Life-history of the parthenogenetic oonopid spider, Triaeris stenaspis (Araneae: Oonopidae)

Abstract

Selected life-history traits of an oonopid spider, Triaeris stenaspis Simon, which has been introduced into greenhouses in Europe, were investigated. Spiders were reared in the laboratory under constant physical and dietary conditions, and followed from egg to death. The spiders passed through 3 juvenile instars, each lasting approximately a month. The adult stage lasted on average 6 months, which is 54% of the entire life cycle. The mortality in each juvenile instar was similar. Five morphological characters were recorded for each instar, which provided a reliable means of identifying the developmental stages. All spiders developed into females and although kept isolated they laid fertile eggs, which indicates thelytokous parthenogenesis. Eggs were always enclosed in a disc-shaped egg-sac, each containing 2 eggs. Total fecundity was on average 27 eggs and rate of laying eggs decreased with age. Fecundity was positively correlated with adult longevity. Fertility was rather low, approximately 59%. It was negatively correlated with fecundity but not related to longevity. Low fertility appears to be the only cost of parthenogenetic reproduction. There was considerable genotypic variation in all traits studied compared to that in sexually reproducing spiders. There were no apparent maternal effects on all the traits studied. Using molecular methods proved that parthenogenesis in T. stenaspis is not induced by the endosymbiotic bacteria, Wolbachia sp. or Cardinium sp.

Full text

INTRODUCTION
Parthenogenesis is a rare phenomenon in the animal
kingdom. It has been frequently studied in insects (Suo-
malainen, 1962), but rarely in arachnids. Except for
mites, where it is rather common (Helle et al., 1980), it is
reported only for a few scorpions (Lourenço, 2002,
2008), harvestmen (Tsurusaki, 1986), schizomids (Red-
dell & Cokendolpher, 1995) and amblypygids (Weygoldt,
2007), but there is little reliable information. Of the more
than 40 000 species of spiders in the world (Platnick,
2008) only a few are parthenogenetic (Lake, 1986;
Deeleman-Reinhold, 1986; Gruber, 1990; Shimojana &
Nishihira, 2000; Edwards et al., 2003).
Parthenogenetic reproduction has long been considered
to be evolutionary short-lived (White, 1973). It is claimed
that the major disadvantage of asexual clones is their
inability to adapt to changes in the environment and resist
infections (Groot et al., 2005). There is a number of
asexual species that are, however, very successful. For
example, acaridid mites not only adapt to new hosts but
also develop resistence to acaricides (Campos & Omoto,
2002). Two alternative hypotheses are proposed to
explain how asexual populations can adapt to different
niches. The General Purpose Genotype model suggests
the existence of generalist clones that are ecologically tol-
erant throughout the entire species range (Lynch, 1984).
The Frozen Niche Variation model postulates that the
asexual species consists of many specialist clones, each
adapted to a particular niche (Vrijenhoek, 1979).
A species can benefit from parthenogenesis. Partheno-
genetic species are considered superior colonizers as they
are able to establish a new population in isolation and
away from their bisexual progenitors (Cuellar, 1977;
Lourenço, 2008). Parthenogenetic animals do not waste
eggs producing male progeny, have no mating costs and
possess a higher reproductive potential (White, 1973). All
of this can give rise to successful species expansion. For
example, recently a parthenogentic scorpion Tityus serru-
latus Lutz & Mello was introduced into Brazil, which dis-
placed a related bisexual species, Tityus fasciolatus
(Pessõa) (Lourenço, 2002).
There are several ways in which parthenogenesis can
evolve. Cytologically parthenogenesis can be apomictic
(ameiotic), automictic (meiotic) or generative (haploid)
(Suomalainen, 1962). Recently, it was discovered that the
latter type can be induced by maternally inherited endo-
symbiotic bacteria, such as Wolbachia and Cardinium.
These bacteria have been detected in many insects and
shown to affect their mode of reproduction (Weeks et al.,
2002; Zchori-Fein & Perlman, 2004). Very recently these
endosymbiotic bacteria have been detected in a number of
spider species (Goodacre et al., 2006), but their effect on
spider reproduction is unknown.
This study focused on an oonopid spider, Triaeris
stenaspis Simon, which was introduced into greenhouses
in central and western Europe (Korenko et al., 2007).
Oonopid spiders are rare in Europe, occuring mostly in
the tropics (Platnick, 2008). Little is known about the
natural history of these species (Burger et al., 2006) as
Eur. J. Entomol. 106: 217–223, 2009
http://www.eje.cz/scripts/viewabstract.php?abstract=1445
ISSN 1210-5759 (print), 1802-8829 (online)
Life-history of the parthenogenetic oonopid spider, Triaeris stenaspis
(Araneae: Oonopidae)
STANISLAV KORENKO, JAKUB ŠMERDA and STANO PEKÁR*
Department of Botany and Zoology, Faculty of Sciences, Masaryk University, KotláĜská 2, 611 37 Brno, Czech Republic
Key words. Araneae, Oonopidae, Triaeris stenaspis, ontogeny, life cycle, development, fecundity, parthenogenesis, endosymbionts,
Wolbachia,Cardinium
Abstract. Selected life-history traits of an oonopid spider, Triaeris stenaspis Simon, which has been introduced into greenhouses in
Europe, were investigated. Spiders were reared in the laboratory under constant physical and dietary conditions, and followed from
egg to death. The spiders passed through 3 juvenile instars, each lasting approximately a month. The adult stage lasted on average 6
months, which is 54% of the entire life cycle. The mortality in each juvenile instar was similar. Five morphological characters were
recorded for each instar, which provided a reliable means of identifying the developmental stages. All spiders developed into
females and although kept isolated they laid fertile eggs, which indicates thelytokous parthenogenesis. Eggs were always enclosed in
a disc-shaped egg-sac, each containing 2 eggs. Total fecundity was on average 27 eggs and rate of laying eggs decreased with age.
Fecundity was positively correlated with adult longevity. Fertility was rather low, approximately 59%. It was negatively correlated
with fecundity but not related to longevity. Low fertility appears to be the only cost of parthenogenetic reproduction. There was con-
siderable genotypic variation in all traits studied compared to that in sexually reproducing spiders. There were no apparent maternal
effects on all the traits studied. Using molecular methods proved that parthenogenesis in T. stenaspis is not induced by the endosym-
biotic bacteria, Wolbachia sp. or Cardinium sp.
217
* Corresponding author: pekar@sci.muni.cz

none of them has been subjected to a detailed study. T.
stenaspis was first described from the Caribbean island of
St. Vincent (Simon, 1891) and according to Platnick
(2008) is distributed from USA to Venezuela, including
the West Indies. Only females of this species are known
and Koponen (1997) suggested that it is parthenogenetic.
The aim of this study was to (1) investigate in detail the
life-history of T. stenaspis, (2) determine its mode of
reproduction, (3) reveal how its life-history traits are
affected by parthenogenesis, and (4) determine whether
parthenogenesis is caused by bacterial endosymbionts.
MATERIAL AND METHODS
Development and reproduction
Adult females (N = 3) of T. stenaspis were collected from a
greenhouse in the Botanical Garden of the Masaryk University,
Brno, Czech Republic. These females laid several eggs from
which 68 spiderlings hatched. These were used in this study,
which lasted for 10 months. Spiderlings were placed singly in
cylindrical containers (diameter 35 mm, height 40 mm) with a
layer of plaster of Paris at the bottom. They were kept at room
temperature, 22 ± 3.5°C. The plaster was moistened every 10-
days. Spiders were fed a surplus of springtails (Sintela sp. and
Sinella curviseta Brook) every 5 days.
Our observations focused on the development after emer-
gence from the egg-sac, i.e. beginning with the first free instar.
Each spider was monitored until it died. For each individual the
number of moults, duration of individual instars and mortality
were recorded by checking the spiders at 1–5 day intervals. For
adults their longevity, time to oviposition, production of eggs
and their time to hatching were recorded. Fecundity was
expressed as the total number of eggs produced during a
female’s life and fertility as the proportion of eggs that hatched.
Morphological characters
For each instar the following parameters were measured: (1)
length of prosoma (along longest axis), (2) outer width of ante-
rior eye region, (3) length of tibia I, (4) number of ventral spines
on patella I, (5) number of ventral spines on tibia I and (6) the
number of abdominal scuta. All these parameters were assumed
to be stage specific. The number of spines is, for example, an
important character used in the identification of oonopid spiders
(Chickering, 1969). In addition the size of eggs and egg-sacs
(longest axis) were measured. All measurements were done
under a stereomicroscope, Olympus SZX9.
Analyses
Statistical analyses were conducted within R environment (R
Development Team, 2007). The confidence intervals for predic-
tion of future measurements of prosoma length, anterior eye
region width and tibia length were estimated from a linear
model based on a normal distribution of the measurements. Sur-
vival was estimated by means of the Kaplan-Meier method
implemented within the survival package (Therneau & Lumley,
2007). Mortality for particular instars was compared using a
3-sample test for equality of proportions. As the variance of the
data was homogeneous the relationships between selected traits
were studied using linear regression (LM) with normal errors.
The temporal change in the rate of egg laying, production of
empty sacs and fertility was modelled using Generalised Least
Squares (GLS) in the nlme package (Pinheiro et al., 2006) in
order to test for autocorrelation in the data. If there was no auto-
correlation of the AR1 process, LM or Generalised Linear
Models with binomial errors (GLM) were used. For each trait a
coefficient of variation (CV) was used to estimate the
variability. CV was computed according to the standard formula
for the coefficient of variation multiplied by 100%. Maternal
effects were analysed either using ANOVA for continuous
measurements (e.g., duration of instars) or GLM with binomial
errors for proportions (fertility).
Endosymbionts
Ten individuals were screened for the presence of two endo-
symbionts using PCR with species-specific primers for Wolba-
chia and Cardinium. Spider DNA was extracted using the
Promega Wizard Genomic DNA Purification Kit according to
the manufacturer’s instruction. DNA was extracted from legs to
exclude the possibility of false-positive samples resulting from
ingested prey and decrease redundant inhibitors of PCR from
abdomen. The DNA quality was tested by amplification of
eukaryotic 18S rRNA gene using the universal primers NSF4/18
and NSF399/19 (Hendriks et al., 1989, 1991). Cardinium infec-
tions were detected using ChF (Zchori-Fein & Perlman, 2004)
and CLO-r1 (Gotoh et al., 2007) primers, and Wolbachia infec-
tions by 16Swolb76-99f and 16Swolb1012-994r primers
(O’Neill et al., 1992). The 20 µl PCR mix consisted of 30–50 ng
DNA, 1× PCR buffer, 1.5 U Taq DNA polymerase, 2.4 mM
MgCl2, 100 µM dNTP and 250 nM of each primer. Thermal
conditions for amplification of all PCRs were 94°C for 2 min
and 35 cycles of 94°C for 30 s, 54°C for 30 s and 72°C for 1
min, followed finally by 72°C for 5 min.
RESULTS
Development
After leaving the egg-sac the spiderlings required 3
moults to reach maturity. The first instar lasted on
average 31.5 days (SD = 9.8, N = 51, CV = 31.1%), the
second 29.8 days (SD = 6.9, N = 37, CV = 23.2%) and
the third 24.5 days (SD = 5.3, N = 31, CV = 21.6%).
Thus the duration of juvenile instars decreased with age.
Average longevity of adults was 101 days (SD = 46.4, N
= 31, CV = 45.9%), with a maximum of 201 days. Lon-
gevity was not related to the duration of juvenile develop-
ment (LM, F1,29 = 0.01, P = 0.9). The total life-span (from
218
Fig. 1. Survival (—) of T. stenaspis (N = 69) from hatching to
adulthood. Dashed lines are the 95% confidence intervals. Ver-
tical dotted lines indicate the means of the times of moulting.

hatching to death) lasted on average 182 days (SD = 46.4,
N = 31) and generation time (from hatching to
egg-laying) was on average 97.5 days (SD = 9.4, N = 31).
For none of these traits was there a significant maternal
effect (ANOVA, P > 0.1).
Survival was similar in all juvenile stages (Proportion
test, F22 = 1.5, P = 0.48), ranging between 74 and 84%
(Fig. 1), and independent of the mother (Proportion test,
F22 = 0.6, P = 0.74).
Morphological characters
The size of the prosoma, tibia I and anterior eye region
increased with the stage (Table 1). The 95% CI for pre-
dictions of length of tibia do not overlap between subse-
quent instars. The number of spines differed between
stages. In the first instar there were 2 pairs of ventral
spines on patella I and 3 pairs on tibia I, in later instars
there were 3 and 5 pairs of spines, respectively (Table 1).
All juvenile instars were pale in colour and their colora-
tion depended on the food. Red-yellowish coloration with
three reddish-brown scuta (dorsal, epigastric and ventral)
on opisthosoma was only present at maturity.
Reproduction
All the spiders developed into females (N = 31).
Despite being kept isolated during their entire life they all
began to lay fertile eggs on average 14.1 days (SD = 5.6,
N = 31, CV = 39.7%) after the last moult. The eggs were
enclosed in disc-shaped pinkish silken sacs. Each egg-sac
contained 2 eggs. The spiders laid on average 0.306 eggs
per day (SD = 0.108, N = 31), which decreased with age
(LM, F1,26 = 27.6, P < 0.0001, Fig. 2). The total fecundity
was on average 27.4 eggs (SD = 13.1, N = 31, CV =
47.8%), with two eggs in each egg-sac. There was a posi-
tive linear relationship between fecundity and longevity
(LM, F1,29 = 51.2, P < 0.0001, Fig. 3).
Fertility was negatively related to fecundity (LM, F1,29 =
7, P = 0.012, Fig. 4) and was on average 0.59 (N = 31).
Fertility was not related to longevity (LM, F1,29 = 0.25, P
= 0.62). Fertility declined slightly with successive egg-
219
335
0.383
(0.368, 0.399)
0.173
(0.163, 0.182)
0.709
(0.675, 0.743)
Adult
035
0.319
(0.304, 0.335)
0.149
(0.139, 0.159)
0.643
(0.609, 677)
3rd instar
035
0.239
(0.223, 0.254)
0.123
(0.113, 0.133)
0.527
(0.493, 0.561)
2nd instar
023
0.190
(0.174, 0.205)
0.106
(0.096, 0.115)
0.482
(0.448, 0.516)
1st instar
ScutaPtTiTi (mm)AER (mm)
Prosoma
(mm)
Stage
TABLE 1. The means and 95% CI (for predicting measure-
ments) of five parameters: length of prosoma (Prosoma), width
of anterior eye region (AER), length of tibia I (Ti), number o
f
pairs of ventral spines on tibia I (Ti), number of pairs of ventral
spines on patella I (Pt) and number of abdominal scuta (Scuta).
N = 10 for all parameters.
Fig. 2. Temporal changes in the rate of egg laying, expressed
as the average number of eggs produced by a female per day
during its life (N = 31). Linear model is presented.
Fig. 3. Relationship between fecundity (total number of eggs
laid) and adult longevity. Linear model is presented.
Fig. 4. Relationship between fertility (proportion of eggs that
hatched) and fecundity (total number of eggs laid). Linear
model is presented.

clutches, but not significant so (GLS, F1,418 = 3.5, P =
0.06, Fig. 5). In fact, the fertility of the last egg clutches
of the few individuals that produced many egg clutches
was high. Fecundity, time to production of first egg-sac
and fertility were independent of the mother (ANOVA or
GLM, P > 0.14).
The average egg-sac was 1.45 mm (SD = 0.1, N = 10)
long. Eggs were on average 0.47 mm (SD = 0.01, N = 10)
in diameter. Incubation time of eggs was on average 32.7
days (SD = 5.4, N = 22, CV = 16.5%). In addition to pro-
ducing egg-sacs they also produced empty sacs, i.e. only
the basal disc. The production of empty sacs increased
with age (GLM, F21 = 38.4, P < 0.0001, Fig. 6).
Endosymbionts
The fragment of eukaryotic 18S rDNA gene from all
samples was amplified, which indicated satisfactory DNA
quality. Neither Wolbachia nor Cardinium were detected
in the tissue samples.
DISCUSSION
At present few species of spider are thought to be par-
thenogenetic and only for two of them is there laboratory
evidence. Suggestive evidence is available for two other
species. A single juvenile female of a sparassid, Holconia
insignis (Thorell), kept in isolation after its final moult
laid a single egg-sac from which spiderlings hatched
(Lake, 1986). Another instance is the troglobiontic
amaurobiid, Coelotes troglocaecus Shimojana & Nishi-
hira, which is found in limestone caves on Okinawa
Island (Japan) and the populations of which consist only
of females. That this spider lives in a isolated habitat in
caves, at low population densities, in the absence of males
and has degenerate reproductive organs (spermathecae
and spermathecal ducts) strongly suggest reproduction by
thelytokous parthenogenesis (Shimojana & Nishihira,
2000). There is laboratory evidence of parthenogenesis,
based on experimental rearing of individuals in isolation,
for only two species of spider. Machado (1964) was the
first to suggest parthenogenesis in the orychoceratid
Theotima minutissima (Petrunkevitch), which was only
recently experimentally confirmed by Edwards et al.
(2003). Deeleman-Reinhold (1986) suggested that a local
parthenogenetic population of Dysdera hungarica Kulc-
zyĔski existed in Europe. Sexual populations occur only
in the eastern part of its range, in the Transylvanian
mountains and northern Bulgaria. Parthenogenesis was
experimentally confirmed in this species by Gruber
(1990).
It is unknown whether T. stenaspis is also parthenoge-
netic in its native area, the Carribic, as the male of this
species has not yet been found (Platnick, 2008). Unlike C.
troglocaecus, but similar to D. hungarica, females of T.
stenaspis possess fully developed copulatory organs
(Korenko et al., 2007). This suggests that reproduction by
parthenogenesis might only occur in geographically iso-
lated populations.
We thought that parthenogenesis might be induced in
these allopatric populations as a result of infection by
some endosymbiotic bacteria (Weeks et al., 2002). How-
ever, neither Wolbachia or Cardinium were detected in T.
stenaspis tissues. Preliminary results indicate automictic
parthenogenesis as meiosis still occurs. The diploid
number of holocentric chromosomes (2n) is 8 and the
likely sex determination system XX (J. Král & J. Musi-
lová, pers. comm.).
To determine the effect of parthenogenesis on the life
history a sexual population of the same species needs to
be studied. As males are not known in T. stenaspis a
closely related species could be used. However, there is
not a single study on the life-history of any other oonopid
spider. Therefore, observed traits had to be compared
with those of an unrelated species of spider.
The number of instars depends mainly on spider size
(Schaefer, 1987). T. stenaspis is on average 2 mm long.
220
Fig. 5. Temporal changes in the average proportion of eggs
that hatched in the successive egg-sacs produced by 31 females.
Fig. 6. Temporal changes in the proportion of empty sacs pro-
duced by females (N = 17). The logit model, where p is the pro-
portion of sacs and x is time, is presented.

Three juvenile instars is the number recorded for other
tiny species of spider (e.g., Schaefer, 1987; Rybak, 2007).
But T. minutissima has at least 5 juvenile instars, though
it is only 0.9 mm long (Edwards et al., 2003). The number
of instars often depends on environmental conditions
(e.g., Higgins & Rankin, 1996). We did not record any
variation in the number of instars, which suggests that
this trait might be canalised in T. stenaspis. The intermolt
interval, however, was very variable suggesting it is plas-
tic. A similar developmental trajectory, with both plastic
and canalised traits, is reported for a few other spiders
(Higgins & Rankin, 1996). As all the T. stenaspis were
reared under similar physical conditions (temperature and
humidity) this variation must be mainly due to genotypic
variation. All individuals were provided with a surplus of
springtails, which are presumably the main prey of this
spider.
Survival is affected by both intrinsic (e.g., nutrition,
hunger) and extrinsic (predation, disease, competition,
temperature, moisture) factors. Studies of survival of spi-
ders under natural conditions reveal survivorship curves
of type II and III (Tanaka, 1992; Boulton & Polis, 1999).
For T. stenaspis a survivorship curve of type I was
recorded. This is typical for species in which mortality is
concentrated towards the end of their life. This is obvi-
ously because T. stenaspis was reared under constant
laboratory conditions, which filtered out the effects of
many extrinsic and intrinsic variables.
Like the rate of development, fecundity is influenced by
physical conditions (e.g., Downes, 1988), prey quality
and quantity (Mayntz et al., 2005). Females of T.
stenaspis laid egg clutches composed only of two eggs,
but produced many egg clutches. Other similar small
sized species of spider have a different reproductive strat-
egy: they produce few egg-sacs each containg several
eggs. For example, the linyphiid Bathyphantes simillimus
(L. Koch) produces 6 egg-sacs each containing an
average of 11 eggs (Rybak, 2007). Edwards et al. (2003)
record that the parthenogenetic females of the tiny T.
minutissima produce 1–2 egg-sacs each containing an
average of 5 eggs.
Several models of the relationship between reproduc-
tive parameters, such as fecundity, egg size, number of
egg-sacs and body size or body mass of females have
been proposed. The model proposed by Marshall & Git-
tleman (1994) for the relationship between fecundity and
the mass of females predicts fewer egg-sacs for T.
stenaspis than were recorded. That of Simpson (1995)
predicts a smaller total fecundity and larger eggs than
recorded. The low number of eggs per egg clutch and
their small size in T. stenaspis is presumably a result of an
interaction between the size of the eggs and morpho-
logical constraints of the abdomen. The abdomen of
females has three scuta, which might restrain enlargement
of the abdomen during egg development. Iteroparous
reproduction (multiple egg-sac production) could have
helped this species to spread so successfully.
The rate at which spiders lay eggs decreases with adult
age (e.g., Miyashita 1988; Fischer & Vasconcellos-Neto,
2005b). A similar pattern was observed in T. stenaspis,
but in terms of the frequency of egg-sac production as the
number of eggs in a sac remained constant (2).
At warmer latitudes spiders tend to be more long-lived,
unlike in the temperate zone where many spiders have
very short adult lives (Schaefer, 1987). Data for species
from warmer climates indicates that the adult stage makes
up more than 50% of the life span. For example, in the
sexually reproducing sicariid Loxosceles intermedia
Mello-Leitão the adult life span is about 60% of the total
lifespan (Fischer & Vasconcellos-Neto, 2005a); in T.
stenaspis it was 54% and in T. minutissima it is 50%
(Edwards et al., 2003). It is suggested that rapid growth
occurs at the expense of longevity. This is not supported
by our data.
Fertility, or hatching success, is usually very high,
especially of laboratory reared spiders. For example, the
fertility recorded for a clubionid (Austin, 1984) and
zodariid spider (Pekár et al., 2005) is more than 90%. In
general, in sexual species of spiders fertility correlates
positively with the duration of mating (Fischer &
Vasconcellos-Neto, 2005b). In L. intermedia the fertility
of the eggs in the first egg-sac is higher than 80% but is
significantly less for those in successive egg clutches
(Fischer & Vasconcellos-Neto, 2005b). In sexually repro-
ducing species this might be due to sperm depletion. In T.
stenaspis this decrease in the fertility of the eggs in suc-
cessive egg clutches was recorded only in females that
had a short or moderate life-span. The fertility of the
females with a long life-span remained constant.
The continuous production of the basal discs of egg-
sacs is interesting. The production of empty egg-sacs is
known to occur in sexually reproducing linyphiid spiders
kept under laboratory conditions (S. Toft, pers. comm.).
Why females of T. stenaspis abandon basal discs is not
clear. External factors, such as insufficient food and dis-
turbance by prey are an unlikely cause as the spiders were
provided with a surplus of prey and the sacs were placed
on the side of the container, an area inaccessible to the
prey. The production of such a large number of egg-sacs
is presumably far beyond their ability so there has never
been a selection for maintaining reproductive capacity for
so long (S. Toft, pers. comm.).
Parthenogenetic reproduction is assumed to produce
genetically homogeneous clones that vary little (pheno-
typically or genetically) in their traits. But Asher (1970)
showed that even parthenogenesis can sustain genetic
plasticity under certain conditions. In this study all indi-
viduals were reared under similar conditions, yet there
was remarkable variation, ranging between 17 and 48%.
We assume this variation is largely genotypic because of
the standardised rearing conditions. This range in varia-
tion is similar to that recorded for the sexually repro-
ducing L. intermedia (Fischer & Vasconcellos-Neto,
2005b). Interestingly, the variation between spiderlings
from a single mother was as large (or larger) as that
between those from different mothers. Thus no maternal
effects on traits were recorded.
221

The results show that T. stenaspis is a parthenogenetic
eurychronous, iteroparous species with about 3 genera-
tions per year. The maintenance of plasticity in partheno-
genetic populations is thought to be costly (Asher, 1970).
We identified one cost, low fertility. This cost seems to
be negligible compared with the benefits as this species
has established viable populations in many greenhouses
in Europe (Korenko et al., 2007). Of the 18 species in the
genus Triaeris only this one has colonized Europe pre-
sumably because of its parthenogenetic mode of repro-
duction.
ACKNOWLEDGEMENTS. We would like to thank S. Toft for
very useful comments on the manuscript. We are greatly
indebted to T. Bilde and J.P. Maelfait for the collembola cul-
tures, P. Weygoldt and N.I. Platnick for providing us with
copies of some rare papers. This study was supported by grants
no. 0021622416 and LC06073 from the Ministry of Education,
Youth and Sports of the Czech Republic. SK was supported by
grant no. 526/09/H025 from the Czech Science Foundation.
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