal
Laboratory,
University
Duisburg-Essen
Medical
School,
45122
Essen,
Germany;
e-mail: [email protected]; [email protected]
Abstract
The organisation and possible function of the pharynx apparatus of Scutigera coleoptrata was first
described in 1967 by Seifert. The pharynx apparatus in the foregut of S. coleoptrata consists of a ballshaped outgrowth of the dorsal pharyngeal wall, the pistil, and a cooperating ventral groove, the pharynx
furrow. Here we present new structural details of the pistil and the pharynx furrow with its underlying
epithelium, using LM, TEM, and SEM techniques.
Keywords: centipedes, foregut, pistil, pharynx furrow, fine structure
1. Introduction
Our knowledge of the digestive system of centipedes is still fragmentary. The more recent
overviews (Lewis 1981, Minelli 1993, Rosenberg 2009) describe the gut as a straight tube
which is clearly divided into an ectodermal stomodeum or foregut, an entodermal mesenteron
or midgut, and an ectodermal proctodeum or hindgut. As ectodermal invaginations, the
foregut and the hindgut are lined by a cuticle. The gross anatomy of the alimentary canals of
representatives of the Lithobiomorpha (Lithobiidae), Scolopendromorpha (Cryptopidae), and
Geophilomorpha (Geophilidae, Himantariidae) was first presented by Plateau (1878).
Subsequent studies focused particularly of anatomical and histological details of the gut of
scolopendromorph (Willem 1889, Balbiani 1890, Kaufman 1962, Jangi 1966, Koch et al.
2009) and geophilomorph (Kaufman 1960) centipedes. The gut of Lithobius forficatus
(Linnaeus, 1758) (Lithobiomorpha) was surveyed by Sograff (1880) and studied in more
detail (anatomy and histology) by Kaufman (1961a) and Rilling (1968). Almost no
information exists for the anatomy of the gut of Craterostigmus tasmanianus Pocock, 1902
(Craterostigmomorpha), apart from Manton´s (1965) short outline. The digestive system of
the Scutigeromorpha has not been described in full detail thus far. Our present knowledge is
largely based on anatomical descriptions on scutigeromorph centipedes by Takakuwa (1955)
and histological descriptions of Scutigera coleoptrata (Linnaeus, 1758) by Kaufman (1961b).
The digestive system as a whole has not yet been described. Verhoeff´s (1902–1925)
description of the centipede pharynx also included the roof of the preoral chamber
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(epipharynx) and its bottom (hypopharynx), which is subsumed as peristomatic organs (Haase
1884, ‘Schlundgerüst’, Verhoeff 1902–25, ‘peristomatische Organe’). Recent investigations
of the epi- and hypopharynx confirmed considerable variability and morphological
complexity of the peristomatic structures in Scutigeromorpha, Lithobiomorpha, and
Scolopendromorpha (Koch & Edgecombe 2006, 2008, Edgecombe & Koch 2008). An
account of the pharynx apparatus in Scutigeromorpha in more detail is still missing.
Seifert (1967) first recognised that the pharynx apparatus in Scutigera coleoptrata
represents no mere prolongation of sclerotised bars of the preoral chamber extending into the
pharynx. He identified a stalked ball-shaped outgrowth of the dorsal pharyngeal wall that he
called the pistil (‘Pistill’) and a ventral groove of the pharynx bottom (‘Mörserrinne’, pharynx
furrow). Here, we present some new structural details on the pharynx apparatus of Scutigera
coleoptrata. We will describe the cuticle of the pistil and of the pharynx furrow with its
underlaying epithelium, using LM, TEM, and SEM techniques.
2. Materials and methods
2.1. Animals
Specimens of Scutigera coleoptrata (Linnaeus, 1758) were collected around Banyuls-sur
Mer, France. The specimens were kept in glass boxes (15 cm x 8 cm x 15 cm) filled with 1
cm garden soil and two pieces of moistened (tap-water) blotting paper (15 cm x 15 cm each).
The animals were fed with wingless Drosophila melanogaster Meigen, 1830 and Musca
domestica Linnaeus, 1758. Specimens of S. coleoptrata were anaesthetised with CO2, cut into
several parts and fixed as described below.
2.2. Light and transmission electron microscopy (TEM)
For TEM investigation, heads of Scutigera coleoptrata were fixed in phosphate-buffered
(pH 7.2) paraformaldehyde (4 %), containing 15 % saturated picric acid and 0.08 %
glutaraldehyde. They were postfixed with 1 % OsO4 in the same buffer and, after alcohol
dehydration, embedded in Epon. Semithin sections (0.5–1 µm) were stained with toluidine
blue (Trump et al, 1961; modified: toluidine blue share 1 %). Sections were studied using a
DMSL-Leica microscope. Ultrathin sections were stained with uranyl acetate and lead citrate
and studied using a ZEISS EM 902 A electron microscope.
2.3. Maceration procedure and scanning electron microscopy (SEM)
The SEM investigation of the cuticle lining of the pharynx furrow requires the removal of
the tissue. We chose a maceration technique using pepsin following Hilken (1994). The
solution of 1.5 g pepsin in 100 ml HCl-solution (1.5 %) was effective with a maceration
period lasting up to 10 to 30 days. SEM specimens were critical-point dried, sputter-coated
with gold, and examined using a CAMSCAN DV4 scanning electron microscope.
3. Results
The foregut of Scutigera coleoptrata with its pharynx apparatus was described on a light
microscopical level by Seifert (1967). He identified a stalked pistil on the dorsal pharyngeal
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Pharynx apparatus of Scutigera coleoptrata
wall and a ventral groove on the bottom of the pharynx. Therefore, we restrict our description
to some features of the pharynx apparatus mainly detected by TEM and SEM-investigations.
The pharynx apparatus (‘Pharynxapparat’, Seifert 1967) of Scutigera coleoptrata is in part
a sclerotised structure on the dorsal and ventral wall of the middle part of the pharynx (Fig.
1A, D, E). It consists of a cuticular groove on the floor of the pharynx, the pharynx furrow
(Fig. 1A–E). This furrow flattens at its posterior end (Fig. 1F). Its length is about 200 µm, its
diameter about 70 µm. Additionally, on the dorsal wall of the pharynx a cuticular ridge that
bears a stalked knob is developed, the so-called pistil (Figs 1E, 2A, 3A, B). In the area of the
pharynx apparatus, the lumen of the foregut is relatively small and U-shaped in comparison
to the posterior part. The roof of the pharynx is typhlosole-like invaginated in the area of the
pistil (Fig. 1D, E).
pf
A
100 µm
C
B
po
po
vi
vi
20 µm
D
10 µm
E
F
nr
dm
p
pf
Fig. 1
50 µm
vi
pf
50 µm
pf
50 µm
A–F: The pharynx apparatus of Scutigera coleoptrata. – A: Foregut in the area of the
pharynx apparatus, from ventral (SEM); B: Porous wall of the pharynx furrow, from ventral
(SEM); C: Detail of B; D: Anterior region of the foregut with the beginning of the pharynx
furrow (cross section, LM); E: Middle region of the foregut with full developed pharynx
furrow and an inserted pistil (LM); F: Posterior region of the foregut with a flattened
cuticular section in the region of the pharynx furrow. Huge dilator muscles are attached
dorsally to the foregut (cross section, LM). dm, dilator muscle; nr, Nervus recurrens; p,
pistil; pf, pharynx furrow; po, pores; vi, ventral invagination.
Gero Hilken & Jörg Rosenberg
330
A
B
C
pf
dm
p
pf
pf
10 µm
D
vi
10 µm
2.5 µm
F
E
pf
c
vi
c
ac
me
me
1 µm
H
G
1 µm
1 µm
I
ml
c
c
po
eg
po
1 µm
Fig. 2
0.2 µm
0.2 µm
A–I: The pharynx apparatus of Scutigera coleoptrata. – A: Pharynx furrow with the inserted
pistil. The nuclei of the epithelial cells are arranged lateral to the furrow (cross section, LM);
B: The two flaps of the pharynx furrow. The pistil is extracted. A dilatator muscle and the
underlaying ‘multi-layered’ epithelium are visible (TEM); C: Detail of B. Numerous pore
canals pass the cuticle of the pharynx furrow. Only in its basal part, the endocuticular
lamellae are packed densely, whereas in the apical part, the distances between the lamellae
are widened (TEM); D: Ventral invagination between the two flaps of the pharynx furrow
and the underlaying ‘multi-layered’ epithelium with rounded cells in cross section (TEM);
E: Detail of the ‘multi-layered’ epithelium at a lateral region of the furrow. The projections
of the cells run oblique around the furrow and are equipped with numerous mitochondria
(TEM); F: In the region where the flaps are connected ventrally, the cuticle is conspicuously
structured. Apically, a broad sclerotised area is observable. Horizontally arranged
sclerotised stripes reaches into the underlaying cuticle (TEM). G: Perpendicular to the
cuticular layers, numerous pore canals cross the cuticle (TEM); H: The cuticle of the
pharynx furrow is covered by a distinct mucous layer. Apically, a small sclerotised cuticular
layer is developed, interrupted by small, unsclerotised sections (TEM); I: Several epidermal
glands are found between the foregut epithelium around the pharynx apparatus (TEM). ac,
aberrant cuticle; c: cuticle; dm: dilatator muscle; eg: epidermal gland; me: ‘multi-layered’
epithelium; ml: mucous layer; p: pistil; pf: pharynx furrow; po: pore canals; vi: ventral
invagination.
Pharynx apparatus of Scutigera coleoptrata
331
The whole gut is surrounded by circular muscles (Fig. 3G). Seifert (1967) described several
muscles (ventral and dorsoventral dilators) that are attached to the cuticle of the foregut. The
attachment of one of the dilator muscles is shown in Fig. 3F. The epithelium of the foregut is
single-layered (Fig. 3E). Scattered epidermal glands are found between the cells of its
epithelium (Fig. 2I). Only in the epithelium around the furrow and the insertion area of the
pistil the epithelium appears ‘multi-layered’. The nuclei in the region of the furrow are
situated laterally (Fig. 2A). The ‘multi-layered’ epithelium is formed by stretched longitudinal
arranged cells; their projections are ventrally directed and equipped with several mitochondria
(Fig. 2D–E). In cross sections of the median area of the furrow, the cellular projections appear
more or less roundish (Fig. 2D). It seems that the projections of these cells run obliquely
around the furrow, as seen in a more laterally situated area (Fig. 2E).
The pharynx furrow is formed by two cuticular flaps, linked ventromedially by a small
differentiated cuticle area (Fig. 2A–C). At its connection, a ventral invagination is developed
(Fig 2B–D). In LM, the cuticle of the pharynx furrow appears strongly sclerotised (Fig. 2A).
However, in TEM, the specialised cuticle of the furrow is not homogeneously structured and
sclerotised. Laterally, the lamellated endocuticle of the pharynx is displaced by the cuticle of
the furrow. Only in its basal part, endocuticular lamellae are densely packed, whereas in the
apical part, the distances between the lamellae are conspicuously widened (Fig. 2C, D). Only
apically is a small sclerotised cuticular layer developed, interrupted by small but not
sclerotised sections (Fig. 2H). Perpendicular to these cuticular layers, hundreds of pore canals
cross the cuticle (Fig. 2C–D). These canals originate from the underlying epithelium and seem
to open at the surface of the furrow (Fig. 2G–H). Also in SEM-images it is observable that the
whole cuticle of the inner side of the furrow is pierced by numerous pores (Fig. 1B–C). The
cuticle of the pharynx furrow is covered by a distinct mucous layer (Fig. 2H). Often bacteria
are attached to this layer (Fig. 3G), that is probably secreted by epidermal glands, located in
the epithelium of the foregut (Fig. 2I). In the region where the flaps are ventrally connected,
the cuticle is conspicuously structured. Endocuticular lamellae and pore canals in this region
are not observable (Fig. 2C, F). Apically, a broad and sclerotised area is observable (Fig.
2B–D). Horizontally arranged sclerotised stripes draw into the underlaying cuticle (Fig. 2F).
The pistil consists of an elongated shaft and a spherical end piece (Figs 1E, 3A, B). The
cuticle of the pistil is mainly sclerotised in its marginal parts; a small sclerotised band is
observable in the middle part of the shaft (Fig. 3A, B). The remaining cuticle of the foregut
is folded and underlain by a subcuticle (Fig. 3C, F). The broad lamellated endocuticle is
covered by a small sclerotised exocuticle (Fig. 3F). Scattered epidermal glands are arranged
between the epithelium of the foregut (Fig. 2I).
Gero Hilken & Jörg Rosenberg
332
A
C
me
me
c
sc
sc
2.5 µm
2.5 µm
D
B
me
p
se
c
2.5 µm
E
F
c
pc
c
cm
2.5 µm
dm
G
2.5 µm
Fig. 3
c
2 µm
b
A–G: The pharynx apparatus of Scutigera coleoptrata (TEM). – A-B: Pistil and the
underlaying ‘multi-layered’ epithelium. The pistil consists of an elongated shaft and a
spherical end piece; C: Transition zone between the ‘multilayered’ epithelium and the
connected single-layered epithelium in the dorsolateral region of the foregut. A distinct
subcuticle is observable beside a lamellated endocuticle; D: Transition zone between the
‘multilayered’ epithelium and the regular single-layered epithelium; E: Insertion of one of
the ventrolateral dilatator muscles; F: Circular muscle, surrounding the cuticle of the
foregut; F: Bacteria, attached to the wall of the foregut. b: bacteria; c: cuticle; cm: circular
muscle; dm: dilatator muscle; me: ‘multi-layered’ epithelium; p: pistil; sc: subcuticle; se:
single-layered epithelium.
Pharynx apparatus of Scutigera coleoptrata
333
4. Discussion
The digestive tract of Chilopoda is divided into a foregut, a midgut, and the hindgut. In the
foregut of different Chilopoda conspicuous cuticular structures are developed. In
Lithobiomorpha the anterior part of the pharynx is armed with backward directed spines
(Gibson-Carmichael 1885) and in representatives of the Scolopendromorpha, the cuticle of
the anterior part of gizzard is characterised by cuticular plates with ridges and backward
directed spines (Balbiani 1890, Jangi 1966, Koch et al. 2009). The pharynx apparatus in the
foregut of notostigmophoran Chilopoda was overlooked by Haase (1884) and Verhoeff
(1902–25); it was first recognised by Seifert (1967).
Why does the Notostigmophora develop such a complicated structure in the foregut? Seifert
(1967) suggests some functional aspects: The pistil seems to fit into the pharynx furrow like
a milling device. This study shows that the pistil is quite a delicate structure that is not as
strongly sclerotised as it seems to be in LM images. Thus, its function as a tool for trituration
remains uncertain. Additionally, the furrow is not suitable to act as a friction surface because
of the relatively less sclerotised cuticle. It seems to be more likely that the pharynx apparatus
may serve primarily as a structure to separate large from small food particles and to grind the
food particles. A gliding of the pistil in the pharynx furrow may be supported by the mucous
layer that acts as a lubricant. The mucus might originate from epidermal glands, located in the
foregut epithelium, and from salivary glands in the head region.
Seifert (1967) stated that the pistil can be shifted into or extracted out of the pharynx furrow
only in its posterior end section. According to our study, the two flaps of pharynx furrow seem
to be moveable. Due to the basal invagination and the specialised cuticle of the connecting
area between the flaps it is likely that they can be pulled apart by ventral dilatator muscles.
These muscles were already described in detail by Seifert (1967). The connecting cuticle
between the two flaps seems to have a high elasticity. Thus, we assume that the pistil can be
inserted and extracted over the whole range of the pharynx furrow, when the furrow opens
dorsally. A main function of the pharynx apparatus as a mill structure appears questionable,
and a main function as a sorting structure seems to be more plausible.
The investigation of the fine structure of the pharynx furrow shows that the wall of the
furrow is pierced by hundreds of pore canals. The function of these canals is not clear yet.
However, the thick mucous layer that covers the cuticle of the furrow apically is probably
formed by the underlying specialised epithelium, although this epithelium shows no glandular
character on its own. Within the lateral wall of the foregut, we documented several epidermal
glands releasing their secretion into the foregut. However, it is not clear, whether these glands
produce digestive enzymes already in this part of the gut.
The described ‘multi-layered’ epithelium in the ventral and dorsal part of the pharynx
apparatus is interpreted as a network of obliquely directed elongated cells. It seems that this
epithelium follow the extensions of the foregut, depending of the filling degree of the foregut.
Similar epithelia in Chilopoda are described as ‘pseudo-multilayered’ in the maxillary organ
of Scutigera coleoptrata (Hilken & Rosenberg 2006). Actually, even this kind of epithelia is
most likely single-layered.
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5. Acknowledgements
We are grateful to Dr Markus Koch (Institute of Biology, Freie Universität Berlin) for his
helpful comments on the manuscript.
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Accepted 25 September 2009