Do cave orb spiders show unique behavioural adaptations to subterranean life? A review of the evidence

Abstract

Interest for subterranean biology has risen sharply in recent years due to the simplicity of the cave environment. However, most studies have focussed on morphology with few studies looking at behaviour. The cave orb spiders show some unique behavioural adaptations compared to other orb spiders, including rudimentary orb webs, off-web foraging and a complex life cycle with a surface phase. Here, we compare these behavioural adaptations in the European Meta menardi and Meta bourneti to similar behaviours in surface-dwelling orb spiders. We find that current data suggest (1) an extreme reduction in the number of frame threads, (2) evidence of capturing non-flying prey, but not necessarily evidence for off-web foraging and (3) dispersal through a surface-dwelling life stage, but with data lacking on the role of ballooning and their return to caves. We conclude that Meta spiders have potential as model organisms for studies on behavioural adaptations and flexibility.

Full text

Behaviour 156 (2019) 969–996 brill.com/beh
Review
Do cave orb spiders show unique behavioural
adaptations to subterranean life? A review of the
evidence
Thomas Hesselberg a,b,c,∗,Daniel Simonsen aand Carlos Juan c,d
aDepartment of Zoology, University of Oxford, South Parks Road, Oxford, OX1 3PS, UK
bDepartment for Continuing Education, University of Oxford, Wellington Square, Oxford,
OX1 2JA, UK
cDepartment of Biology, University of the Balearic Islands, 07071 Palma de Mallorca,
Spain
dIMEDEA (CSIC-UIB), Mediterranean Institute for Advanced Studies, Esporles, 07190,
Spain
*Corresponding author’s e-mail address: Thomas.hesselberg@zoo.ox.ac.uk
Received 19 November 2018; initial decision 8 May 2019; revised 19 May 2019;
accepted 21 May 2019; published online 1 July 2019
Abstract
Interest for subterranean biology has risen sharply in recent years due to the simplicity of the cave
environment. However, most studies have focussed on morphology with few studies looking at
behaviour. The cave orb spiders show some unique behavioural adaptations compared to other orb
spiders, including rudimentary orb webs, off-web foraging and a complex life cycle with a surface
phase. Here, we compare these behavioural adaptations in the European Meta menardi and Meta
bourneti to similar behaviours in surface-dwelling orb spiders. We find that current data suggest
(1) an extreme reduction in the number of frame threads, (2) evidence of capturing non-flying prey,
but not necessarily evidence for off-web foraging and (3) dispersal through a surface-dwelling
life stage, but with data lacking on the role of ballooning and their return to caves. We conclude
that Meta spiders have potential as model organisms for studies on behavioural adaptations and
flexibility.
Keywords
behavioural flexibility, cave spiders, dispersal, orb web geometry, prey capture, Tetragnathi-
dae.
©The authors, 2019 DOI 10.1163/1568539X-00003564
This is an open access article distributed under the terms of the prevailing CC-BY-NC license at the time of publication.
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970 Behavioural adaptations in cave orb spiders
1. Introduction
Subterranean habitats are some of the least studied terrestrial ecosystems and
we have little current knowledge on the natural history and behaviour of the
majority of organisms found in them. However, interest has grown in recent
years and cave ecosystems are now generally recognised as natural ecolog-
ical and evolutionary laboratories due to the isolation of their organisms,
the fewer and more simple biotic interactions due to low species richness
and abundance and the number of similar morphological adaptations found
in a range of organisms (Poulson & White, 1969; Pipan & Culver, 2013;
Mammola, 2018). Cave ecosystems are characterised by stable environmen-
tal conditions, low light levels and as a consequence usually no primary
production and they are as a result generally nutrients poor (Culver & Pi-
pan, 2009). Caves though are not homogenous in their ecology, but can be
divided into three zones. The entrance zone is generally defined as the area
immediately near the entrance which is reached by direct sunlight and is
characterised by being relatively nutrient rich due to the number of epigean
animals that accidentally enter the cave. The twilight zone is characterised
by very low light levels and a much lower abundance of organisms. Finally,
the dark zone starts where light can no longer be detected by the human eye
and stretches until the end of the cave. This is usually the largest and most
nutrient poor zone, but it is also where the largest number of cave adapted
species can be found. Following the traditional speleological terms (Sket,
2008), organisms with adaptations, in almost all cases this refers to morpho-
logical adaptations, to caves and other subterranean habitats are classified
as troglomorphs. All troglomorphs are also troglobionts, which are organ-
isms that are obligatorily associated with subterranean habitats for all life
stages. In contrast, troglophiles usually spend some time outside them, while
trogloxenes are visitors to the subterranean habitat, but spend most of their
life outside.
Unsurprisingly, spiders are generally well represented in caves given their
exaptations to subterranean life including, for most families, low reliance on
vision and low metabolism allowing them to survive months without feed-
ing (Foelix, 2011). There are more than 1000 species of troglobiont spiders
and at least an equal number of troglophile spiders belonging to 48 fami-
lies out of 47 000 described species of spiders in 113 families (Mammola &
Isaia, 2017). In Europe, 195 troglobiont and 291 troglophile species are cur-
rently recognised in 22 families (Mammola et al., 2017a). The most species
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 971
rich families are Linyphiidae with 222 subterranean species (including the
almost exclusively troglobiont genera Centromerus and Troglohyphantes),
Dysderidae with 56 species and the predominantly subterranean Nesticidae
with 50 species (Mammola et al., 2017a).
Surprisingly, given the scarcity of flying insects, orb spiders in the fam-
ily Tetragnathidae (genera Meta and Metellina) are common in European
caves, although all are troglophiles and mainly limited to the twilight zone
(Mammola & Isaia, 2014). Orb webs are two-dimensional highly organised
structures consisting of radii running from the central hub to the frame and
overlaid by the capture spiral that are adapted to intercept and retain flying
insects (Eberhard, 1990b). Thus, the orb web can be viewed as a physi-
cal record of the spider’s foraging behaviour, which together with the ease
of getting spiders to construct orb webs in the laboratory and the highly
quantifiable nature of the web have made orb spiders and their webs model
organisms for the study of a wide range of different behaviours (Zschokke
& Herberstein, 2005; Hesselberg, 2015). Orb spiders show a remarkable be-
havioural flexibility and modify their web-building and foraging behaviour
in response to a wide range of internal and external factors including micro-
climatic conditions (Vollrath et al., 1997; Liao et al., 2009; Turner et al.,
2011; Wu et al., 2013), prey capture experience (Pasquet et al., 1994; Heiling
& Herberstein, 1999; Blamires, 2010), leg loss (Vollrath, 1987; Pasquet et al.,
2011) and spatial constraints in the micro-habitat (Krink & Vollrath, 2000;
Barrantes & Eberhard, 2012; Harmer et al., 2012; Hesselberg, 2013). Despite
their relatively simple and small brains, they show impressive cognitive abil-
ities (Hesselberg, 2015; Japyassu & Laland, 2017) including spatial learning
of prey impacts (Nakata, 2013), alertness to web damage and faster repairs
in windy conditions (Tew et al., 2015), simple numerosity (Rodríguez et al.,
2015), and memories of previous web-building behaviour (Eberhard, 1988)
and prey capture (Rodríguez & Gamboa, 2000; Rodríguez et al., 2013).
We know almost nothing about the behaviour of any cave spider (Mam-
mola & Isaia, 2017), but the cave orb spiders have been alleged to possess a
number of unique behavioural adaptations to subterranean life not found in
terrestrial orb spiders. Here, we review the evidence for adaptive modifica-
tions of the standard orb web and for unusual foraging as well as the highly
interesting life cycle and dispersal behaviour and contrast it to closely related
epigean tetragnathid spiders. Our aim is to highlight the research potential of
studying the behaviour of these large, conspicuous and relatively abundant,
but overlooked, troglophiles.
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972 Behavioural adaptations in cave orb spiders
2. Cave orb spiders
The largest family of orb spiders is Araneidae followed by Tetragnathidae
and the cribellate Uloboridae (Foelix, 2011), but the only orb spider family
with described representatives in the subterranean environment is Tetrag-
nathidae. This is a large family with 987 species in 46 genera (Kallal &
Hormiga, 2018). Currently, 7 cave-dwelling spiders are known in the genus
Meta and two in Metellina (Table 1), although it is likely that there are sub-
terranean species in other tetragnathid genera; for example, Orsinome has
been reported in caves in Tasmania (Eberhard, 1992). The genus Meta cur-
rently has 23 species, but as it has been treated as the ‘waste-bin’ of the
Metaine subfamily this is likely to change. In 2018 alone, 11 species were
transferred from Meta to other genera (mainly Metellina and Tetragnatha)
(Kallal & Hormiga, 2018; Marusik & Larsen, 2018). Given that Marusik &
Larsen (2018) use a relatively large size as a criterion for including species
in Meta (and in Metellina), many of the Asian species are likely incorrectly
placed in Meta (Table 1). Meta meruensis for example most likely belongs
in the genus Leucauge (Marusik & Larsen, 2018). In general, we know very
little about the natural history of most Meta spiders and for many species,
we have not even been able to determine if they are cave-dwellers or not (Ta-
ble 1). Although most of the species with known habitats are subterranean,
this appears not to be a key characteristic of the genus as M. stridulans is an
endemic of Madeira, where it inhabits laurel forests (Crespo et al., 2014).
In this review, we will focus on the two species of Meta spiders that we
know most about — the common European cave orb spiders M. menardi and
M. bourneti — with a brief mention of Metellina merianae, which, together
with M. villersi (Denis, 1955), is the only known Metellina regularly found
in caves. Meta bourneti (Simon, 1922) (Figure 1) is a large spider (the male
is 10–13 mm and the female 14–16 mm in total length) with a dark brown
cephalothorax and a usually uniformly yellowish brown to black abdomen.
It is classified as a troglophile and usually found in the twilight zone of
caves, where it builds relatively small orb webs compared to closely related
epigean tetragnathids. It is widely distributed in southern Europe and around
the Mediterranean including northern Africa. It is also found in southern
parts of the UK, where it is likely to be an introduced species (Mammola,
2017). Meta menardi Latreille, 1804 (Figure 1), also sometimes referred to
as the European cave spider, is a large spider (the male is 10–12 mm and the
female 12–15 mm in total length). It has a red-brown cephalothorax and a
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 973
Table 1 .
Overview of species in the genus Meta.
Species Authority Known distribution Carapace
width (mm)
Total length
(mm)
Number of
publicationsaHabitat
Meta abdomenalis Patel & Reddy, 1992 India 1.2 3.8 1 Unknown
M. birmanica Thorell, 1898 Myanmar 1 Unknown
M. bourneti Simon, 1922 Europe to Georgia, North Africa 6.0 14.524C
AVE S
M. dolloff bLevi, 1980 USA 6.3 14.02CAVE S
M. japonica Tanikawa, 1993 Japan 10.95CAV E S
M. manchurica Marusik & Koponen, 1992 Russia, Korea 4.4 12.27CAV E S
M. menardi (Latreille, 1804) Europe to Turkey 6.0 16.0 124 CAVE S
M. meruensiscTullgren, 1910 East Africa 8.5 5 Unknown
M. mixta O. Pickard-Cambridge, 1885 China 1 Unknown
M. monogrammata Butler, 1876 Australia 13.0 1 Unknown
M. montana Hogg, 1919 Indonesia 1.5 5.0 1 Unknown
M. nebulosadSchenkel, 1936 China 1 Unknown
M. nigridorsalis Tanikawa, 1994 China, Japan 1.8 5.84C
AVE S
M. obscura Kulczy´
nski, 1899 The Canary Islands, Madeira 6.5 2 Unknown
M. ovalis (Gertsch, 1933) North America 4.5 13.719C
AVE S
M. qianshanensis Zhu & Zhu, 1983 China 4.5 12.3 4 Unknown
M. rufolineata (Urquhart, 1889) New Zealand 2.0 6.8 3 Unknown
M. serrana Franganillo, 1930 Cuba 1 Unknown
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974 Behavioural adaptations in cave orb spiders
Table 1 .
(Continued.)
Species Authority Known distribution Carapace
width (mm)
Total length
(mm)
Number of
publicationsaHabitat
M. shenae Zhu et al., 2003 China 1 Unknown
M. simlaensis Tikader, 1982 India 3.2 9.4 1 Unknown
M. stridulans Wunderlich, 1987 Madeira 3.9 11.01LaurelForest
M. trivittata Keyserling, 1887 Australia 1.8 5.4 1 Unknown
M. turbatrix Keyserling, 1887 Australia 2.4 6.4 1 Unknown
Metellina merianae Scopoli, 1763 Europe and the Middle East 2.4 9.048C
AVE S
Metellina villiersi Denis, 1955 Guinea 8.01CAV E S
Taxonomic status and distribution come from the World Spider Catalog (2019). Information on habitat and morphology (adult female size
is indicated by total length and cephalothorax width in the table) comes from a range of papers cited in World Spider Catalog (2019):
aInformation on number of publications is the combined number of taxonomic publications listed in World Spider Catalog (2018) and
publications on ISI Web of Science (species names searched in topics across all databases as of September 2017).
bOnly Meta spider that is on the IUCN red list as ‘Vulnerable’ (but added in 1996 so needs updating).
cThis species is likely to belong in Leucauge (Marusik & Larsen, 2018).
dThis species has apparently been described from juveniles alone (Schenkel, 1936).
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 975
Figure 1. Distribution of the European cave spiders (2nd row: known distributions taken from
the World Spider Catalogue (2018) and from Araneae – Spiders of Europe (Nentwig et al.,
2018)) and cave location preference (3rd row: the entrance zone is the first few metres inside
the cave (light grey), the twilight zone (dark grey) stretches 10–15 m, depending on cave
structure, before the dark zone starts (black)). 1st column: Meta bourneti Simon, 1922. Photo
with permission from Didier Petot. 2nd column: Meta menardi Latreille, 1804. Photo with
permission from Thomas Hesselberg. 3rd column: Metellina merianae Scopoli, 1763. Photo
with permission from Richard McMellon.
yellow abdomen with darker stripes or large spots. Similarly, to M. bourneti,
M. menardi is a troglophile that is predominantly found in the twilight zone
of caves, although it is also recorded from other dark places including mines,
cellars, manholes and in crevices and boulder fields (R˚užiˇ
cka et al., 2013). It
is considered common across Europe including Scandinavia up to near the
Arctic circle and as far east as Turkey. Metellina merianae Scopoli, 1763
(Figure 1) is a medium sized spider (the male is 7–8 mm and the female
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976 Behavioural adaptations in cave orb spiders
6–10 mm in total length) with a yellow-green cephalothorax and a green-
brown abdomen overlaid with a darker chequer pattern. It is found in the
entrance zone and the first part of the twilight zone in caves and other dark
places, but unlike the two Meta species, it can also be found in terrestrial
habitats including damp and shady woodland areas. There is no evidence
of any behavioural adaptions to subterranean habitats in this species. It is
common and widespread in Europe extending as far east as Caucasus and
Iran.
Interestingly, although there is a large overlap in the distribution of all
three species (Figure 1), none of them are sympatric on the micro-scale.
Instead they show a significant degree of niche separation. Meta menardi
and Meta bourneti are never found in the same cave (Mammola & Isaia,
2014). M. bourneti has a relatively wide tolerance of microclimatic condi-
tions, whereas M. menardi has a relatively narrow range preferring cooler
caves with high humidity and in the favourable habitats M. menardi appar-
ently outcompetes M. bourneti (Mammola & Isaia, 2014). On the other hand,
Meta menardi are found to co-occur with Metellina merianae with limited
spatial niche separation, although M. merianae is usually found closer to
the cave entrance than M. menardi (Novak et al., 2010). Instead the two
species show a significant temporal niche separation with the number of
juveniles of M. merianae showing large peaks at the end of the summer,
whereas juvenile abundance of M. menardi is more evenly distributed with
minor peaks in winter and spring (Novak et al., 2010). Throughout the rest of
this review we compare aspects of the behaviour of Meta cave spiders with
those of terrestrial tetragnathids; especially with the closely related Metellina
(Alvarez-Padilla & Hormiga, 2011; Kallal & Hormiga, 2018; see Figure 2).
3. The orb web: behavioural adaptation or flexibility?
The orb web is one of the most conspicuous and geometrically ordered
structures in nature. As mentioned in the introduction, the archetypical orb
web consists of a central hub from which radii radiate outwards towards the
frame and upon which the capture spiral is overlain (Figure 3A). This gen-
eral structure is conserved across all the major orb-spider families, but with
variation in the detailed geometry and in the material properties of the dif-
ferent silk types used (Vollrath & Selden, 2007; Sensenig et al., 2010). There
are, however, minor differences among the families (Foelix, 2011). Tetrag-
nathid webs, in contrast to araneid webs, tend not to have any threads in
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 977
Figure 2. Simplified phylogenetic tree of orb web spiders in the family Araneidae and Tetrag-
nathidae highlighting subfamilies and genera of interest for the discussions presented in
this paper. Adapted from the maximum likelihood analysis (Figure 1) of Kallal & Hormiga
(2018).
the centre of their hubs and tend to be more horizontally inclined with the
woodland species Metellina mengei, for example, having webs that varied
in inclination with horizontal from 5 to 85° (Tew & Hesselberg, 2018). The
Figure 3. Schematic drawing of orb webs based on measurements of typical webs (Simonsen
& Hesselberg, data not shown). Arrows points to frame threads, radii and the capture spiral.
The vertical scale bars at the righthand side of each web is 20 mm in length. (A) Metellina
mengei web; (B) Meta menardi web.
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978 Behavioural adaptations in cave orb spiders
differences are not immediately apparent from a casual glance, but webs of
Meta menardi and M. bourneti differ from the typical orb web in that they
have very few frame threads (Figure 3B). The web is also smaller and less
complex with fewer radii and capture spiral turns, which have led some au-
thors to conclude that the web is rudimentary in nature and does not play any
role in prey capture (Eckert & Moritz, 1992; Smithers, 1996). This has been
refuted by Fritzen & Kaponen (2011), who observed a M. menardi catching
a mosquito that flew into its web. In the Japanese cave spider Meta japon-
ica, prey was also caught when intercepted by the web in similar manner
to epigean orb spiders (Yoshida & Shinkai, 1993). A comparative quantita-
tive study (Simonsen & Hesselberg, data not shown) on the web geometry of
M. menardi and epigean tetragnathids demonstrated that while M. menardi
webs have fewer radii (mean ±SE: 14.9 ±3.1, N=23) than Metellina
mengei (mean ±SE: 18.6 ±2.9, N=29) (Figure 3), their webs have almost
the same number of radii as the webs of another common northern Euro-
pean epigean tetragnathid; Tetragnatha montana (mean ±SE: 15.9 ±2.3,
N=37). M. menardi webs did have fewer capture spiral turns (mean ±SE:
6.1 ±2.0, N =23) than both M. mengei (mean ±SE: 15.0 ±3.6, N=29)
and T. montana webs (mean ±SE: 14.0 ±3.1, N =38). This is, however,
in contrast to the Asian cave orb spider Meta japonica, which built larger
webs but with a similar number of radii (mean ±SE: Number of radii: 17.0
±4.3, N=47 and number of capture spirals: 14.6 ±5.3, N=42) (Yoshida
& Shinkai, 1993).
Thus, while the overall web geometry of Meta is probably not an adap-
tation to the subterranean habitat, the omission of frame threads is highly
unusual. The terrestrial tetragnathids, M. mengei and T. montana had an al-
most 1:1 ratio between the number of frame threads and the number of radii,
while M. menardi and M. japonica in contrast had ratios of 1:10 and 1:15,
respectively (Yoshida & Shinkai, 1993; Simonsen and Hesselberg, data not
shown). Most radii attach directly to the cave wall or ceiling, rather than
to frame threads with the web then attaching to the substrate with a few
long anchor threads as is usually the case in orb spiders (Zschokke, 2000;
Foelix, 2011; Hesselberg, 2013). As discussed in the next section, this might
be an adaptation to capture walking prey using the radii as trip wires to
alert the spider of passing prey. However, most Meta webs are built across
small crevices in the cave wall or ceiling with the capture spiral rarely being
far from the substrate, so it is possible that spiders face spatial constraints
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 979
when constructing their webs. Orb spiders generally adjust to spatial con-
straints in the laboratory by changing the shape of their webs and reduce
the spacing between spiral turns, but otherwise keep the web structure in-
tact (Ades, 1986; Vollrath et al., 1997; Krink & Vollrath, 2000). However,
the araneid Eustala illicita sometimes appeared to attach radii directly to the
frame wall when building webs in narrow, elongated vertical frames in the
laboratory (Hesselberg, 2013). Similarly, the tetragnathid Leucauge argyra
was observed to occasionally attach radii directly to small-diameter cylin-
drical tubes (Barrantes & Eberhard, 2012). It is therefore possible that Meta
spiders are just showing extreme behavioural flexibility to spatial constraints
rather than showing specific adaptations to the cave environment. This is sup-
ported by observations that when M. menardi did build frame threads, these
were constructed in the parts of the web that was furthest away from the
cave wall (D. Simonsen, Pers. Obs.); potentially suggesting that the frame
building behaviour occurs when M. menardi are released from the spatial
constraints of the small crevice. None of the epigean spiders tested in the
laboratory, however, showed the extreme reduction in frame threads seen
in Meta spiders, so it remains most likely that this is an adaptation rather
than behavioural flexibility. Manipulative laboratory experiments of Meta
building in environments of varying spatial complexity, following the guide-
lines given by Zschokke & Herberstein (2005), are needed to conclusively
answer this question. Such experiments might also shed light on the web-
building behaviour of Meta, which has not been described so far. Given that
both araneid and tetragnathid orb spiders construct their frame threads at the
same time as they construct the radii (Eberhard, 1990a; Zschokke & Voll-
rath, 1995; Foelix, 2011), it would be interesting to see how Meta spiders
have adapted the otherwise stereotypic building sequence to eliminate frame
thread construction. They could also reveal what potential factors are in-
volved when Meta spiders do build frame threads, particularly whether this
behaviour is influenced by the environment.
4. Foraging behaviour
The main function of the orb web is to intercept and retain flying insects
(Eberhard, 1990b). To do so, the impact energy is dissipated through de-
formations of primarily the radii and in most cases through aerodynamic
damping on the whole vibrating web (Lin et al., 1995; Sensenig et al., 2012;
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980 Behavioural adaptations in cave orb spiders
Zaera et al., 2014), while the glue droplets on the capture spiral and its re-
markable extensibility, retain the prey in the web long enough for the spider
to run out and catch it (Nentwig, 1982; Swanson et al., 2007; Sahni et al.,
2010). During prey interceptions, the spider relies on vibrational informa-
tion relayed through the radii to give it information on prey location and
prey size (Klärner & Barth, 1982), before rushing out from the hub (or via
a signal thread from its retreat) towards the prey (Hesselberg & Vollrath,
2006; Turner et al., 2011). Depending on the size and defensive capabil-
ities of the prey, the spider either directly bites the prey (small, harmless
prey) or first wraps the prey in additional silk before approaching for the
bite (larger, potentially dangerous prey) (Robinson et al., 1969; Hénaut et al.,
2001, 2014). Similar, behavioural flexibility was shown by the Japanese cave
spider Meta japonica in response to dipterans and ants being thrown into the
web (Yoshida & Shinkai, 1993). It has though been claimed that Meta orb
webs are rudimentary and do not play a role in prey capture and that spi-
ders instead engage in off-web foraging to capture prey such as snails and
myriapods, which crawl on the cave wall (Eckert & Moritz, 1992; Smithers,
1996).
The Meta orb web, however, clearly can function as a trap as the Japanese
cave orb spider example above shows (see also Fritzén & Koponen, 2011).
Studies of the prey actually consumed by spiders find that while dipterans
(especially mosquitoes, gnats and crane flies), as in other orb spiders, is a
major prey type (accounting for between 7 and 39% of all prey), between
36 and 69% of prey does not fly including snails, arachnids and myriapods
(Table 2). This value may be even higher as some of the prey classified as
flying in Table 2, spend more time resting or walking on the cave wall than
flying (L. Knight, pers. obs.). It is unlikely that prey would fall from the
cave ceiling into the web in such numbers, so the Meta spiders must engage
in unusual foraging behaviour. However, we have been unable to find any
published recordings or observations of actual off-web hunting, or of spiders
with prey not on an orb web. This is of course not evidence that the behaviour
does not take place. It could be rare and short in duration, and as M. menardi
has been found to be more active at night (Mammola & Isaia, 2018), off-
web foraging might take place at times when cave biologists are unlikely
to be there to observe it. If it does take place, it would constitute a very
interesting departure from normal foraging behaviour in orb spiders as the
standard prey capture behaviour described above would not work outside
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 981
Table 2 .
Prey consumption of the cave orb spider Meta menardi.
Prey Smithers (2005a)aNovak et al. (2010)bC. Terrell-Nield (unpubl)c
Location Mine, UK Caves, Slovenia Caves, UK
Flying
Dipteraa7% 30% 39%
Caddisflies 16% 0% 10%
Hymenopterans 0% 0% 0%
Lepidoptera 2% 0% 15%
Otherb6% 1% 0%
Total flying 31% 31% 64%
Non-flying
Slugs and snails 19% 0% 1%
Myriapodsc36% 11% 20%
Isoptera 0% 5% 2%
Arachnidsd7% 43% 5%
Beetles 7% 10% 8%
Total non-flying 69% 69% 36%
Diptera mainly includes fungus gnats and mosquitoes; other flying prey includes
lacewings, crickets and unidentified remains; Myriapods mainly includes millipedes; and
Arachnids mainly includes spiders and harvestmen.
aBased on observations of prey in webs over a year (prey N=67, note there is a mistake
in the total number of prey and hence of the relative frequencies given in Table 1 of Smithers,
2005a).
bBased on observations of prey in webs from 1977 to 2002. Relative frequencies were
reported, so to estimate percentage, the following number of prey (mean value of band) was
assumed for each of the study’s frequency bands: 1 =1prey,2=6prey,3=20 prey, 4 =
40 prey and this was then expressed as a percentage (total prey N=156).
cBased on a combination of direct observations of prey in webs and analysis of prey
remains dropped from the web and intercepted by nets placed underneath the web in autumn.
of a web. Instead Meta spiders would have to display the more ancestral
stalking or sit-and-wait behaviour found in web-less spiders (Foelix, 2011).
However, there are examples of tetragnathid species not building webs as
adults including the aphid specialist Pachygnatha degeeri (Madsen et al.,
2004; Harwood et al., 2005) and others in the same genus (Alvarez-Padilla
& Hormiga, 2011), but nothing is known about their detailed prey capture
behaviour. They are though often found wandering on the ground and are
frequently captured in pitfall traps in agricultural land (Meek et al., 2002;
Alvarez-Padilla & Hormiga, 2011), so presumably actively search for prey.
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982 Behavioural adaptations in cave orb spiders
An alternative explanation to off-web foraging relates to the behavioural
adaptations of the web itself as discussed in the previous section. The radii
that directly attach to the cave wall could act as trip lines alerting the spi-
der to passing prey that it can then rush out along the radii to attack. This
would be similar to the foraging behaviour of some trapdoor spiders includ-
ing Nemesia meridionalis (Family Nemesiidae) and Idiosoma rhaphiduca
(Family Idiopodae) that construct signal or trip lines radiating out from the
opening of their burrow (Buchli, 1969). When an invertebrate triggers one of
the lines by touching it, the ensuing vibrations cause the spider to rush out of
its burrow and seize the prey. A similar use of the radii as signal or trip lines
may be a more likely explanation for prey capturing in Meta spiders than
off-web foraging. It explains why no observations of the behaviour exist and
partly explains the elimination of frame threads as their higher pre-stress may
lower the vibrational transmission strength going along the radii to the hub
through leaking, as is found with the higher pre-stressed non-sticky spirals
in Nephila webs (Wirth & Barth, 1992; Landolfa & Barth, 1996; Mortimer
et al., 2016). In addition, this prey capture strategy would also retain many
of the normal orb spider prey capture behaviours (i.e. respond and reorient
to vibrational information from radii and rush out along the radii towards the
prey), although behavioural modifications to allow the spider to attack prey
not caught up in a web are still required.
5. Life cycle, dispersal and population genetics
The cave orb spiders have a complex and unusual life cycle in that they have
a mandatory hypogean and a mandatory epigean phase (Smithers, 2005b;
Mammola & Isaia, 2014) (Figure 4). Cocoons are laid relatively close to
the cave entrance, in locations experiencing moderate winds, to facilitate
the dispersal phase and possibly to wind dry the cocoons to avoid exces-
sive wetting (Mammola & Isaia, 2014; Chiavazzo et al., 2015). The cocoons
have a characteristic tear drop shape with a length of about 30 mm and
hang from the cave ceiling with a 20 mm long connecting thread (Figure 4).
Each cocoon contains about 250 eggs (Eckert & Moritz, 1992). After several
months, the spiderlings hatch and moult into the 2nd instar before emerging
from the cocoon. The spiderlings look very different from the adults with
a contrasting white and a dark pattern on the abdomen (Pennington, 1979)
(Figure 5). They show positive phototaxis, but remain in tight clusters in the
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 983
Figure 4. The life-cycle of Meta menardi and Meta bourneti. From Mammola & Isaia (2014)
with permission from Stefano Mammola.
cave entrance zone probably awaiting optimal epigean environmental con-
ditions before dispersing from the cave (McIndoo, 1910; Smithers, 2005b).
M. menardi spiderlings then spend 1 to 2 months outside, catching prey in
orb webs alongside M. merianae and other terrestrial tetragnathids (Penning-
ton, 1979), where they moult once or twice before returning to the cave as
subadults. The exact timing of this varies, but in the temperate M. menardi,
cocoons are usually laid in early summer, hatch in early autumn with the spi-
derlings leaving the subterranean habitat in early spring (Smithers, 2005b),
before returning in the summer. Inside the cave, the subadults undergo a
number of further moults before mating. The males probably die relatively
soon after mating, although it is not known if they mate once or multiple
times with different females. The female matures her eggs and constructs the
cocoon, which she guards until her death 2–3 months later (Eckert & Moritz,
1992).
According to some studies, after leaving the subterranean habitat, M.
menardi spiderlings disperse by ballooning (Smithers, 2005b; Novak et al.,
2010; Mammola & Isaia, 2014), although this has not been directly observed.
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984 Behavioural adaptations in cave orb spiders
Figure 5. 2nd instar spiderling of Meta menardi. Photo by Didier Petot.
However, juveniles of many species of spiders engage in ballooning to dis-
perse away from siblings and to find new suitable habitats (Foelix, 2011).
Ballooning involves the juveniles taking up a specialised tip-toe stance and
releasing silk into the air using cues from ambient wind speed and atmo-
spheric electric fields (Weyman, 1993; Morley & Robert, 2018). Once the
silk thread is long enough, wind-induced drag will lift the spider up into the
air and carry it significant distances. Spiders have been reported at an alti-
tude of several kilometres and spiders have been found on islands hundreds
of kilometres from the mainland (Bell et al., 2005). Ballooning spiders have
no control over the direction or length of their dispersal and might land in
unsuitable or lethal habitats (Bell et al., 2005). Ballooning is thus a particu-
larly risky strategy for spiders with very narrow habitat requirements such as
cave spiders. Given the scattered and relatively sparse occurrence of caves
and other large suitable subterranean habitats, how do juvenile Meta spi-
ders find their way back into caves? One solution might be that juveniles
do not necessarily have to return to caves. M. menardi adults have report-
edly been found in smaller crevices in scree, rocks and possibly hollow trees
as well as a range of smaller man-made habitats (Penney & Ramsey, 1994;
Smithers, 2005b; Fritzen & Kaponen, 2011; R˚užiˇ
cka et al., 2013). Although
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 985
such smaller and potentially much more numerous micro-habitats may prove
suitable for the development of juveniles into adults, it is unlikely they would
allow for reproduction as this would require both a male and a female find-
ing and co-inhabiting these small spaces. Thus natural selection is likely to
act against juveniles settling in isolated small dark spaces. Another solu-
tion might be that juveniles randomly search for suitable habitats through a
combination of walking and ballooning. Some researchers have speculated
that the diurnal and seasonal air ventilation in larger caves may allow juve-
niles to detect the presence of caves (De Freitas et al., 1982; Lunghi et al.,
2014), although it is difficult to imagine that this airflow would be detectable
from any significant distance by small spiders moving on the ground or in
vegetation. It remains a (remote) possibility that this air turnover might suck
nearby ballooning spiders into caves, which could potentially be investigated
with aerial traps near cave entrances. In any case random searching is likely
to have a very low chance of success with a resultant abnormally high spi-
derling mortality and thus require a large production of juveniles. However,
Meta spiders do not lay cocoons with more eggs than other orb spiders of
comparable size. One study found that M. menardi cocoons have 254 ±82
eggs (Eckert & Moritz, 1992), while another reported between 300 and 400
eggs per cocoon (Smithers, 2005b). In comparison, one cocoon of Argiope
bruennichi had 250 overwintering spiderlings (Bergthaler, 1995) while Ar-
giope aurantia cocoons contain between 300 and 1400 eggs (Hieber, 1992).
The most likely answer is therefore that only a small percentage of the ju-
veniles disperse by ballooning with the majority remaining in the vicinity
of their natal cave and returning to it after their terrestrial phase (possibly
with some never leaving the cave in the first place). The evolution of the
terrestrial phase could still be explained by the occasional successful bal-
looning and by the scarcity of suitable small prey items in the cave habitat
(Smithers, 2005a, b). The lack of whole-scale ballooning is further supported
by the fact that juveniles in webs have only ever been recorded in the imme-
diate vicinity of suitable subterranean habitats (Fritzen & Kaponen, 2011),
although this could perhaps be explained by sample bias. It is even possible
that M. bourneti from Mediterranean karst environments do not balloon and
possibly never leave the cave environment, as the very arid environment out-
side the caves is extremely hostile for the spiderlings (C. Ribera, personal
communication).
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986 Behavioural adaptations in cave orb spiders
It is clear that much more research is required into the dispersal phase of
Meta spiders in order to evaluate any of the hypotheses mentioned above.
However, as juveniles are far too small to carry tracking tags, it remains dif-
ficult to directly monitor their movements out of and back in to caves. More
indirect methods such as population genetics studies might shed some light
on the degree of dispersal (e.g., through gene flow; Slatkin, 1987) between
spider populations in neighbouring caves and those further afield. Molecu-
lar markers have been used in many arthropod species, including spiders,
to deduce effective population sizes and/or population structure (Moya et
al., 2004; Vandergast et al., 2004; Flagel et al., 2014; Hjalmarsson et al.,
2015; Sappington, 2018; Schwentner & Giribet, 2018). However, few studies
have examined genetic differentiation among orb-weaving spiders (Lee et al.,
2004; Kuntner & Agnarsson, 2011a,b). Troglobiont cave taxa usually occupy
narrow distributions and usually exhibit poor dispersal abilities (Trontelj et
al., 2009, Juan et al., 2010 and references therein), although the physical
characteristics of subterranean habitats may condition the extent of gene
flow (Rizzo et al., 2017). Troglophile and trogloxene species, in particular,
should in theory exhibit higher levels of gene flow than troglobionts, albeit
this is contingent on the ecological requirements and the geographic/cli-
matic characteristics of each species (Caccone, 1985). Allele frequencies
and DNA sequence differences among samples collected at different geo-
graphic scales are used to estimate gene flow levels to explain the observed
patterns. Pairwise FST distances are typically used to quantify differentiation
between sampled populations (Slatkin, 1987) along with descriptive popula-
tion parameters such as haplotype (h) and nucleotide (π) diversity values, the
latter defined as the average number of pairwise differences between DNA
sequences (Nei & Lil, 1979). The fast nucleotide substitution rate of mi-
tochondrial DNA sequences and its maternal inheritance have made these
markers particularly popular for these estimations, allowing detection of re-
cent dispersal and vicariant events. Microsatellite DNA markers and, more
recently, modern DNA sequencing technologies (next-generation sequenc-
ing) can provide genome-wide markers able to be used in phylogeography
and population studies (Eaton & Ree, 2013). In particular, techniques able
to scan a reduced representation of the genome, such as restriction-site-
associated DNA sequencing (RADseq) (Miller et al., 2007), genotyping-by-
sequencing (Elshire et al., 2011) along with several modifications of the two
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 987
original techniques are the most cost-effective approaches (Eaton & Ree,
2013; Andrews et al., 2016).
Different population structure patterns would be expected depending on
the dispersal mechanisms. A dispersal predominantly by ballooning should
render a homogenous genetic structure, even at considerable geographical
distances, so that low FST, haplotype and nucleotide diversity values would
be expected for both nuclear and mitochondrial markers. In contrast, a cer-
tain amount of population structure may be expected (very low differentia-
tion within cave sites but higher between caves from different regions with
isolation-by-distance) if most juveniles return to the natal caves or they es-
sentially remain in their own caves for their entire life-cycle. A sex-biased
dispersal model with sedentary females and dispersive males is common in
orb spiders (Foellmer & Fairbairn, 2005; Kasumovic et al., 2007), but there
is no evidence of males dispersing out of caves in Meta (Rector, 2009). How-
ever, such a behaviour could be detected by molecular ecology methods, as it
would give contrasting patterns for mitochondrial and nuclear markers, with
the former showing reduced gene flow compared to the latter (Miles et al.,
2018).
6. Conclusion
The interest in using the subterranean environments and its inhabitants as
model systems for studies in ecology and evolutionary biology has been
growing in recent years (Culver & Pipan, 2009; Juan et al., 2010; Juan &
Emerson, 2010; Trontelj et al., 2012; Pipan & Culver, 2013; Mammola,
2018). This is also true for spiders, where a large number of studies from
Central and Southern European subterranean systems have been published
(see, for example, R˚užiˇ
cka et al., 2013; Manenti et al., 2015; Mammola et
al., 2017a; Mammola & Isaia, 2017). As some of the largest and most con-
spicuous spiders in caves, Meta spiders have received considerable attention.
However, almost all studies focus on ecology, biodiversity and biogeography,
but as we have demonstrated in this review, there is a large untapped potential
for using Meta as a model organism in behavioural studies as well. Although
more quantitative research is needed especially on the non-European Meta,
we found good evidence for cave orb spiders showing a number of specific
behavioural adaptations to subterranean life. These include an unusual life-
history, a somewhat modified orb web and potentially a novel prey capture
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988 Behavioural adaptations in cave orb spiders
strategy involving both on- and off-web foraging. To fully explore the de-
scribed behaviour, a truly interdisciplinary approach is needed that combines
field studies in situ in subterranean habitats with molecular biological tech-
niques and carefully controlled laboratory studies.
To determine the degree to which these behavioural adaptations are unique
to cave spiders or alternatively are extensions of the natural behavioural flex-
ibility of orb spiders, more comparative studies between the hypogean Meta
spiders and closely related epigean species. A suitable candidate is Metellina
mengei with recent studies shedding light on its mating behaviour (Prenter et
al., 1994; Bridge et al., 2000), web structure and behavioural flexibility (Tew
& Hesselberg, 2017, 2018), and foraging behaviour (Richards and Hessel-
berg, data not shown). Even more interesting would be comparisons between
hypogean and epigean Meta spiders with at least one species, Meta stridu-
lans, confirmed to be exclusively epigean as it inhabits the laurel forests
of Madeira, Portugal, although almost nothing is currently known about its
behaviour (Crespo et al., 2014). Obtaining a more detailed understanding
of the behaviour and biology of Meta spiders may also shed light on the
wider ecological relationship and dynamics of subterranean habitats, since
they can potentially be considered an indicator species due to their relative
abundance and role as top predators in the twilight zone. Although caves
are to some extent shielded from the anthropogenic threats facing many
terrestrial habitats, the subterranean habitat is intrinsically linked to what
is happening aboveground. A recent study, for example, highlights the ex-
tinction risks facing cave spiders from global warming, which, with up to
decades delay, also results in warmer subterranean temperatures (Mammola
et al., 2017b). Similarly, pesticides, heavy metals and organic pollution can
enter the subterranean environment through occasional trogloxene visitors
and underground streams (Wood et al., 2008; Medellin et al., 2017; Modrá et
al., 2018). Epigean spiders are known to modify their behaviour and orb web
geometry in response to pesticides (Samu & Vollrath, 1992; Benamú et al.,
2013; Pasquet et al., 2016) and pesticide and heavy metal pollution can be
determined from absorbed residues in spider silk (Samu et al., 1992; Xiao-Li
et al., 2006; Tahir et al., 2018). Thus, it is an intriguing possibility that Meta
webs and silks can be used as bioassays for assessing wider cave pollution
levels.
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T. Hesselberg et al. / Behaviour 156 (2019) 969–996 989
Acknowledgements
The authors would like to thank Lee Knight, Stefano Mammola and
Christopher Terrell-Nield for valuable discussions. Didier Petot and Richard
McMellon for permission to use their photographs, and Stefano Mammola
for permission to use his Meta life-cycle drawing. TH would also like to
thank the Department for Biology at the University of the Balearic Islands
for funding allowing him to visit.
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