Arthropod Structure & Development 38 (2009) 401–416
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Arthropod Structure & Development
journal homepage: www.elsevier.com/locate/asd
Early development of the anterior body region of the grey widow spider
Latrodectus geometricus Koch, 1841 (Theridiidae, Araneae)
Yu Liu*, Andreas Maas, Dieter Waloszek
Workgroup Biosystematic Documentation, University of Ulm, Helmholtzstrasse 20, Ulm, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 November 2008
Accepted 1 April 2009
We document the early morphogenesis of Latrodectus geometricus, particularly of the anterior body
region. Significant changes in the development of the external prosomal structures revealed with
scanning electron microscopy (SEM) images include: (1) reorganisation of each pre-cheliceral lobe by
subdivision and internalisation of its central area; (2) shortening of the ventro-median bridge connecting
the pre-cheliceral lobes and its eventual disappearance; (3) appearance and expansion of a prospective
mouth region between the pre-cheliceral lobes with a recessed median area surrounded by lip-like
borders, the anterior lip-part developing into the hypostome; (4) reduction of the mouth region to an
area around the hypostome and the lip-like latero-posterior border of the mouth opening; (5) change of
the position of the mouth region from anterior to the insertions of the chelicerae to posterior to them; (6)
eventual shortening of the mouth opening to a slit overhung by the hypostome; (7) origination of the
prosomal shield from the anterior margin of the pre-cheliceral lobes and the tergal portions of the four
posterior-most prosomal segments; and (8) expansion of a ‘ventral sulcus’ from the cheliceral to the fifth
opisthosomal segment separating the sides of these segments. Embryonic features are compared across
the Chelicerata and discussed briefly in a phylogenetic context.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Prosoma
Spider embryos
Chelicerata
Phylogeny
1. Introduction
Crown-group chelicerates (¼Euchelicerata, following the
system proposed by Chen et al., 2004) have a specific body tagmosis
not found in other euarthropods (trilobites, crustaceans, myriapods
and insects): the anterior region, prosoma, comprises seven
segments, and the posterior region, opisthosoma, 13 segments. The
prosoma encompasses the ocular segment, and the segments
carrying chelicerae, pedipalps and the other four pairs of limbs. Like
the head of other arthropods, the prosoma is a character-rich region
for reconstructing relationships, in this case amongst chelicerates
(Anderson, 1973, p. 442). Early embryos show striking similarities
between the prosoma of chelicerates and the head of other
arthropods (Anderson, 1973). This provides an opportunity to
compare the developing morphology and topology of structures of
the anterior region of the embryos across arthropods. These
embryological data can contribute to our understanding of
arthropod phylogeny.
Among the crown-group chelicerates, embryos of spiders have
played an important role in recent studies about arthropod
* Corresponding author. Tel.: þ49 731 503 1006; fax: þ49 731 503 1009
E-mail address: yu.liu@uni-ulm.de (Y. Liu).
1467-8039/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.asd.2009.04.001
evolution. For example, numerous molecular studies on the
embryos of Cupiennius salei Keyserling, 1877 (Ctenidae, Entelegynae) make this animal a well-established model species (Stollewerk
et al., 2001; Damen, 2002; Schoppmeier and Damen, 2005; Janssen
et al., 2008; Stollewerk and Seyfarth, 2008; McGregor et al., 2008).
Historically, basic studies on the embryonic development of
spiders, focusing on morphogenesis, have been carried out since
the 19th century (e.g. Herold, 1824; Claparède, 1862). This early
research remained of interest until well into the second half of the
20th century (see Anderson, 1973). Surprisingly, subsequent to the
review of Anderson (1973), interest in this issue declined significantly with the exception of a brief paper by Marechal (1994) that
paid special attention to the visual system of a mygalomorph
spider, Ischnothele guyanensis (Walckenaer, 1837).
Existing knowledge about spider embryonic development can
be briefly summarised as follows. Once cleavage has finished, the
yolk-rich spider eggs enter a blastula stage, at which most of the
blastoderm cells begin to converge at one side of the egg to form
the germ disc (Holm, 1952, 1954; Rempel, 1957; Suzuki and Kondo,
1995). The centre of the germ disc then has a bulged appearance
due to the development of the caudal cumulus underneath the
germ cells (Holm, 1952, 1954; Rempel, 1957; Suzuki and Kondo,
1995; Akiyama-Oda and Oda, 2003). The caudal cumulus later on
migrates to the periphery of the egg and its migrating route finally
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Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
splits the germ disc (Akiyama-Oda and Oda, 2003). The two separated parts of the germ disc migrate away from each other until the
whole germinal area is band-like in shape (Holm, 1952, 1954;
Rempel, 1957; Suzuki and Kondo, 1995; Akiyama-Oda and Oda,
2003). Thereafter, the germinal area is called germ band. The germ
band elongates itself around the egg and stops elongating when its
anterior and posterior ends almost touch (Holm, 1952; Rempel,
1957; Chaw et al., 2007; McGregor et al., 2008). In some species, the
distance between the two ends is not that short, because the
second half of the opisthosoma does not attach to the egg surface
but folds anteriorly (e.g. Holm, 1940; Yoshikura, 1955). At this time
in development, a process called ‘inversion’ (or reversion) begins.
This process occurs in most spiders except those of the Mesothelae
and certain species from the Orthognatha (Foelix, 1996, p. 216). The
inversion in spider embryology means that the germ band, which is
initially situated on one side of an egg, is separated into two halves
longitudinally by a furrow appearing in the midline of the germ
band. The furrow was termed a ‘ventral sulcus’ by various embryologists (see Anderson, 1973). The two halves of the germ band are then
pushed away from each other by the widening of the ventral
sulcus, and they finally reach a position on the lateral side of the
egg (Holm, 1952, 1954; Rempel, 1957; Yoshikura, 1954, 1955, 1958,
1961, 1972; Suzuki and Kondo, 1995). Thereafter, a thin epidermal
layer starts to grow on the ‘dorsal’ side of the egg to connect the
two halves of the germ band. This process is called ‘dorsal
closure’, at the end of which the body form of the hatchling,
resp. the adult spider emerges (Holm, 1940, 1952, 1954; Rempel,
1957; Yoshikura, 1954, 1955, 1958, 1961, 1972).
In order to understand the development of the prosoma of
chelicerates, and to compare it with the head formation of
Fig. 1. Latrodectus geometricus Koch, 1841. A. Arrangement of a Step 5 embryo growing around the egg. B. Scheme demonstrating the range of angles covered by SEM photographs
obtained from the different parts of the embryo. Labels C–E correspond to the subsequent photographs. C. Anterior area ranging from posterior tip of the telson to the walking limb
L2. D. Mid-level area ranging from the hypostome to the walking limb L4. E. Posterior area ranging from the walking limb L4 to the tip of the hypostome. Scale bars ¼ 100 mm.
Abbreviations: CH: chelicerae, e: pedipalpal endite, EP: epithelial portion, HY: hypostome, L1–4: walking limbs 1–4, M: mouth opening, OLB1–4: opisthosomal limb buds 1–4, PCL:
pre-cheliceral lobes, PP: pedipalp, TE: telson, VS: ventral sulcus, Y: yolk.
Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
other arthropods, we employed scanning electron microscopy to
investigate embryos of the grey widow spider Latrodectus geometricus Koch, 1841 (Theridiidae, Araneae) at an ultrastructural
level. Several details found herein are first documented for spider
embryos: (1) a ventro-median bridge connecting the pre-cheliceral
lobes and its development; (2) three pores being arranged like the
mathematical ‘because’ sign (q) in the prospective mouth region;
and (3) the prosomal shield is developed from the fusion of the
anterior margin of the pre-cheliceral lobes and the tergal portions
of the four posterior-most prosomal segments. We discuss our data
in the light of earlier investigations on the embryogenesis within
the Chelicerata. Common features found in early embryos of
various chelicerates are further discussed. Such discussion provides
preliminary information for mapping the embryonic features into
ground pattern conditions, as what has been briefly carried out by
Anderson (1973) and Yoshikura (1975).
2. Materials and methods
Living material of Latrodectus geometricus was kindly provided
by Martin Thierer-Lutz from the born to be eaten Insektenzucht
GmbH, Schnürpflingen, Germany. The selection of this species is
mainly based on three reasons: (1) easy lab culture of the adults; (2)
the large amount of eggs produced by the females all through the
year; and (3) the embryonic development of this species has never
been described before. Eggs were removed from the cocoons and
fixed with Carnoy’s fixative (100% alcohol: chloroform: glacial
acetic acid ¼ 6:3:1) for 30 min. The fixed samples were then rinsed
two times in 90% ethanol. After the fixation, the outer chorion was,
in most cases, ruptured so that it could easily be removed from the
yolk resp. embryo by gentle pipetting. The inner vitelline
membrane was removed manually in 0.1 M phosphate-buffered
saline (PBS) by using tissue forceps under a binocular. No postfixation was applied. The dissected embryos were dehydrated
through a graded series of ethanol. The samples were incubated in
hexamethyldisilazane for 10 min and then air-dried overnight
(Nation, 1983a,b; Giammara et al., 1987). After mounting on stubs
with black wax, the samples were sputter-coated with a mixture of
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gold and palladium, and observed under a scanning electron
microscope (SEM; Zeiss DSM 962).
The embryo of Latrodectus geometricus grows around the egg
such that the anterior and posterior ends approach each other
(Fig. 1A). The provided diagram (Fig. 1B) includes the three standard
angles that we used to document photographically the anterior,
median and posterior parts, respectively (Figs. 1C–E). Of these, we
focus on the anterior view, as in Fig. 1C. The digital SEM images
obtained were trimmed in Adobe PhotoshopÔ and arranged into
plates in Adobe IllustratorÔ.
We adopted mainly the terminology and staging of spider
embryos introduced by Anderson (1973), Suzuki and Kondo (1995),
Akiyama-Oda and Oda (2003) and Chaw et al. (2007) for embryonic
terms and general chelicerate/arachnid and arthropod terminology.
We use the term ‘step’ for describing every single specimen that was
fixed at a certain moment of development. The term ‘stage’ is avoided
because it represents a period of time. Concerning the phylogeny
within Arachnida we follow the system put forward by Ax (1999), in
which the taxon Scorpionida is the sister taxon to the remaining
arachnids, the Lipoctena (sensu Weygoldt and Paulus, 1979).
3. Results
The current work mainly presents investigations on the postgerm disc development of the anterior region of the widow spider
embryos (Figs. 2A and 3–5; see the supplementary data for brief
observations on the germ disc phase). The dorsal side of the
embryos at selected steps is presented in order to document the
formation of the prosomal shield (Fig. 5). Two diagrams are
provided to explain the details (Figs. 6 and 7). In total, 16 embryonic
steps are recognised with significant changes between consecutive
steps (a detailed description of each step identified is included in
the supplementary data). Embryonic development of the widow
spider results in a hatchling (Fig. 8). We herein provide a summarised description of the major events occurring in the early development of the anterior body region of Latrodectus geometricus.
A pair of lobe-like structures comprises the anterior-most part of
the spider embryo. We herein use the term ‘pre-cheliceral lobes’ for
Fig. 2. Latrodectus geometricus Koch, 1841. SEM pictures of a Step 1 embryo. A. Antero-lateral view demonstrating the pre-cheliceral lobes (PCL) and the following limb buds (CH, PP,
L1, L2). B. Postero-lateral view demonstrating the last four pairs of prosomal limbs (L1–4) and the opisthosoma (OP). Note that the walking limb L4 is presented as a single swelling,
from which the two limb buds will differentiate at the next step. Abbreviations other than used in the previous figure: An: anterior.
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Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
highlighting the location of those structures. The development of the
pre-cheliceral lobes starts with the occurrence of an initially smooth
area (Figs. 2A, 3A,B and 6A,B), which changes into an area with
distinct structures such as median and lateral subdivisions (ms, ls in
Figs. 3, 4 and 6; marked in yellow in Fig. 6; see also Table 1) and three
pairs of furrows appearing successively in the central area (a, b, c in
Figs. 3, 4 and 6; Table 1). Subsequently, the two subdivisions merge
into each other to form the single central area of each pre-cheliceral
lobe (yellow in Fig. 6). The central area later recesses or sinks
underneath the anterior margin (AM in Figs. 3–6) of each precheliceral lobe through the former anterior furrow (a in Figs. 3, 4
and 6), and then becomes internalised. Eventually, the entire central
area vanishes from the surface of the embryo (Figs. 4D–H and 6L–P;
Table 1). The anterior margin becomes more and more prominent,
and it folds slightly dorsally and posteriorly. It eventually fuses with
the developing dorsal (tergal) area of the four posterior-most prosomal segments, and forms the anterior part of the prosomal shield
(Fig. 5; Table 1; see the following for more details).
In the earliest recognised step, only pre-cheliceral lobes are
present as the anterior-most structures (Figs. 2A and 3A). A smooth
sub-rectangular area between the lobes appears at the next step
(Fig. 3B, red in Fig. 6B; see also Table 2). This area has a significant
differentiation during later development (red in Figs. 6C–H). One
feature is that a shallow depression first appears with three small
pores (Figs. 3C, 6C and 7B), which are bordered anteriorly by a pair
of swellings, the prospective hypostome (HY in Figs. 3C and 7B).
Subsequently, the depression deepens to form the mouth opening,
while the prospective hypostome rises to form a uniform lobe
eventually. The posterior border of the mouth opening develops
into a lip-like ridge (Figs. 3D–G, 6D–G and 7C,D). The mouth
opening becomes relatively shorter, and widens to a slit. Together
with the posterior ridge around it, the mouth opening becomes
overhung by the developing hypostome (Figs. 3H, 6H and 7E). The
hypostome eventually represents the only visible structure left of
the entire initial mouth area (red in Figs. 6H–P). We did not detect
the fate of the three pores after Step 4 or Step 5 (Figs. 3D and 6D), so
we have no clues yet of their structural nature and function. In
summary, in L. geometricus, a mouth area does not develop until the
germ band has reached its full length and until the buds of the
prosomal limbs have all appeared (Figs. 3A,B and 6A,B). Therefore
we term the smooth area of Step 2 ‘initial mouth area’. This area and
the three pores within the initial mouth region have rarely drawn
attention before.
Dorsally, after the central area of each pre-cheliceral lobe has
internalised, the marginal area represents the major remains of
each lobe (Fig. 4D, green in Fig. 6L). As the anterior part of the
marginal area, i.e. the anterior margin grows dorsally and posteriorly, its anterior rim starts to get fused with the developing tergal
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area of the four posterior-most prosomal segments (compare the
open arrowhead in Figs. 5C–F). This fusion results in a shield
covering the entire prosomal region. Hence, the shield is called
prosomal shield, of which the anterior-most part is developed from
the anterior margin of the former pre-cheliceral lobes. On the other
hand, the posterior rim of the pre-cheliceral lobes secondarily
develops into the anterior border of the prosomal shield (compare
the black arrowhead in Figs. 5A–F; see also Figs. 3C–H, 4 and 6C–P).
The complete outline of the prosomal shield is visible from Step 12
on (Figs. 4D, 5D and 6L). The tergal area of the cheliceral and
pedipalpal segments is undetectable due to the forward shift of the
attaching position of those appendages. In summary, formation of
the prosomal shield of L. geometricus is completed before the
embryonic phase ends. The prosomal shield develops from two
different structures: (1) the anterior margin (AM in Figs. 3–6) of the
pre-cheliceral lobes; and (2) the tergal area of the four posteriormost prosomal segments.
A longitudinal furrow is present at the ventral midline, labelled
ventral sulcus by many authors (e.g. Anderson, 1973), and it is
present from Step 2 to Step 14 (Figs. 3B–H, 4A–F and 6B–N; see also
Table 3). From the cheliceral segment to about the fourth or fifth
opisthosomal segment, the embryonic body primordium is divided
into two halves longitudinally by the ventral sulcus at the ventral
midline (Figs. 1C–E). As the two separate halves of the embryonic
median body primordium migrate to the sides, the width of the
ventral sulcus is increasing, being widest at the border between
prosoma and opisthosoma. In parallel, the anterior and posterior
ends of the embryo are drifting apart from each other passively as
a result of the width expansion of the ventral sulcus, since the
length of the embryonic body primordium does not increase
throughout the entire post-germ disc phase (Figs. 5A–C). At this
step, the differentiation of the prosoma and the opisthosoma can be
recognised by the fact that on each half of the germ band, the area
at the boundary between the prosoma and the opisthosoma has
been narrowed much more than the remaining area. The area at the
boundary between the prosoma and the opisthosoma later on
becomes a thin stalk bridging these two tagmata.
A single ventral sternal (cuticular) plate with a smooth surface
begins to form at the time when the two halves of the embryonic
body primordium have been pushed sideways by the widening of
the ventral sulcus to eventually reach their final positions. This
formation occurs through an expansion of the ventral epithelial
portion of the four posterior-most prosomal segments (EP in Figs.
4C,D, see also Figs. 4E–G and 6M–P). Ventral epithelial portions (EP
in Figs. 3 and 6B) of the cheliceral and pedipalpal segments are
visible at Step 2–Step 8 (Figs. 3B–H and 6B–H), and almost undetectable in later steps (e.g. Figs. 4A–H and 6I–P). There are many
modifications of the ventral inter-limb area among the extant
Fig. 3. Latrodectus geometricus Koch, 1841. SEM pictures of Step 1–Step 8 embryos. Anterior view. A. Step 1 embryo: two pre-cheliceral lobes (PCL) with smooth surface are separated
by a concave region (open triangle), all limb buds are located posterior to the pre-cheliceral lobes, the buds of each limb pair approaching each other basally (asterisks); B. Step 2
embryo: enlarged concave region (open triangle) between the pre-cheliceral lobes (PCL). The ventral sulcus (VS) and epithelial portion (EP) keep the limb buds much further apart
than before. C. Step 3 embryo: two subdivisions (ms, ls; highlighted also by white solid lines) differentiate in the central area of each pre-cheliceral lobe. A mouth region appears
with the prospective hypostome (HY) anteriorly, three small pores (white arrowheads) and a U-shaped swelling posteriorly. The ventral sulcus starts to widen. D. Step 4 embryo: the
anterior margin (AM) starts to differentiate due to the development of an anterior furrow. (a) A lateral furrow (b) is present in each pre-cheliceral lobe. The hypostome (HY) has
a median furrow (white arrowhead) indicating its bilobed appearance. The area bearing three pores recesses within a single mouth opening (inset: magnified image from another
specimen). E. Step 5 embryo: in each pre-cheliceral lobe, the two furrows (a, b) deepen, the median subdivisions (ms) bulge, while the lateral subdivisions (ls) flatten. The
hypostome (HY) loses its bilobed appearance. F. Step 6 embryo: in each pre-cheliceral lobe, the anterior furrow (a) deepens while the lateral furrow becomes narrower than in the
previous step. Note that up to this step the chelicerae (CH) are located posterior to the mouth opening (M). G. Step 7 embryo: both subdivisions in each pre-cheliceral lobe flatten.
The chelicerae (CH) are para-oral. The hypostome (HY) has a straight posterior border, and the mouth opening (M) is slit-like. On the outer side of the basal portion of each pedipalp,
note the newly developed cone-like projection (white circle, hypothesised to penetrate the egg shell at hatching, see Discussion). H. Step 8 embryo: in each pre-cheliceral lobe, the
anterior furrow (a) is groove-like, while the lateral furrow (b) is elongated. The anterior margin (AM) is more prominent than in the previous steps (compare ‘AM’ in D–H). A newly
developed furrow (c) is situated posterior to the median subdivision (ms). The mouth opening (M) is overhung by the hypostome (HY). The ventral sulcus is wider than in all
previous steps (compare ‘VS’ in C–H). In all pictures, the dashed line marks the posterior margin of one of the pre-cheliceral lobes; the open star labels the pre-hypostomal region
(the star in F also labels the pre-hypostomal region, but the vertical ridge here is interpreted as a shrinkage artefact); black arrows mark the paired pores close to the base of the
chelicerae; white arrows point to the inner borders of the ventral sulcus; white (open) and black arrowheads mark the anterior and posterior rim, respectively, of the (prospective)
anterior margin (AM) of the left pre-cheliceral lobe. Scale bars ¼ 100 mm (inset in (D) ¼ 10 mm).
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Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
arachnid taxa (see, e.g., Siewing, 1985, his Fig. 841; Westheide and
Rieger, 1996, their Fig. 619). Due to insufficient information about
the embryonic development of some arachnid taxa, it is difficult to
draw any phylogenetic conclusions about the formation of the
sternites or sternum in arachnids at the moment.
The six pairs of prosomal limb buds together with the precheliceral lobes are the first recognizable external structures in
early embryos of Latrodectus geometricus (Figs. 2 and 3A). At the
earliest recognised step herein for L. geometricus, the left and right
buds of the first five pairs of prosomal limbs are very closely
apposed (Figs. 2A, 3A and 6A). The two limb buds of the sixth (¼the
last) prosomal limb pair (L4 in Fig. 2B) are undifferentiated from
a single swelling. At the next step, the two limb buds are clearly
developed. The distance between the left and right buds of all
prosomal limbs enlarges due to the development of the inter-limb
sternal region of the segments and the widening of the ventral
sulcus. In parallel, the limb buds of the pedipalps and following
walking legs become progressively more elongated and segmented
(Figs. 3B–H and 4). An endite starts to develop on the inner side of
the basal (¼basipodal) podomere of the pedipalps from Step 3
onwards (Figs. 3C and 6C). All limb buds are initially post-oral (Figs.
2A and 3A), and this situation does not change until Step 7, at which
the chelicerae are located on the sides of the mouth opening
(Fig. 3G). In the next steps, the chelicerae are located anterior to the
hypostome, and even the pedipalps finally reach a position anterior
to the mouth opening (e.g. Figs. 4D–H; see also Section 4.1.2). A pair
of small cone-like structures develops on the outer side of the
pedipalpal podomere carrying the endite from Step 7 onwards
(Figs. 3G–H, 4 and 6G–P). This structure is considered an egg tooth
and is probably used by the animal to penetrate the eggshell when
hatching (cf. Kästner, 1949; Yoshikura, 1955, p. 22). The egg tooth is
not present in any of the hatchlings of L. geometricus (unpublished
data). It is obvious that a pair of small pores (black arrow in Figs.
3D,E,G) develops close to the base of, and anterior to the chelicerae
from Step 4 to Step 7. No earlier investigation has documented these
pores. The functional significance of the pores remains unclear.
The developmental events described above result in a hatchling
(Fig. 8). At this step, the dorsal closure has been finished. The
opisthosoma seems to be swollen with yolk (Fig. 8A). In the anterior
region of the prosoma, the chelicerae are closely situated in front of
the hypostome. The mouthparts form a pre-oral tube consisting of
the hypostome anteriorly, the so-called labium posteriorly, and the
pedipalpal endites laterally (Fig. 8B). A large and distinctive
sternum is present posterior to the labium (Fig. 8A).
4. Discussion
4.1. Embryonic development of Latrodectus geometricus
4.1.1. Pre-cheliceral lobes
In embryos of L. geometricus, each of the pre-cheliceral lobes can
be recognised as two areas: (1) a marginal region that later on
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becomes the anterior part of the prosomal shield; and (2) a central
area that is subdivided into two portions that later fuse and separate from the marginal region by three furrows. The central area of
each pre-cheliceral lobe finally gets internalised. Similar findings
have been reported and discussed by several previous authors.
Pross (1966) recognised two pairs of somites in front of the
chelicerae in the embryos of the lycosid spider Pardosa hortensis
Thorell, 1872. He further suggested the two pairs of somites to be
two pre-cheliceral segments: the protocerebral and the deutocerebral segments (Pross, 1966, his Figs. 25a–c). Subsequently, the
cheliceral segment was considered as being tritocerebral. The
findings of Pross (1966) match well with our discovery of the two
subdivisions in the central area of each pre-cheliceral lobe of the
widow spider embryos. In histological sections, the central area
(¼the subdivisions) of the pre-cheliceral lobes appears to give rise
to pre-cheliceral ganglia (Figs. 20–25 in Yoshikura, 1955, 1958; Figs.
4a,b in Marechal, 1994) or nerve components (Kästner, 1950). Based
exclusively on the existence of the neuro-related subdivisions, we
hesitate to consider those subdivisions as representatives of
segments, which normally contain also muscles and appendages
(in most cases). Interestingly, two subdivisions of the central area of
each pre-antennal lobe, visualised by the expression of the neurorelated genes Pax6.1 and Pax6.2, are reported in the embryos of
a diplopod (Prpic, 2005). Three subdivisions were also found in
the same region of collembolan (Uemiya and Ando, 1987) and
thysanuran embryos (Larink, 1970; Machida, 1981). Even more,
Kishimoto and Ando (1985) reported four subdivisions in the preantennal lobes of the embryos of a stonefly. In all, it turns out to be
difficult to compare and homologise segments and their appendages in the anterior region of arthropods, if each subdivision found
in the pre-cheliceral/pre-antennal lobes of different arthropod
embryos is regarded as representing one segment. We herein
intend to homologise the pre-cheliceral and pre-antennal lobes of
various arthropod embryos as the most parsimonious explanation.
Recently, it could be demonstrated by different methods, such as
the neuro-system staining techniques, palaeontological approaches
and hox gene expressions, that the chelicerae of chelicerates are
homologous to the antennulae or first antennae in all the rest of the
euarthropods, and both are innervated by the second neuromere,
the deutocerebrum (e.g. Mittmann and Scholtz, 2003; Chen
et al., 2004; Jager et al., 2006).
The anterior furrow (a in Figs. 3D–H, 4A–C, 5B,C and 6D–K) has
been found to be significant for the brain formation of a spider.
According to the histological sections presented by Yoshikura
(1955, his Figs. 20–25), the anterior furrow (‘cerebral groove’ in
Yoshikura, 1955, 1958) develops into a deep depression through
which the central area (¼the subdivisions) of the pre-cheliceral
lobes ‘sinks’ into the yolk, the internalisation process. Another two
furrows (b, c in Figs. 3–6) in each pre-cheliceral lobe help laterally
and posteriorly isolating the central area from the marginal region
of the pre-cheliceral lobes. A similar internalisation can also be
observed in other spiders (Yoshikura, 1955, 1958, 1972) and other
Fig. 4. Latrodectus geometricus Koch, 1841. SEM pictures of Step 9–Step 16 embryos. Anterior view. A. Step 9 embryo: the median and lateral subdivisions (ms, ls) of each precheliceral lobe start to merge into each other. B. Step 10 embryo: the anterior margin (AM) of each pre-cheliceral lobe is much more differentiated than in all previous steps. C. Step
11 embryo: the width of the anterior part of the pre-cheliceral region is reduced. A ridge (white arrowhead) is located between the pre-cheliceral lobes. D. Step 12 embryo: the entire
pre-cheliceral region is highly reduced in size (the white arrowhead points to the highly reduced median ridge). The outline of the prospective prosomal shield is marked by
a dashed line (see also Fig. 5D). Note that the former central area (ms þ ls) of each pre-cheliceral lobe has internalised and only triangular holes are left as the remains. E. Step 13
embryo: the ridge (white arrowhead) between the two pre-cheliceral lobes is highly reduced. Two slits represent the remains of the internalised central area (ms þ ls). A
prospective sternum (pSTN) is developed from the growth of the former epithelial portion (see EP in C, D). F. Step 14 embryo: the anterior margins (AM) of the pre-cheliceral lobes
medially fuse with each other. The remains (ms þ ls) of the internalised central area of each pre-cheliceral lobe are hole-like. G. Step 15 embryo: the hole-like remains (ms þ ls) of
the internalised central area of each pre-cheliceral lobe are smaller than those in the preceding step. Two circular regions (asterisk) are situated laterally to the medially fused
anterior margins (AM) of the pre-cheliceral lobes. A labium (LB) starts to develop at the anterior end of the prospective sternum (pSTN). H. Step 16 embryo: the prosomal shield is
well-developed, with a clearly defined anterior border. The cone-like projection is still present on the outer side of the basal portion of each pedipalp. In all pictures, white arrows
point to the borders of the ventral sulcus; white (open) and black arrowheads mark the anterior and posterior rim, respectively, of the anterior margin of the left pre-cheliceral lobe;
white circles mark the cone-like structure developed on the outer side of the basal portion of each pedipalp. Note that the cone-like structures (white circles; cf. Fig. 3H) do not
change their morphology from Step 9 to Step 16. Scale bars ¼ 100 mm. Abbreviations other than used in the previous figure: pSTN: prospective sternum.
Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
arachnids (Kästner, 1948, 1949, 1950; Yoshikura, 1961; Weygoldt,
1975; Moritz, 1957; Brauer, 1895; Farley, 1998, 2001; Thomas and
Telford, 1999), although the anterior furrow was termed differently,
e.g. ‘fovea’ (Rempel, 1957), ‘semilunar groove’ (Holm, 1940, 1954;
Farley, 1998, 2001; adopted also by Anderson, 1973), and ‘cerebral
groove’ (Yoshikura, 1954, 1955). The lateral furrow (b in Figs. 3–6)
was observed in Ischnothele guyanensis and termed as ‘lateral
vesicles’ by Marechal (1994), while the posterior furrow (c in Figs. 3,
4 and 6) has never been mentioned before.
In the embryos of Latrodectus geometricus, a sub-triangular
median area, the postero-median bridge (light green in Fig. 6)
connects the pre-cheliceral lobes (dark green in Fig. 6). This bridge
retains a medial connection after the appearance of the mouth area
(red in Fig. 6), but later on it is separated by the development of the
mouth region and vanishes eventually (Fig. 6). The reduction of the
bridge indicates the shift of the mouth region from anterior to
posterior. Unfortunately, information about the bridge is so rare in
existing works that further comparison cannot be made for the
moment.
Many small pore-like depressions distributed all over the
surface of the pre-cheliceral lobes (Figs. 3D,G) have also been found
on the surface of not only the pre-cheliceral lobes but also the area
between the two buds of the same appendages that belong to the
more posterior segments in early embryos of a scorpion (Farley,
2001, his Figs. 9–11). Those depressions were thought to suggest
‘invagination of epithelial primordia for internal structures’ (Farley,
2001, p. 73).
Within the Lipoctena, data from Palpigradi, Solifugae and Ricinulei, are still missing. Information from the Pantopoda (e.g.
Morgan, 1891) and the Xiphosura (e.g. Sekiguchi et al., 1982) are not
sufficient to confirm the presence or absence of the structures and
processes discussed above. Therefore, it can only be suggested that
the subdivisions, the internalisation, and the associated furrows of
the pre-cheliceral lobes belong to the ground pattern of, at least, the
Arachnida.
4.1.2. Mouth region
A structure that refers to what we call ‘hypostome’ in L. geometricus has been observed by all other investigators of chelicerate
embryology, such as Farley (1998, 2001) in a scorpion, who took the
term ‘rostrum’ for it after Yoshikura (1955) and Foelix (1996).
Another term, ‘labrum’, was largely applied to our ‘hypostome’,
mainly by zoologists or embryologists working on extant animals
(e.g., Anderson, 1973). ‘Hypostome’ is commonly found in the
palaeontological literature, established there for the pre-oral sclerotic plate between the antennulae of, e.g., Trilobita. The detailed
data of mainly the three-dimensionally preserved Cambrian ‘Orsten’
arthropods (summary on the ‘Orsten’ in Maas et al., 2006) demonstrated, however, the distinction between the hypostome and the
labrum on the ventral side of the arthropod head (e.g., Walossek and
Müller, 1990). In fact, a ‘labrum’ can be understood as an expansion
of the mouth membrane at the rear end of the (also present)
hypostome. Furthermore, a labrum develops only in the in-group
crustacean taxon Labrophora, hence, is proposed as one of the
autapomorphies of this taxon (Maas et al., 2003; Siveter et al., 2003).
Indeed stem taxa of the Crustacea ‘‘still’’ have the ‘hypostome’ only,
e.g. the ‘Orsten’ forms Goticaris longispinosa Walossek and Müller,
409
1990 and Cambropachycope clarksoni Walossek and Müller, 1990
(both re-studied in Haug et al., in press), Oelandocaris oelandica
Müller, 1983 (re-studied in Stein et al., 2008) and Henningsmoenicaris
scutula Walossek and Müller, 1990 (re-studied in Haug et al., pers.
comm). Therefore, ‘hypostome’ and ‘labrum’ are not equivalent or
interchangeable, do not exclude each other, but are two different
structures next to each other, in a particular euarthropod taxon only
(e.g., Waloszek, 2003). In consequence, the terms ‘hypostome’ and
‘labrum’ need to be treated with care since the original terminology
might be as much misleading as described structures with the same
name may not be homologous (see also Maas et al., 2003; Scholtz
and Edgecombe, 2005, 2006).
Excluding the nature of the hypostome, the question remains
what it actually is and belongs to. Our data suggest that it is
a structure appearing and developing in the mouth region, hence,
not part of the pre-cheliceral lobes (¼ocular or protocerebral
segment), neither the sternal (¼inter-appendage) region of the
antennular (¼cheliceral or deuterocerebral) segment. Its development starting after all the prosomal limb buds have developed and
the lack of expression of neuro-related genes Pax6.1 and Pax6.2 in
the hypostome of Glomeris marginata (Villers, 1789) embryos (Prpic,
2005) are arguments against the hypothesis raised by some authors
considering the hypostome as a pair of appendages (e.g., Butt, 1960;
Haas et al., 2001a,b; Boyan et al., 2002, 2003 [considering the
hypostome to be related to the tritocerebral segment ¼ pedipalp
segment of chelicerates]; Kimm and Prpic, 2006).
The increasing distance between the hypostome and the anterior margin of the pre-cheliceral lobes suggest that the whole
mouth region changes its position in ventral and posterior direction
during embryonic development (Figs. 3B–H, 4A–C and 6B–K). This
gives the impression that the attaching position of the chelicerae
and the pedipalps ‘move’ from being post-orally located (Figs. 6B–G)
to their final positions of being anterior to the hypostome and
parallel to the mouth opening, respectively (Figs. 6L–P and 8). It is
possible that, together with cell proliferation and apoptosis during
the inversion (Prpic and Damen, 2005), the internalisation of the
central area (¼the subdivisions) of the pre-cheliceral lobes motors
the change of the positions of the mouth region and the prosomal
appendages. A similar development can also be observed in the
embryos of Limulus polyphemus Linnaeus, 1758 (Mittmann, 2004),
two scorpions (Brauer, 1895; Farley, 1998, 2001), a mite (Thomas
and Telford, 1999), and a pantopod (Morgan, 1891, his pl. IV, Figs. I
and II). However, unfortunately, not much attention has been paid
to the position changes.
We conclude that the shift of the entire mouth region from
anterior to posterior belongs to the ground pattern of, at least, the
Chelicerata, or it is even more plesiomorphic than this level.
Information about the presence of the three small pores and the
latero-posterior border of the mouth opening cannot be found in
earlier literature on other chelicerates. Therefore, further phylogenetic comparison across the Chelicerata in this specific respect
cannot be made.
4.1.3. Formation of the prosomal shield
In embryos of L. geometricus, the prosomal shield consists of the
marginal region of the previous pre-cheliceral lobes anteriorly and
the tergal area of the four posterior-most prosomal segments.
Fig. 5. Latrodectus geometricus Koch, 1841. SEM pictures of embryos at selected steps. Dorsal view. A. Step 3 embryo: the anterior and posterior ends of the embryo are approaching
each other. B. Step 8 embryo: the anterior and posterior ends of the embryo are much more separated than before. C. Step 10 embryo: the anterior and posterior ends of the embryo
have the maximum distance from each other. D. Step 12 embryo: the outline of the developing prosomal shield is marked by a dashed line. E. Step 14 embryo: the anterior margin
(AM) of the pre-cheliceral lobes starts to become the anterior-most part of the prosomal shield through the fusion of its anterior rim (white open arrowhead) with the tergal area of
the four posterior-most prosomal segments (compare A–E). The posterior rim (black arrowhead) of the anterior margin (AM) of each pre-cheliceral lobe represents the anterior rim
of the entire prosomal shield (compare A–E). F. Step 15 embryo: the formation of the prosomal shield completes, with the anterior margin (AM) of the pre-cheliceral lobes being the
anterior-most part of the prosomal shield. The anterior rim (white open arrowhead) of the former anterior margin (AM) of the pre-cheliceral lobes is almost undistinguishable. The
open arrow in E and F points to the posterior border of the prosomal shield. Other labels correspond to those in Fig. 4. Scale bars ¼ 100 mm.
410
Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
Fig. 6. A–P. Diagrams of embryos of Latrodectus geometricus in the developmental steps following the germ disc phase. Each drawing represents one of the steps (1–16) described
earlier with SEM pictures. Meaning of the colours: dark green – the pre-cheliceral lobes in A, B, and the marginal region of the lobes in C–P when the differentiation of the central
area (¼ms þ ls, marked as yellow) is present; light green – the postero-median bridge connecting the pre-cheliceral lobes; dark yellow – the furrows around the central area of the
pre-cheliceral lobes; red – the mouth region; orange – the chelicerae; light blue – the pedipalps; grey – the tergite area of the four posterior-most prosomal segments. In each
image, arrows mark major structures of each step; open arrowhead labels the anterior rim of the anterior margin (AM) of the pre-cheliceral lobes, corresponding to the white open
arrowheads in Figs. 3–5. Other labels and abbreviations correspond to those in Figs. 3–5. Scale bars ¼ 100 mm.
Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
411
Fig. 7. Diagrams of the mouth region of the embryos of Latrodectus geometricus Koch, 1841. A. Step 2 embryo: the mouth region is undifferentiated. B. Step 3 embryo: a pair of lobelike structures, the hypostome (HY), develops in the frontal part of the mouth region. Three pores develop posterior to the hypostome (HY). A U-shaped swelling, the lower lip,
develops posterior to the pores. C. Step 4 embryo: the hypostome (HY) and the lower lip approach each other to form the mouth opening that encloses the three pores. Note that the
hypostome (HY) still has a bilobed appearance. D. Step 7 embryo: the hypostome (HY) has lost its bilobed appearance. The mouth opening is slit-like, due to the approaching of the
hypostome (HY) and the lower lip. E. Step 8 embryo: the mouth opening and the lower lip is overhung by the hypostome (HY), hence are invisible.
Similar processes of the prosomal shield formation to what we
report have also been documented for other Araneae (Yoshikura,
1954, 1955; Holm, 1940, 1954) and other arachnids such as uropygids (Kästner, 1948, 1949, 1950; Yoshikura, 1961), harvestmen
(Moritz, 1957), amblypygids (Weygoldt, 1975), and scorpions
(Farley, 1998, his Figs. 21 and 23). To our knowledge, comparable
information has not yet been yielded for the Xiphosura and
Pantopoda.
Phylogenetically, the existing data indicate that the prosomal
shield consisting of the anterior margin of the pre-cheliceral lobes
and the tergite regions of the last four prosomal segments, and the
developmental processes of the prosomal shield can be considered
to be part of the ground pattern of, at least, the Arachnida.
4.1.4. Ventral sulcus and inversion
The ventral sulcus and its correlated inversion process observed
in embryos of L. geometricus are very common in various
lipoctenans (Weygoldt, 1975; Yoshikura, 1961; Kästner, 1948, 1949,
1950; Moritz, 1957; Anderson, 1973; Thomas and Telford, 1999)
with exceptional absence only in some spiders from the Mesothelae
and Orthognatha (Foelix, 1996). Staining and sectioning methods
on embryos of Cupiennius salei have demonstrated that neural
commissures of the appendage-bearing segments are present in
this area (unpublished data from Linne and Stollewerk, personal
communication; see also Stollewerk et al., 2001).
The ventral sulcus and the inversion previously found in other
species are basically the same structure and phenomenon as what
we document herein, with only very detailed differences. For
example, Holm (1954) reported for the embryos of Ischnothele
karschi Strand, 1907 that the ventral sulcus first appears as
a median thinner area in the 7th and 8th segments, which later on
extends anteriorly and posteriorly and finally divides the whole
germ band into two lateral rows of segmental plates. In L. geometricus, however, according to our investigation, the ventral
sulcus first appears in Step 2 embryos as a longitudinal furrow
extending from the cheliceral segment back to the opisthosoma.
Fig. 8. SEM pictures of a fresh hatchling of Latrodectus geometricus. A. Ventral view. B. Magnified view of the mouthparts of the specimen shown in (A). The chelicerae are located
anterior to the hypostome (HY). Note the pre-oral tube formed by the hypostome (HY) anteriorly, the labium (LB) posteriorly, and the pedipalpal endites (e) laterally. Scale
bars ¼ 100 mm. Abbreviations other than used in the previous figure: STN: sternum.
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Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
Table 1
Summary of the development of the pre-cheliceral lobe (PCL) in Latrodectus geometricus embryogenesis.
Step 0
A
PMB
1
2
3
4
5
A
B
C
D
E
90
45
30
60
60
6
F
60
7
G
80
8
H
–
9
I
–
present
present
present
present
median
part
shorter
median
part
shorter
median
part
absent
completely
absent
–
10
J
–
–
11
K
–
–
12
L
–
–
13
M –
–
14
15
N
O
–
–
–
–
16
P
–
–
Subdivisions of the central area
a
b
c
d
d
d
shallow, elliptic
deepened
d
d
d
d
d
d
d
median subdivision (ms) present
ms more distinct than before, slightly raised
ms distinctly humped
d
d
lateral subdivision (ls) present d
ls more distinct than before
shallow, elliptic
more flattened than ms
crescentic
ms even more distinct than before
more distinct than before
crescentic, deeper deeper
d
ms flattened again
more flattened than before
un
width 50%
d
deep groove
narrow, slit-like
present
un
longer
un
un
un
un
un
un
un
indistinguishable
indistinguishable indistinguishable
–
–
–
–
–
–
–
–
–
–
–
–
ms separated from anterior margin (AM) of the un
pre-cheliceral lobe (PCL)
ms and ls merge to form a single, rounded triangular central area of each PCL,
slightly smaller than before
rounded triangular, smaller;
AM partly slightly overhanging the recessed central area, particularly anteriorly
central area more recessed than before, smaller, triangular, slightly more
posteriorly and inwardly located against sharply rising ridge in the ventral midline
central area invisible, only a small triangular hole remaining between AM
and the sharply rising ridge, triangular area recessing underneath AM
posterior margin of each PCL significantly shorter than before, forming
a shallow valley with 2 slit-like holes separated by a thin ridge
AM less sharply marked than in Step 13, holes rounded
entire anterior area narrower and shorter than before, AM less clearly marked,
holes smaller
posterior margin of pre-cheliceral lobes small, AM being the anterior-most of
prosomal shield,
holes absent
Columns: ‘‘step’’ – steps as described in text; ‘‘0’’ – letters corresponding to those of Figure 6; ‘‘A’’ – orientation of the long axis of the PCL relative to the long body axis; ‘‘PMB’’ –
postero-median bridge; ‘‘a’’ – anterior furrow; ‘‘b’’ – lateral furrow; ‘‘c’’ – posterior furrow. Grey filling of boxes marks significant changes. Abbreviations: un ¼ unchanged;
– ¼ inapplicable; d ¼ absent.
In the mite Hyalomma dromedarii Koch, 1844 (Anderson, 1973) and
the whip spider Tarantula marginemaculata Koch, 1840 (Weygoldt,
1975), the ventral sulcus was demonstrated as separating left and
right parts of the embryonic body primordium behind the precheliceral lobes, but also even the pre-cheliceral lobes in very early
embryos.
The embryonic body primordium of L. geometricus completely
curves around the yolk, i.e. attaching its dorsal side to the yolk (Figs.
1D,E). In the embryos of many other spiders and arachnids the
terminal part of the opisthosoma posterior to the ventral sulcus folds
ventrally, such as in Segestria bavarica Koch, 1843 (Haplogynae;
Holm, 1940, Figs. 5 and 6), Ischnothele karschi Dönitz, 1887(Holm,
1954, Fig. 3; see also Anderson, 1973, Fig. 141d), Atypus karschi
Dönitz, 1887 (Yoshikura, 1958, Figs. 6B,C), Ummidia fragaria Dönitz
1887 (Yoshikura, 1972, Figs. 2–3), Hyalomma dromedarii (Anderson,
1973, Figs. 148c–e), and Paruroctonus mesaensis Stahnke, 1957 (Farley, 2001, his Fig. 12). In xiphosuran embryos the terminal part of the
opisthosoma does not fold; the embryos do not grow around the egg
but develop on one side of the egg (Sekiguchi et al., 1982; Mittmann,
2004). In all, it seems that differences from our investigation on
L. geometricus are due to the possibility of rather taxon-specific
modifications of their development, even despite close phylogenetic
relationship, such as in the araneids.
Outside Lipoctena, studies by Brauer (1895) and Farley (2001)
did not confirm the presence of a ventral sulcus for scorpion
embryos. Also xiphosuran embryogenesis seems not to include the
formation of a ventral sulcus (Sekiguchi et al., 1982; Mittmann,
2004). Data on pantopods are still needed. The embryonic development of the pre-cheliceral lobes and the existence of the ventral
sulcus are unclear from the existing data on pseudoscorpions
(Weygoldt, 1964). In the embryos of Archegozetes longisetosus Aoki,
1965 (Acari) the ventral sulcus is probably present (Thomas and
Telford, 1999, their Fig. 5A). Further comparison cannot be made for
the moment, mainly due to the difficulty in correlating the various
studies, in which not only parts were omitted in the line drawings
made by many previous authors. Anderson (1973), for example,
termed the dorsal parts of the embryonic epidermis ‘dorsal extraembryonic ectoderm’ (embryonic cells outside the embryo is
contradictory).
Summing up, it is likely that the embryonic ventral sulcus is an
autapomorphy in the ground pattern of the Lipoctena. Outside
Chelicerata a ventral sulcus-correlated phenomenon has been
reported for centipede embryos, e.g., Scolopocryptops rubiginosus
Koch, 1878 (Scolopendromorpha, Myriapoda) by Sakuma and
Machida (2004) and Sakuma (2005), and in Orchestia cavimana
Heller, 1865 (Isopoda, Crustacea) by Ungerer and Wolff (2005).
Considering the large phylogenetic gap among these taxa and
focusing on the Chelicerata, assumptions of convergence appear to
be most plausible to us.
4.1.5. Formation of the sternitic region and the nature of the labium
In the hatchlings of L. geometricus, a triangular lobe (LB in
Fig. 8B) seems to be articulated with the anterior part of the
sternum and points against the hypostome. This structure is usually
termed ‘labium’ by arachnologists (e.g., Foelix, 1996), and its
development was mentioned by few previous authors, e.g., Yoshikura (1955). Its abaxial joint with the sternum is right at the
posterior end of the insertions of the pedipalps. To our knowledge,
no earlier literature has ever mentioned the development of the
labium in spiders. According to our investigation, the labium first
Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
413
Table 2
Summary of the development of the mouth region in Latrodectus geometricus embryogenesis.
Step
0
Pre-hypostomal portion
Hypostome (HY)
Mouth opening
Lip-like bulge lateral and
posterior to mouth opening
1–2
3
A–B
C
d
Smooth rectangle
d
Pair of slightly enhanced lobes
connecting medially
d
Slightly bulged
4
D
Triangle; length tripled from
Step 3
Much raised, bilobed
5
E
2 triangles standing on
the tips of each other
Overhanging the mouth opening;
bilobation absent
d
Present as a slightly depressed area
surrounded by HY anteriorly and the
lip-like bulge laterally and posteriorly
3 pores in the area being arranged as
the mathematic because sign q
More depressed and more enclosed by
the approaching of HY and the lip-like
bulge, 3 pores still present
More depressed 3 pores not detected from
this step onwards
6
7
F
G
Un
Similar to Step 5–Step 6, but with
a straight posterior border
Un
Slit-like, on level with the insertions
of chelicerae
Un
Much less prominent
8
H
9
10
I
J
Un
Posterior triangle more elongated
than in Step 6, anterior one
unchanged
Posterior triangle more elongated
than in Step 7, anterior triangle
unchanged
Indistinguishable
–
Slightly posterior to the insertions of
chelicerae and covered by HY, not
traced from this step onwards
?
?
Covered by the
hypostome, not traced
from this step onwards
?
?
11
12
K
L
–
–
?
?
?
?
13
M
–
?
?
14–16
un
–
Lobe-like; with a convex tip
pointing backwards and covering
the mouth opening
Un
Located significantly posterior to
the insertions of chelicerae
Un
Almost completely covered by the
chelicerae
Located on level with the
insertions of pedipalps
Un
?
?
Slightly more elevated
Un
Columns: ‘‘step’’ – steps as described in text; ‘‘0’’ – letters corresponding to those of Fig. 6. Bold highlighted text marks significant changes. Abbreviations: Un ¼ unchanged;
? ¼ unknown condition; – ¼ in applicable; d ¼ absent.
appears in the late embryogenesis (LB in Figs. 4G,H) as an anterior,
pointed tip of the sternal plate, i.e. the labium derives from the
sternal plate and, hence, has a cuticular nature.
It is noteworthy that the term ‘labium’ is used differently in
arthropod terminology. In entomology, ‘labium’ refers to the
medially fused second maxillae in insects, hence has an appendicular origin. In crustacean terminology, ‘labium’ is used by some
workers on specific in-group taxa, e.g. by ostracodologists, for the
sternum as the fusion product of the sternites of the mandibular
and maxillulary (1st maxilla) segments (cf. Maas et al., 2003; Maas
Table 3
Summary of the development of the prosomal appendages and the ventral body region in Latrodectus geometricus embryogenesis.
Step
0
Chelicerae (CH)
Pedipalps (PP)
Walking limbs 1–4
Ventral body region
A
1
A
Cone-shaped anlage
08
B
Elongated, unsegmented
Cellular band broader, ventral sulcus (VS)
and sternal portions (ST) present
Un
3
4
C
D
Ovoid buds with a bifid tip
Un
Segmented, endite medially
Un
First three pairs present
as paired anlagen; 4th pair
as a single swelling
Elongated, unsegmented;
4th pair present as
paired anlagen
Segmented
Un
Narrow cellular band in the intra-appendage
region of the segments
2
Cone-shaped anlage
posterior to
pre-cheliceral region
Spherical, unsegmented
Un
108
5
6
E
F
Tip tripartite
Un
Un
Endite more significant
Un
Un
7
G
H
Un
VS slightly wider than in Step 7
Un
9
I
Cone-like structure
(egg-tooth) present
Egg-tooth with wrinkled
surface
Un
Un
8
Un
Un
458
10
11
12
13
J
K
L
M
Located lateral to mouth
opening
Longer, tips approaching
HY medially
¼ longer than in Step 8, tips
superposing HY laterally
Located anterior to HY
Un
Un
Un
Width of VS doubled from previous step, ST un
VS slightly wider than in Step 3 and widening
more and more towards the posterior
VS slightly wider than in Step 4
VS slightly wider than in Step 5;
region posterior to the CH segment
wider than pre-cheliceral region
VS slightly wider than in Step 6, trapezoidal
Lateral to mouth opening
Un
Anterior to mouth opening
Located almost lateral to CH
Un
Un
Located postero-laterally to PP
Un
Un
Un
608
Un
14
15
N
O
P
Un
Right and left buds closer
to each other than before
Un
Un
Un
16
Un
Right and left buds closer
to each other than before
Un
VS slightly wider than in Step 9
Un
Un
VS un; ST forming a single plate
of sternum (STN)
VS un; STN more developed
VS covered by STN; STN suboval, expanding
over entire prosomal area ventrally
Un
Un
208
Un
308
Un
Un
Un
Columns: ‘‘step’’ – steps as described in text; ‘‘0’’ – letters corresponding to those of Fig. 6; ‘‘A’’ – angle of insertions of prosomal appendage to the ventral midline. Bold
highlighted text marks significant changes. Abbreviations: Un ¼ unchanged.
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Y. Liu et al. / Arthropod Structure & Development 38 (2009) 401–416
and Waloszek, 2005), so being again differently used. However,
according to our investigation, we exclude the possibility of an
appendicular origin of the labium in spiders.
4.2. Some more notes on the embryonic development of other taxa
of Chelicerata (Chelicerata s. str. sensu Chen et al., 2004)
Of the Pantopoda, many species hatch at the so-called protonymph stage, carrying three pairs of appendages, i.e. the chelicerae
and another two (Bain, 2003; Vilpoux and Waloszek, 2003;
Waloszek and Dunlop, 2002), while in others the hatchlings are
‘small adults’, such as in Pallene empusa Wilson, 1880 (Morgan,
1891). The earliest embryonic stage of P. empusa that Morgan (1891)
documented shows a germ band growing around the egg, which
exhibits, from anterior to posterior, a pair of pre-cheliceral lobes,
a mouth region being located between the two limb buds of the
chelicerae and buds of the following walking legs (Morgan, 1891,
his pl. IV, Figs. I and II). This is similar to the early embryos of
L. geometricus in terms of the existence of the same body elements,
i.e. the pre-cheliceral lobes, mouth region and so on. However, the
position of the mouth region in between the cheliceral limb buds is
dissimilar to early embryos of L. geometricus but similar to older
ones of Step 7–Step 8 recognised here (cf., e.g., Figs. 3D,H). This may
indicate that there are more embryonic stages with recognizable
external structures in P. empusa earlier than those documented by
Morgan (1891). We suppose that the mouth region in the embryos
of P. empusa is also initially located anterior to the cheliceral limb
buds, as is the case in the embryos of L. geometricus and other
chelicerates, e.g., scorpions (Farley, 2001). Studies of the embryogenesis of pantopods with protonymph hatchlings are not available
yet.
In embryos of Limulus polyphemus (Xiphosura), all limb buds are
originally located posterior to the mouth opening, which is situated
in-between a pair of pre-cheliceral lobes (Sekiguchi et al., 1982;
Mittmann, 2004). The pre-cheliceral lobes in the embryos of
L. polyphemus are not as prominent as those in spider embryos. Like
in L. geometricus, the position of the mouth region of L. polyphemus
embryos also shifts from anterior to posterior during embryogenesis. Details of the development of the pre-cheliceral lobes remain
unknown.
5. Conclusions and outlook
The detailed investigation on the embryonic development of
Latrodectus geometricus enables a comparison between our data
and those from existing literature. We are now able to characterise
several features found in the embryos of various chelicerates and to
further achieve new understanding of the phylogenetic status of
those features. New understanding are: (1) in the pre-cheliceral
lobes, the two subdivisions and their later internalisation, as well as
the associated furrows are currently considered to be part of the
ground pattern of the Arachnida. Investigation of such lobes in
embryos of xiphosurans and pantopods is essential to further
determine the status of those features. (2) The shift of the entire
mouth region from anterior to posterior belongs to the ground
pattern of, at least, the Chelicerata, or is even more plesiomorphic
than this level. (3) The prosomal shield forming from the fusion of
the anterior margin of the pre-cheliceral lobes and the tergal region
of the four posterior-most prosomal segments plus the entire
formation process of the prosomal shield are regarded as belonging
to the ground pattern of the Arachnida. (4) It seems reasonable to
consider the embryonic ventral sulcus as an autapomorphy for the
Lipoctena.
The embryonic morphogenesis data are able to demonstrate the
initial situation and the developmental processes of various
structures. Similar studies have already been done by many authors
in the 19th and the first half of the 20th century. At that time, they
mainly presented light-microscopic and histological investigations.
With modern techniques, such work can be expanded to visualise
more details of the external and internal development of arthropod
embryos. We propose that better understanding of arthropod
evolution can be achieved by performing such studies on various
arthropods.
Acknowledgements
We are grateful to Martin Thierer-Lutz, Schnürpflingen and his
colleagues from born to be eaten for providing us all living material
used herein. Professor Ryuichiro Machida, Sugadaira, Japan and his
students, and Professor Roger Farley, Riverside, USA are thanked for
sharing their experience of preparing arthropod embryos for SEM
observations to one of the authors (YL). Joachim T. Haug, Ulm,
Viktoria Linne, Mainz, and Angelika Stollewerk, London, provided
valuable, still unpublished information. We are grateful to Nigel
Hughes, Riverside, and Ronald Jenner, London, for correcting the
language. Two anonymous reviewers gave valuable suggestions
and comments. We also thank the Central Unit for Electron
Microscopy at the University of Ulm for supporting our SEM work.
Gerd Mayer, University of Ulm, is appreciated for his help with
ordering literature. Particular thanks are due to the EU Programme
‘‘Marie Curie Host Fellowships for Early Stage Research Training
(EST)’’ for funding Yu Liu as a PhD fellow within the MOLMORPH
network under this programme (contract number MEST-CT-2005020542).
Appendix A. Supplemental material
Supplementary information for this manuscript can be downloaded at doi:10.1016/j.asd.2009.04.001.
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