Table Of Content1999. The Journal of Arachnology 27:123-128
VENTRAL MESOSOMAL CHANGES IN EMBRYOS
FROM THREE SCORPION FAMILIES:
lURIDAE, BUTHIDAE AND VAEJOVIDAE
Roger D. Farley; Department of Biology, University of California, Riverside,
California 92521 USA
ABSTRACT. The scanning electron microscope was used to examine embryos at a stage when book-
lungs and spiracles are forming. Earlier studies with scorpion fossils suggest there was ventral mesosomal
transition from gills or booklungs above ventral plates to stemites, booklungs and spiracles. In Hadrurus
arizonensis (luridae), ventral plates and then sternites are formed on the ventral surface of mesosomal
segments before spiracles appear. Bilateral invaginations in body segments XII-XV apparently give rise
to the booklungs, with spiracles formed lateral to the site ofinvagination. Sternites with bilateral depres-
sions were also present before spiracles in embryos of the buthid Centruroides exilicauda. In the devel-
opmental stages herein examined, spiracles were formed in embryos ofParuroctonus mesaensis (Vaejov-
idae); but there was no indication ofventral plates or sternites on the ventral mesosoma. Spiracles appear
in the intersegmental area posteriorto body segments XIII-XV. Booklungs may formlaterfromprimordia
associated with bilateral depressions observed in a later stage in these segments.
The earliest scorpion fossils (Silurian) sug- METHODS
gest these animals were aquatic, while&all sur- The composition of physiological saline
viving species are terrestrial (Selden Jeram
1989; Sissom 1990; Jeram 1994). A critical and the procedures for collection and main-
tenance of specimens were described in an
stage in scorpion evolution was the change
earlier publication (Farley 1987). Specimens
(ventral mesosoma) from gills to booklungs,
of Paruroctonus mesaensis Stahnke 1957
probably in the Permian and Carboniferous
were collected in the Colorado Desert nearIn-
periods. Kjellesvig-Waering (1986) provided
some evidence that aquatic scorpions had gills dio and Palm Springs, California. Specimens
of Hadrurus arizonensis Ewing 1928 (Wil-
above ventral plates in the ventral mesosoma. &
He proposed that there was gradual reduction liams 1970; Francke Soleglad 1981) and
Centruroides exilicauda Wood 1863a (Wood
of these plates and formation of stemites,
booklungs and spiracles. Selden & Jeram 1863b; Ewing 1928; Williams 1980) were col-
(1989) and Jeram (1990) described a fossil lected in Arizona. Specimens of all three spe-
Carboniferous scorpion with booklungs rather cies are in the California Academy ofScience,
than gills above ventral plates. San Francisco.
Scorpion embryos were examined with the Tissues were flushed with saline to remove
possibility they might provide some informa- debris as animals were dissected with micro-
tion about the water-to-land transition (Farley scissors and forceps. The ovariuterine tubules
1999a, b). These initial observations showed were opened and embryos removed. Sur-
some differences in the ventral mesosoma rounding membranes (amnion, serosa) were
during spiracle and booklung formation in pulled away with microprobe and forceps.
embryos of the vaejovid, Paruroctonus me- Tissues were fixed (6-10 h, 23-25 °C) with
M
saensis, and the iurid, Hadrurus arizonensis. 4% glutaraldehyde in 0.1 cacodylate buffer
The present study is an extension ofthat work, with one drop ofcalcium chloride for each 10
including embryos of Centruroides exilicau- ml of solution (Lane et al. 1981). The tissues
da, a buthid. The latter was examined since were washed in cacodylate buffer-NaCl solu-
buthids are considered most primitive among tion and postfixed (2 h, 23-25 °C) in 1% os-
M
extant scorpion families (Stockwell 1989, mium tetroxide in 0.2 cacodylate buffer
1992; Sissom 1990), and mesosomal changes with NaCl. The concentrations of these solu-
may reflect the ancestral condition. tions were adjusted to approximate the os-
123
124 THE JOURNAL OF ARACHNOLOGY
molality of scorpion blood (630 mOsm; Yok“ available to determine if booklungs and new
Ota 1984). Tissues were dehydrated in spiracles form at these depression sites, or if
acetone, critically^point dried (Balzers, CDD the initial spiracles (Fig. 4) move to the adult
020) and sputter-coated (EMscope SC500) position, farther anterior and lateral in the seg-
with 20 nm thickness of gold/palladium. Tis- ment (Farley 1990a, b). The early spiracles
sues were examined at 12-15 KV with a Phil- differed in shape among the embryos, butusu-
ips 15 scanning electron microscope (SEM). ally had a smooth, apparently cuticularmargin
RESULTS and a slit-like opening (Fig. 4), in comparison
with the oval shape in the adult.
At a stage before spiracle andbooklung for- Although ventral plates or stemites are not
mation, embryos ofH. arizonensis have plates evident in embryos ofP. mesaensis when spi-
demarcated on the ventral surface of meso- racles first appear (Fig. 4), the ventro- poste-
somal segments. Initially, only a narrow ridge rior margin of each mesosomal segment was
outlines the ventral plates, with the delineated examined for indications of invagination or
region much smaller than the ventral surface gill-like structures. In some embryos, the lat-
of the segment. The outlined region becomes eral intersegmental area shows differentiation
a flap-like structure (Fig. 1) fused to the body suggestive ofinfolding, with vertical striations
wall anteriorly but free at the lateral and pos- (Fig. 4). The spiracles form in the medial as-
terior margins. The early ventral plates do not pect of this distinctive intersegmental area.
extend the full width of the mesosoma nor Embryos were not sectioned, but during
overlap antero-posteriorly. Embryos were not dissection in transmitted light, some internal
sectioned, but no indications of an opening or structures can be seen. In embryos of P. me-
gill-like structures were observed at the pos- saensis, there was no indication of a thick-
terior margin of the ventral plates. Paired in- ening or density anterior to the spiracles, as
dentations in body segments XII-XV (Hjelle wouldbe expected ifbooklungs were forming.
1990) are presumably booklung primordia. The spiracle site in the intersegmental area
In later stages, the invaginations in seg- (Fig. 4) did not appear to be a region of in-
ments XII-XV become more prominent (Fig. vagination as occurs in the bilateral depres-
2), and the ventral cuticle increases in length sions seen in the mesosomal segments of the
and width, forming structures that resemble iurid and buthid embryos (Figs. 1-3).
adult stemites with the perimeter joined to
DISCUSSION
pleural or intersegmental integument. The
stemites extend the full width of the meso- In H. arizonensis and C. exilicauda, book-
soma and overlap in the longitudinal axis. In- lung and spiracle formation appears to be like
trastemal spiracles eventually form at the that described by earlier workers in species
adult location (Farley 1990a, b),just lateral to from the families Buthidae (Abd-el Wahab
the site of booklung invagination. Booklungs 1951), Chactidae (Laurie 1890; Brauer 1895)
do not develop in segment XVI; the indenta- and Scorpionidae (Metschnikoff 1871; Laurie
tions (Fig. 2) eventually disappear, leaving no 1892). The bilateral depressions evident in
external trace. Mesosomal development in the mesosomal segments in Figs. 1-3 appear to
buthid, C. exilicauda, appears to be similar to be sites ofinvagination, and spiracles are later
that of H. arizonensis. In Fig. 3, an embryo formed here at the location seen in adults
of the former species has stemites with bilat- (Farley 1990a, b). The early demarcation of
eral depressions, presumably for booklungs. flap-like structures (Fig. 1) supports the notion
In embryos ofP. rnesaensis, there is no de- that ventral plates preceded (Kjellesvig-Waer-
marcation of ventral plates or stemites at the ing 1986) or occurred with booklungs in an-
time when spiracles first appear (Fig. 4). In cient scorpions (Selden & Jeram 1989; Jeram
the stages observed in this study, spiracles 1990).
were seen near the mesosomal midline in the Differences were reported among scorpion
intersegmental tissue posterior to segments species in the shape and texture ofthe cuticle
XIII-XV. In later stages, bilateral depressions of adult booklung lamellae (Lankester 1885;
were seen in segments XII-XVI, butthere was Berteaux 1889; Laurie 1896a, b). These were
still no indication ofventral plates or stemites. proposed as taxonomic criteria, but other fea-
Advanced embryos of P. mesaensis were not tures subsequently found acceptance (Stock-
FARLEY—VENTRAL MESOSOMA IN SCORPION EMBRYOS 125
—
Figures 1, 2. SEMs ofventral surface ofmesosoma ofembryos ofHadrurus arizonensis. 1. Flap-like
ventral plates (P) are fused to the body wall anteriorly and free at the posterior margin. Bilateral invagi-
nations (arrows) are present where spiracles and booklungs will eventually form. Teeth (T) are evident at
the posterior edge ofthe pectines. XIV, body segment; 2, Later stage. The ventral cuticle ofeach segment
has broadened and is now a stemite (S) attached aroundthe entireperimeter. Bilateralinvaginations (black
arrows) have deepened. Shallow depressions occur in body segment XVI, but booklungs do not develop
in this segment. The white arrow indicates apair of small, transitory appendages ofunknown significance
between gonopore and pectine. A, remnants of amnion not removed during preparation; T, pectinal teeth.
Scales, 0,5 mm.
126 THE JOURNAL OF ARACHNOLOGY
—
Figures 3, 4. SEMs of ventral surface of mesosoma of embryos. 3, Centruroides exilicauda. Each
segment has a sternite (S) like that of the irurid embryo ofFigure 2. Bilateral invaginations (arrows) are
presumably the site ofbooklung formation. Depressions are evident in body segment XVI although book-
lungs do not form in this segment. L, fourth walking leg. T, pectinal teeth. Scale, 0.5 mm; 4, Paruroctonus
mesaensis, left side ofventral mesosoma. No ventral plates or stemites are evident, but spiracles (arrows)
are present at the posterior margin ofbody segments XIII-XV. The spiracles are at the medial end of an
invaginated intersegmental region () with vertical striations. L, fourth walking leg. Scale, 0.2 mm.
FARLEY—VENTRAL MESOSOMA IN SCORPION EMBRYOS 127
well, 1989, 1992; Sissom 1990)= Developing am (1989) considered it more likely that ven-
booklungs were previously described as bilat- tral plates later became stemites by fusion
eral invaginations in the ventral mesosoma with the body wall. The latter proposal is sup-
(Metschnikoff 1871; Laurie 1890, 1892; ported in the present study in embryos of H.
Brauer 1895; Abd-el Wahab 1951). Tissue arizonensis. Small regions, initially outlined
sections showed that sac-like invaginations by a ridge on the ventral surface of mesoso-
extend anteriorly in the segment from the ini- mal segments, become flap-like plates (Fig. 1)
tial site ofingress, which remains open to be- and then the ventral cuticle is broadened to
come the spiracle. A few lamellae are initially form stemites (Fig. 2). There was no indica-
formed in the horizontal plane. These laterro- tion of reduction or loss of the ventral plates,
tate 90° to the dorso-ventral axis, along with resulting in exposure of overlying stemites.
development of many more lamellae (Laurie ACKNOWLEDGMENTS
1890, 1892).
There may be absence or delay of ventral The author wishes to thank KariCJ.MeWest
plates, and stemites may form late in embryos forproviding gravid specimens of exilicau-
of P. mesaensis in comparison with the iurid da. Thanks also to Morice A. Izmane for as-
and buthid embryos. Among scorpion fami- sistance in collecting the other species. This
lies, heterochrony occurs in embryogenesis in research was supported by intramural funds
relation to the mode of maternal nourishment from the University of California.
ofthe embryos (Matthew 1959; Farley 1999a, LITERATURE CITED
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