Table Of Content2005. The Journal of Arachnology 33:604-612
FIRST ULTRASTRUCTURAL OBSERVATIONS ON THE
TARSAL PORE ORGAN OF PSEUDOCELLUS PEARSEI
AND R BONETI (ARACHNIDA, RICINULEI)
Giovanni Talarico\ Jose G. Palacios-Vargas^, Mariano Fuentes Silva^ and Gerd
Alberti^: ^Zoological Institute & Museum, Ernst-Moritz-Arndt-University
Greifswald, J.-S.-Bach-Str. 11/12, D-17489 Greifswald, Germany. E-mail:
[email protected]; ^Laboratorio de Ecologia y Sistematica de Microartropodos,
Departamento de Ecologia y Recursos Naturales, Facultad de Ciencias, UNAM,
Mexico.
ABSTRACT. Due to their relative rareness and restricted distribution, little is known about the ultra-
structure of ricinuleids. In particular, sense organs have not been the subject of electron microscopic
research until now. Ricinuleids use their forelegs to explore their surroundings with tentative movements.
The distal tarsomeres of legs I and II of two cavernicolous Mexican species, Pseudocelluspearsei from
the Yucatan Peninsula and Pseudocellus honeti from GueiTero, were examined in this study with light
microscopy, scanning (SEM) and transmission electron microscopy (TEM). A conspicuous feature ofthe
distal tarsomeres of legs I and II is a single circular opening that extends as a deep tube-like pit into the
tarsus. This pore organ is lacking in the 6-legged larvae. Comparable organs are present in Araneae,
Scoipiones, Amblypygi and Anactinotrichida. The tarsal organs of the mentioned groups possess several
types of sensilla (olfactory, thermo- and hygrosensitive and mechanosensitve). The pore organ is located
in the distal third of the dorsal half of the tarsus. In longitudinal sections it shows a long oval shape. In
cross sections it is nearly circular. The pore organ contains a large number of long, slightly curved setae.
These setae are localized on the bottom and the lower two thirds of the wall of the pit and project into
the lumen. The upper third of the wall is free of setae and shows folds which extend parallel to the
opening. All setae inside the pit seem to be of the same type. In sections they show a complex inner
structure and likely represent chemoreceptive wall pore single-walled (wp-sw) sensilla. This indicates a
possible olfactory function. The pore organ is underlain by numerous gland cells which represent char-
acteristics of unicellular “class I” gland cells.
Keywords: Tarsus, ultrastructure, sensory organs
The order Ricinulei Thorell 1892 is one of relatively few old fundamental studies (e.g.,
the smallest arachnid groups. Only 56 recent Hansen & Sprensen 1904; Millot 1945, 1949).
species, all belonging to the family Ricinoidi- Until recently, little has been known about the
,
dae Ewing 1929, have been described. The re- ultrastructure ofricinuleids as there have been
cent species are divided into three genera. Ri- few scanning and transmission electron micro-
I
cinoides Ewing 1929 is from Western Central scopic studies on this animal group (for SEM
Africa, and Cryptocellus Westwood 1874 and see Legg 1976, 1977; Dumitresco & Juvara-
Pseudocellus Platnick 1980 are both from Bals 1977; Platnick & Shadab 1976, 1977; |:
Central America. Ricinuleids inhabit humid Harvey 1984; Adis et al, 1999; for TEM see j
layers of soil and litter in tropical rainforests Alberti & Palacios-Vargas 1984; Ludwig & ;'
or caves (Cooke 1967; Mitchell 1970; Adis et Alberti 1990; Ludwig et al. 1994). In partic-
f
al. 1989). They pass through 5 postembryonic ular, sensory organs have not been subjected
life stages: a 6-legged larva, 3 nymphal stages to electron microscopic research until now. j!
(proto-, deuto- and tritonymph) and the adult Ricinuleids use their forelegs, especially the
,!
stage (Mitchell 1970). elongated second leg, to explore their sur-
i
Most available studies about ricinuleids are roundings with tentative movements (Pollock
taxonomic (Mitchell 1970). The knowledge 1967). Hence the presence ofdifferent sensilla ‘|
about their internal morphology is based on on the distal tarsomeres ofthe forelegs can be
|j
604
I
TALARICO ET AL.—TARSAL PORE ORGAN OF TWO RICINULEIDS 605
expected. Some authors identified different (1960). All sections and voucher specimens
types of setae and other surface structures on are housed in the Zoological Institute & Mu-
the tarsi and expected them to be sensilla (e.g., seum of the University of Greifswald.
Hansen & Sprensen 1904; Pittard & Mitchell
& RESULTS
1972; Dumitresco Juvara-Bals 1973, 1976;
Legg 1976), but information about their ultra- The pore organ is located in the distal third
structure and possible function are still not of the dorsal half of the distal tarsomeres of
available. In the present work, we intend to legs I and II (Figs. 1, 3). The width of the
present the first ultrastructural study of the opening is about 32 pim in P. pearsei and 36
pore organ of the foreleg tarsi of Ricinulei. jam in P. boneti (Figs. 2, 4). The edge of the
METHODS opening differs slightly in both species. In P.
pearsei the edge is smooth without any pro-
The distal tarsomeres of leg I and II oftwo jections (Fig. 2), while in P. boneti there are
cavernicolous Mexican species were exam- some short and thin microtrichae which pro-
ined in this study. Specimens of Pseudocellus ject radially into the center of the opening
&
pearsei (Chamberlin Ivie 1938) from Yu- (Fig. 4). Except for the larvae (Figs. 5, 6), this
catan peninsula were collected in three differ- structure is present on the forelegs ofeach in-
ent caves, Gruta Actun Chen (Quintana Roo; vestigated life stage and both sexes ofP. pear-
20° 20H3" N & 87° 20' 45" W), GrutaX-Caret sei and P. boneti. The pore organ extends as
(Quintana Roo; 20° 33' 54"N & 86° 58' 49" a deep tube-like pit into the tarsus. In longi-
W) and Gruta Sabac-Ha (Yucatan; 20°10'18"N tudinal sections it shows a long oval shape
&
89°16'03"W). Pseudocellus boneti (Bolivar (Figs. 7-10). In cross sections it is nearly cir-
and Pieltain 1941) from Guerrero was col- cular (Figs. 11, 12). Sexual dimorphism could
lected in the caves Grutas de Acuitlapan not be observed in the present material.
(Mexico; 18°38'00" N & 99° 31' 55'" W). In both species the pore organ contains a
Both species have been found in bat guano or large number of long slightly curved setae.
under flat stones. For SEM, 7 specimens ofP. These setae are localized on the bottom and
pearsei (1 larva, 1 protonymph, 1 deuto- the lower two thirds of the wall of the pore
nymph, 3 adult males and 1 adult female) and organ and project into the lumen but do not
4 specimens of P. boneti (1 larva, 1 trito- reach the opening (Figs. 2, 4, 13). The upper
nymph and 2 adult males) stored in ethanol third of the wall is free of setae and shows
(70%) were dehydrated in graded ethanols, folds which extend parallel to the opening
critical-point dried and coated with gold-pal- (Fig. 13, 14). Some small openings in the wall
ladium. Examination was performed on a are visible (Fig. 13, 14). In SEM micrographs,
LEO DSM 940. For TEM the distal tarsomer- the setae show a great number of wall pores
es I and II of 5 specimens of P. pearsei (3 (Fig. 15) but in some parts of the shaft the
deutonymphs and 2 adult males) were dis- openings of these pores are covered by drop-
sected in ice-cold Sorensen phosphate buffer lets ofdifferent size (Figs. 16, 17). In P. pear-
(pH 7.4; 0.1 M) and then fixed in 3.5% glu- sei all setae inside the pore organ seem to be
taraldehyde buffered in Sorensen phosphate ofthe same type (Fig. 12). Sections reveal the
buffer overnight. Further processes included complex wall ofthese setae. It consists oftwo
postfixation with OSO 2%) for two hours, layers: a thick inner wall with up to 25 pores
4 (
rinsing in buffer, dehydration in graded etha- per section and a thin outer wall with a similar
nols and embedding mainly in Spurrs medium number of pores which are plugged by elec-
(Spurr 1969) and alternatively in Epon-Aral- tron dense bodies (Figs. 18-20). Some setae
dite. Ultrathin sectioning with a Diatome di- are partly surrounded by secretions (Figs. 19).
amond knife took place on a Leica Ultracut. This is very evident in the basal part of the
Sections were stained with saturated uranyla- pore organ where most of them arise (Figs.
cetate (in 70% methanol) for 5 minutes and 23, 26). The sockets ofthe setae are inflexible
lead citrate according to Reynolds (1963) for (Fig. 26). The setae are innervated by 4-7 out-
15 minutes. The sections were examined with er dendritic segments (Figs. 21, 22). These are
a Zeiss EM 10 A. For general orientation sem- surrounded by an enveloping cell and many
ithin sections (400-700 nm) were used which densely arranged microvilli (Fig. 21). The lat-
were stained according to Richardson et al. ter are formed by the tormogen cell which
606 THE JOURNAL OF ARACHNOLOGY
—
Figures 1-6. ^Distal tarsomeres. 1. Tarsus II of Pseudocellus pearsei (adult male). Scale bar = 100
(xm. 2. Pore organ opening of that tarsus. Scale bar = 10 p.m. 3. Tarsus I of Pseudocellus boned (adult
male). Scale bar = 50 fxm. 4. Opening of the pore organ of tarsus I of P. boneti (tritonymph). Note the
small microtrichae (arrow). Scale bar = 10 pm. 5. Tarsus I of P. pearsei (larva). Scale bar = 100 pm.
6. Tarsus II of P. pearsei (larva). Scale bar = 100 pm. Note the dorsofrontal region of the tarsi without
pore organ (arrows).
TALARICO ET AL.—TARSAL PORE ORGAN OF TWO RICINULEIDS 607
—
Figures 7-12. Light and TEM micrographs of the pore organ of Pseudocellus pearsei. 1. Sagittal
section oftarsus 11. Scale bar = 50 (xm. 8. Detail of pore organ with longitudinal and oblique sections of
sensilla. Scale bar = 10 ixm. 9. Horizontal section oftarsus 1. Scale bar = 50 pm. 10. Detail ofthe pore
organ base and some oblique sections of sensilla. Scale bar = 10 pm. 1 1. Transversal section oftarsus 1.
Scale bar = 50 pm. 12. Detail of the lumen with cross sections of sensilla. Scale bar = 10 pm. Abbre-
viations; Cl = claw, Cu = cuticle, PS = pore organ-sensilla. Sec = secretion.
608 THE JOURNAL OF ARACHNOLOGY
—
Figures 13-17. Surface of the pore organ integument and the pore organ-sensilla. 13. Longitudinal
section of the pore organ of tarsus I of Pseudocellus pearsei (adult female) with three lateral inserted
sensilla and a gland opening (arrow). Scale bar = 30 pm. 14. Detail of the integument with folds and a
gland opening (arrow). Scale bar = 3 pm. 15. The sensilla shaft of P. pearsei with many wall pores.
Note the damaged area (arrow). Scale bar = pm. 16. Some sensilla with numerous droplets covering
1
the wall pores. Scale bar = pm. 17. Pore organ-sensillum ofPseudocellus boneti with a totally covered
1
surface. Scale bar = pm.
1
TALARICO ET AL.—TARSAL PORE ORGAN OF TWO RICINULEIDS 609
produces the slightly electron dense receptor pore structures and the function of arthropod
lymph (Figs, 21, 26), A dendritic sheath is sensilla (e.g., Altner 1977; Altner & Prillinger
lacking. The dendritic segments terminate in 1980; Tichy & Barth 1992; Steinbrecht 1997;
the basal part of the shaft. Pore tubules be- Hallberg & Hansson 1999), three main types
neath the wall pores are lacking. The apical of olfactory sensilla are known: 1) single-
part of the shaft is completely filled with re- walled sensilla with simple wallpores, 2) sin-
ceptor lymph of different electron densities gle-walled sensilla with plugged wallpores
(Fig, 18), and 3) double-walled sensilla with spoke ca-
Large gland cells which are formed by nals. Single-walled sensilla with plugged
modified epidermal cells occur between the wallpores are present in the capsule ofHaller’s
sensilla forming cells (Figs. 23, 26). The organ (Foelix & Axtell 1972). The wall ofthe
glands appear sack-like and each one forms a pore organ-setae ofP. pearsei differs in struc-
large secretion reservoir which is filled with tural details from the main types described
an almost electron lucent material (Figs. 23, above and also from the capsule-sensilla of
26). Large nuclei, numerous mitochondria, se- Ixodida. Although wall pores with some kind
cretion vesicles and microvilli, which project of pore plugs are clearly present, the complex
into the reservoirs, are present in these cells thin outer layer (Figs. 18-20), which may con-
(Figs. 23, 26). The secretion seems to be de- sist of another type of secretion instead ofcu-
livered through at least 1 pore, partly filled ticle, makes it difficult to assign the pit-setae
with granular material, into the lumen of the to one type ofsensilla. Foelix & Axtell (1972)
tarsal pore organ (Figs. 24, 25) but it can not described a thin layer of “extracellular mate-
be excluded that a gland cell exhibits more rial” which often covers the capsule-sensilla,
than 1 pore. but this layer has no complex structure. It is
not clear whether this layer consists of recep-
DISCUSSION
tor lymph or other secretions but the authors
The first short description of the tarsal pore note that it was only prominent after simul-
organ was given by Pittard & Mitchell (1972). taneous glutaraldehyde-0s04 fixation which
They named it “deep pit” and found that this was not performed in this study. Indeed the
structure is not present in the larva but on the phenomenon ofdroplets appearing on the sur-
distal tarsomeres of leg I and II of all further face of sensilla (Figs. 16, 17) is explained as
life stages. Our observations confirm these re- dried receptor lymph (Foelix & Schabronath
sults for P. pearsei and partly for P. boneti. 1983). Altner (1977) pointed out that pore
&
Dumitresco Juvara-Bals (1973) suggested structures exist which do not fit to the classi-
the “organe tarsal” may be comparable to the fication system of sensilla types. However, the
tarsal organs of other Arachnida. These are presence of wall pores and innervating den-
present in Araneae, the tarsal organs on palps drites (Figs. 15-22) in the pore organ-setae of
I
and walking legs (e.g., Blumenthal 1935; Foe- P. pearsei indicate an olfactory function. The
f lix & Chu-Wang 1973), Scorpiones (Foelix & limited material does not allow the reconstruc-
Schabronath 1983), Amblypygi (Foelix et al. tion of the exact innervation pattern (e.g.
1975) and in Anactinotrichida, the well known number and organization of neurons) of this
Haller’s Organ on tarsus I ofIxodida and Hol- organ. Therefore further investigations are
I othyrida and the telotarsal organ on tarsus I needed. According to Foelix & Axtell (1972)
I of Opilioacarida (summarized by Alberti & and with regard to the more or less endogeous
j' Coons 1999; Coons & Alberti 1999). The tar- living of ricinuleids we believe that the tarsal
sal organs of the mentioned groups possess pore organ serves, similar to the capsule of
I several types of sensilla. Olfactory, thermo-/ Haller’s organ of Ixodida, mainly as a protec-
hygrosensitive and mechanosensitve receptors tive device for numerous olfactory sensilla,
could be identified in numerous studies. which could easily be damaged mechanically
' The tarsal pore organ of ricinuleids shows ifbeen exposed to the tarsal surface. However,
similarities to the proximal part ofHaller’s or- only electrophysical proofs can verify the sen-
j gan, the capsule, in Ixodida. These capsules sory function of an organ (see e.g., Dumpert
i bear 2-7 sunken sensilla (see Foelix & Axtell 1978; De Bruyne & Guerin 1994).
1972; Coons & Alberti 1999) which have ol- In Ixodida a large multicellular gland be-
factory function. According to the concepts of neath the capsule is known (Foelix & Axtell
610 THE JOURNAL OF ARACHNOLOGY
—
Figures 18-26. Ultrastructure ofthe tarsal pore organ ofPseiidoceUiispearsei. 18. Cross section ofa
pore organ-sensillum (apical shaft). Scale bar = 0.5 pm. 19. Cross section ofthe basal shaft with droplets
of secretion. Scale bar = 0.5 pm. 20. Detail of the wall. Scale bar = 0.2 pm. 21. Transverse section of
a sensillum socket with 4 outer dendritic segments (inset). Scale bars = 0.5 pm, 0.2 pm. 22. Horizontal
section ofdendrites beneath a sensillum. Scale bar = pm. 23. Horizontal section ofthe pore organ base
1
with gland cells between the sensilla forming cells (asterisks). Scale bar = 5 pm. 24. Transverse section
of pores (arrows) in the integument between sensilla sockets. Scale bar = 0.5 pm. 25. Horizontal section
of a pore filled with granular material (arrow). Scale bar = 0.5 pm. 26. Detail of Fig. 23. Scale bar = 2
pm. Abbreviations: Cu = cuticle, eC = enveloping cell, gR = glandular reservoir, iL = inner layer. Mi
= mitochondria, Mv = microvilli, N = nucleus, oD = outer dendritic segment, oL = outer layer, PP =
pore plug, RLy = receptor lymph. Sec = secretion, tC = tormogen cell.
TALARICO ET AL.—TARSAL PORE ORGAN OF TWO RICINULEIDS 611
1972), Their glandular openings were found scher. G.T, P.-V. and G.A. remember Mariano
in the capsule walL It was suggested that this Fuentes Silva who died in April 2004.
gland might be the origin of the material sur^
LITERATURE CITED
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between the sensilla and on their surface the soil indicate facultative plastron respiration
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—
612 THE JOURNAL OF ARACHNOLOGY
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