Passive dispersal in arachnids

27 Pages·2008·2.19 MB·English

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PASSIVE DISPERSAL IN ARACHNIDS
BIOLOGICALLETT.2007,44(2):75(cid:150)101
Availableonlineathttp://www.biollett.amu.edu.pl
Passive dispersal in arachnids
PAWE£ SZYMKOWIAK1, GRZEGORZ G(cid:211)RSKI2 and DARIA BAJERLEIN1
1 Department of Animal Taxonomy and Ecology, Institute of Environmental Biology,
Faculty of Biology, Collegium Biologicum, Umultowska 89, 61-614 Poznaæ, Poland;
[email protected], [email protected]
2 Department of Comparative Anatomy, Institute of Zoology, Jagiellonian University, Ingardena 6,
30-060 Krak(cid:243)w, Poland; [email protected]
(Received on 17 December 2006; Revised on 5 December 2007;
Accepted on 17 December 2007)
Abstract: Some arachnids (spiders, mites and pseudoscorpions) are able to use both active and passive
dispersal. The best-known passive dispersal method in arachnids is called (cid:145)ballooning(cid:146) and starts with
(cid:145)tiptoe behaviour(cid:146). Using threads of silk, spiders can move from place to place with air currents. Usually
the spider aeronauts are small, but sometimes larger ones can also be transported in this way. Some
mites spread passively also by using threads of silk, or use their own body as a carrying surface.
A great number of mite species intentionally use other animals (e.g. insects) as carriers, and this
phenomenon is known as phoresy. Similarly, pseudoscorpions sometimes infest insects. The most
probable reasons that may cause passive dispersal are: overpopulation, changeable physical conditions,
and ecocatastrophe. Passive dispersal is one of the major ways of colonization of new areas and habitats,
which extends geographical ranges of species. The oldest traces of passive dispersal come from the
Tertiary period, but it is thought to have existed also in the late Mesozoic. From an evolutionary point
of view, the most interesting is the phoront-host relationship, as a result of co-evolution (e.g. Rhinoseius
mites with hummingbirds).
Keywords: passive dispersal, ballooning, phoresy, spiders, mites, pseudoscorpions
INTRODUCTION
It is difficult to provide accurate definitions of the basic terms connected with
the movement of animals, such as dispersal. The most popular one was suggested
by HOWARD (1960): (cid:145)The dispersal of a single vertebrate is the movement the ani-
mal makes from its birthplace to the area where it will breed, survive and find its
partner(cid:146). According to GREENWOOD (1980), immature specimens tend to wander to
their first breeding site, or potential first breeding site. Another form of dispersal is
called (cid:145)breeding dispersal(cid:146). This appears when an animal produces offspring between
one breeding site and another. If dispersal ends in successful(cid:160)breeding, it is called
(cid:145)effective dispersal(cid:146), and if not, it is then called (cid:145)gross dispersal(cid:146) (GREENWOOD 1980).
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P. Szymkowiak, G. G(cid:243)rski and D. Bajerlein
Differences between dispersal, movement and migration can be described as
follows:
(cid:150) migration: the periodic passage of groups of animals (especially birds or fish)
from one region to another for feeding or breeding;
(cid:150) movement: motion, i.e. the act of changing location from one place to an-
other;
(cid:150) dispersal: the permanent emigration of individuals from a population by
movement of an animal away from its previous home range. The term (cid:145)dispersal(cid:146)
often refers to the movement of a young animal away from the home range where
it was born when it matures. In population ecology, it is the movement of individ-
ual organisms towards different localities. In biogeography, it is the extension of
the geographic range of a species by movement of individuals (on the basis of var-
ious glossaries).
There are several reasons for animals to move from one place to another. These
can be physiological or environmental factors (e.g. a decline in living conditions or
reaction to photoperiod). There are also intrapopulation factors, such as: predatory
pressure, competition for food, and seeking for a mate.
Passive and active dispersal differ considerably in energetic costs, patch choice,
control of flight parameters, etc. Although there is a lot of data concerning the rea-
sons and ways of dispersal of mammals or birds, the knowledge about passive dis-
persal mechanisms of small-sized invertebrates is rather poor. That is why in this
article we attempted to summarize the current knowledge about such mechanisms in
arachnids.
AERIAL DISPERSAL
Arachnids do not have any organs that would enable them to move up active-
ly into the air and to cover long distances. Spiders have developed passive aerial
dispersal called (cid:145)ballooning(cid:146) (BRISTOWE 1929, 1939, BRAENDEGAARD 1937, PRESTON-
MAFHAM & PRESTON-MAFHAM 1984), but among mites aerial dispersal is not
so common. Both groups of arachnids disperse in the same way, but the term (cid:145)bal-
looning(cid:146) should be reserved to spiders (EVANS 1992).
Preparation to the flight
The stimulus that is of great importance for initiation and continuation of pre-
flight behaviour is a blast of wind. Duration of time spent on attempting to take off
is diverse. Spiders usually continue pre-ballooning behaviour even when the stimu-
lus is removed (FAP = fixed action pattern), and time to cessation of this behaviour
is up to several hours (WEYMAN 1995). Usually spiders climb up to the nearest place
exposed to winds (e.g. tree branches, blades of grasses, or fence). Then, standing on
its (cid:145)toes(cid:146), with legs stretched to the maximum, the spider raises the body; this is
called (cid:145)tiptoe behaviour(cid:146) (BRISTOWE 1939) (Fig. 1). The spider releases then a thread
of silk from spinnerets, which is next carried by air currents and, if the wind blows
strong enough to carry the weight of the small animal, the spider rises into the air.
So-called bridge threads are also used for horizontal movement over short distances
as a walking line, without becoming airborne (TOFT 1995). Some scientists expressed
doubts about the spider(cid:146)s ability to spin silk actively, because of the lack of muscles
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PASSIVE DISPERSAL IN ARACHNIDS
around the spinnerets that would enable them to secrete silk out (FOELIX 1996,
DUFFEY 1998). DUFFEY (1998) reports on the observation of sudden movements of
the posterior spinnerets, helping to spin silk from the middle spinnerets. According
to FOELIX (1996), spiders can control the amount of expelled silk by valves situated
in front of the spigots (termination of spinnerets). Spiders usually spin simultane-
ously 6(cid:150)10 threads (DUFFEY 1998).
Fig. 1. Juvenile lycosid spider revealing a tip-toe behaviour, Antelope Island Causeway, Davis County,
Utah, USA. Photo courtesy of NICKY DAVIS
Most mites dispersing via air currents belong to the family Tetranychidae.
Panonychus citri (McGregor, 1919) and Oligonychus ununguis (Jacobi, 1905)
(Prostigmata) produce in unicellular glands, placed at pedipalps, a silk thread that is
used for getting down from high places, e.g. leaves. The threads are attached to
something on the ground, and a gust of wind carries them up. Adult females of
Tetranychus urticae Koch, 1835 (Prostigmata), seldom immature specimens, and only
rarely mature males, strike a pose to be carried off the ground. They stand toward
the wind, raise their front legs, and fly away when the wind is strong enough. Those
standing in a normal position are not raised by the wind. Eriophyoid mites, mainly
older females, stand in an upright position and leaning upon their anal lobe and caudal
seta, wave their front legs trying to increase their exposure to the wind (EVANS 1992).
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P. Szymkowiak, G. G(cid:243)rski and D. Bajerlein
Similar behaviour was found in a much larger predatory mite, Amblyseius fallacis
(Garman, 1948) (Mesostigmata: Phytoseiidae). In the laboratory, when the wind
reached 0.45(cid:160)m/s, females before and during the breeding period stopped usual
movements and gathered at the edge of the experimental platform. Without moving
their front legs, they raised them together with another leg pair and the front part of
the body. Although phytoseiid mites are known to disperse in a passive way, there
is no certainty of the function of taking-off behaviour (SABELIS & AFMAN 1994).
Methods of taking off
BRAENDEGAARD (1937) described two methods, which spiders use to rise. In
the first method, the thread is anchored to the substrate and the spider lengthens it
until it creates a sufficient carrying surface. Then the wind breaks the thread and
lets the animal rise into the air. Other spiders go down the thread, swaying from
side to side, up and down, until it breaks. This phenomenon is called drop-and-swing
dispersal and characterizes juvenile forms, which can travel only over a rather short
distance (BARTH et al. 1991, BARTH 2002). BARTH (2002) found that more than
70% of 9-day-old spiders of Cupiennius show such behaviour. Among others, young
spiders from the family Ctenizidae use the latter method. They go down the thread,
e.g. from higher twigs, and the wind places them horizontally, tightening the thick
drag line of 0.5(cid:150)1.0 mm until the wind breaks it and sweeps away the spider (HAR-
VEY 1994). The fact that there are many threads of various length (20(cid:150)25 cm) at-
tached to blades of grass, suggests that the point in which the thread breaks can be
in most cases within this range.
Conditions of the flight
During the flight the spider walks on, and stretches the thread, trying to con-
trol the parameters of flight (GERTSCH 1979). Landing technique is not complicat-
ed: the spider coils the thread or cuts it off, falling onto the ground (MIKULSKA
1953). Nevertheless, according to THOMAS (1996), it is unclear whether spiders do
actively cut their silk, and even if they do, then what kind of stimulus (e.g. time
spent airborne or a visual stimulus) makes them do it. Such a mechanism enables
spiders to cover considerable distances. GRESSIT & YOSHIMOTO (1963) explained
the origin of Polynesian fauna by long-distance dispersal. CARLQUIST (1965) esti-
mates that they can cover a distance of 3,000 km, but dispersal over such long dis-
tance seems unlikely. Doing so, spiders would have to fly at a height of 8 km above
the poles and 15 km above the equator. Dispersing by air currents when the temper-
ature drops to (cid:150)60(cid:176)C seems to be lethal for such small spiders (DECAE 1987). Charles
Darwin observed flying spiders attached to the rig when the ship (cid:145)Beagle(cid:146) was
100 km off the South American coast, and they were often seen at a distance of 300
km off the same coast (DECAE 1987). OKUMA & KISIMOTO (1981) collected airborne
spiders 400 km offshore from the mainland of China. The majority of specimens were
alive and they were identified as Tetragnatha. BRIGNOLI (1983), on the basis of the
observation on Linyphiidae, claims that despite the great potential of ballooning, it
does not extend the geographic range of the species because of ecological and histor-
ical constraints. In spite of the fact that spiders cover long distances, effective and
intensive migration takes place on short ones (2(cid:150)5 km) (THOMAS 1996, THORBEK et
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PASSIVE DISPERSAL IN ARACHNIDS
al. 2002). Spiders(cid:146) flights usually take place at a height of 6(cid:150)15 m, but they have
also been caught about 3,300 m above sea level (PRESTON-MAFHAM & PRESTON-
MAFHAM 1984). HINGSTON (1920) found spiders from the family Salticidae on Mount
Everest, at an altitude of almost 6,600 m. Probably the spiders reached high moun-
tains by vertical ballooning, and they originated more from the local fauna than from
distant places, although otherwise they can be a stable element of the poorly studied
spider fauna of the high Himalayas.
The most favourable conditions for aerial dispersal are during calm, sunny and
warm days following cool nights, when rising air currents necessary for successful
flight appear. No influence of circadian rhythm and darkness on this phenomenon
was observed (WEYMAN 1995). Gusts of wind blowing at a speed of over 3(cid:160)m/s
will decrease the number of flying spiders (VUGTS & WINGERDEN 1976, WINGER-
DEN & VUGTS 1979, GREENSTONE 1990), but 5(cid:160)m/s disables the taking off (BONTE
& MAELFAIT 2001). Stegodyphus mimosarum Pavesi, 1883 becomes airborne when
the wind blows horizontally 0.1(cid:150)3 m/s, so HENSCHEL et al. (1995) estimated the
length of span thread (12(cid:150)665 m) as required to raise.
Colonization of new areas
Spiders play a significant role in colonizing new areas. They are often the first
colonizers of arising or erupting islands. After volcanic eruptions, usually the fauna
inhabiting the neighbouring islands (re)colonizes such islands. Sumatra and Java were
the main sources of colonization after a volcanic eruption on Krakatau (flABKA &
NENTWIG 2002). The first invertebrate found after Krakatau eruption was a spider
(NEW & THORNTON 1992). Spider families, such as Lycosidae, Salticidae and Tetrag-
nathidae, included many of the pioneers in colonization of the island (BRISTOWE
1931, NEW & THORNTON 1988, flABKA & NENTWIG 2002). Very often, the first
colonizers are widespread species. EDWARDS & THORNTON (2001) listed Trochosa
papakula (Strand, 1911), Tetragnatha ceylonica O. P.-Cambridge, 1869, Tetragnatha
mandibulata Walckenaer, 1842, and Tetragnatha nitens (Audouin, 1826) as pioneer
spiders of Momot (an emergent island). Various groups of arthropods, including
spiders, were first colonizers of lava flows before macroscopic plants appeared.
A native wolf spider (Lycosa sp.) was one of the first colonizers of the very recent,
unvegetated lava flows on Hawaii Island (HOWARTH 1979). Although it is some-
times difficult to define if species colonizing an island are new early colonizers
or recolonizers, in most cases spiders are numerous and rich in species among
colonizers (NEW & THORNTON 1992, EDWARDS & THORNTON 2001, flABKA &
NENTWIG 2002).
Distribution of spiders in Polar Regions
The biota of the Arctic, Antarctic and Sub-Antarctic areas are very poor. Spi-
ders are not very numerous there, because of severe climate, scarce food resources,
and lack of typical living habitats (GRESSITT 1970). MARUSIK & KOPONEN (2002)
recorded 70 spider species occurring in Greenland, and only 14 species in Svalbard.
Spiders that occur in the coldest areas of the Southern Hemisphere are also less
numerous (GRESSITT 1970, 1971, FORSTER 1970, 1971). Some species can colonize
these areas via air currents (e.g. Erigone autumnalis Emerton, 1882) (FORSTER 1971),
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P. Szymkowiak, G. G(cid:243)rski and D. Bajerlein
by bird transportation, or by floating in logs, but there is doubt if the indigenous
spider fauna of many Sub-Antarctic islands originated in such a way. In fact, they
may be relics of former landmasses. Nevertheless, the most important types of dis-
persal on Sub-Antarctic islands are: by air, sea and birds (GRESSITT 1970). FOR-
STER (1970) reported that the spiders of Arctic islands are often endemic. That
author, on the basis of available collections of spiders from South Georgia Island,
identified 4 endemic species of spiders representing the Micryphantidae. Indigenous
spiders of Antarctica perhaps do not exist, though FORSTER (1970) studied a speci-
men of an unknown spider from Terra Nova Bay, Antarctica and decided to assign
it to the Micryphantidae. He stated that the specimen seems to be a remnant of a
windborne spider or exuviae. Most spiders inhabiting Polar Regions are species in-
troduced by man. Good examples are spiders recorded on South Shetland Island (S`IZ
et al. 1970, ROUNSEVELL 1978).
Which spiders do balloon?
There is no clear-cut answer to the question of whether young or mature spi-
ders (cid:145)fly(cid:146) more often (WEYMAN et al. 1995, BONTE et al. 1998), or whether males
or females do it more frequently (MIKULSKA 1953, GREENSTONE et al. 1987, WEY-
MAN et al. 1995, THOMAS & JEPSON 1999, WEYMAN et al. 2002). Observations of
GREENSTONE et al. (1987) on the flights of spiders in Missouri (USA) and Tragne
(New South Wales, Australia) showed that most of specimens caught in the air were
young spiders. Linyphiidae was the dominant family (44.9% in Missouri and 64.2%
in Tragne). Araneidae, Oxyopidae, Thomisidae, and Tetragnathidae constitute 3(cid:150)18%,
while Lycosidae account for 3(cid:150)5%, and Salticidae, Philodromidae, Clubionidae, Gna-
phosidae, Pisauridae, and Mimetidae for 0.1(cid:150)1.8% of all air-dispersed spiders
(GREENSTONE et al. 1987). Similar results were obtained by KAJAK (1959), GREEN-
STONE et al. (1985, 1991), TOPPING & SUNDERLAND (1995), and PEARCE et al.
(2005). Our long-term study (1992(cid:150)1998) on spiders of the Baltic Sea beaches
(Ko‡obrzeg, Poland) revealed a high proportion of tiny spiders among spider aero-
nauts. There were mainly linyphiids among ballooning spiders: Bathyphantes graci-
lis (Blackwall, 1841), Erigone atra Blackwall, 1833, Gnathonarium dentatum (Wider,
1834), Porrhomma convexum (Westring, 1861), and Porrhomma pygmaeum (Black-
wall, 1834). Moreover, Pachygnatha degeeri Sundevall, 1830 was one of the most
abundant species among aeronauts. We found also some other species, which usual-
ly use other methods of moving and their appearance on the beach was unusual:
Araniella cucurbitina (Clerck, 1758), Clubiona subtilis L. Koch, 1867, Philodromus
fallax Sundevall, 1833, Robertus lividus (Blackwall, 1836), Segestria senoculata (Lin-
naeus, 1758) and Xysticus cristatus (Clerck, 1758) (SZYMKOWIAK unpubl. data). The
studies of BLANDENIER & F(cid:220)RST (1998) confirm that young spiders tend to fly more
often than mature ones, but adults of Linyphiidae, because of their small body size,
are especially numerous among the aeronauts (Fig. 2). The exceptional frequency
of Linyphiidae in aerial catches is due to their ability to disperse both as juveniles
and as adults, and to their high abundance in grasslands (cid:150) one of most suitable hab-
itats for ballooning (BELL et al. 2005). All instars of Erigone arctica (White, 1852)
balloon, but mostly third and fourth instars, so this suggests that a biotic factor may
be involved in stimulation of ballooning behaviour (WINGERDEN & VUGTS 1974).
This way of aerial dispersal is used not only by the juveniles but also by males and
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PASSIVE DISPERSAL IN ARACHNIDS
medium-sized females (FOELIX 1996). Sex ratio in aeronauts is an individual fea-
ture of each species. For example, mature females of Araeoncus humilis (Blackwall,
1841) accounted for 63%, Meioneta rurestris (C.L. Koch, 1836) for 58%, whereas
Porrhomma microphthalmum (O.P.-Cambridge, 1871) for only 49.5% of mature
specimens. The sex ratio may vary because of differences in dependence of both
sexes on food resources and microclimate, especially during mating, egg produc-
tion, etc. (THOMAS & JEPSON 1999). BLANDENIER & F(cid:220)RST (1998) recorded the
typical aeronauts: Araeoncus humilis, Erigone atra, Erigone dentipalpis (Wider,
1834), Lepthyphantes tenuis (Blackwall, 1852), Meioneta rurestris, Porrhomma mi-
crophthalmum (all from the family Linyphiidae), Pachygnatha degeeri (Tetragnath-
idae), Mangora acalypha (Walckenaer, 1802) (Araneidae), Robertus arundineti (O.P.-
Cambridge, 1871) (Theridiidae), and Philodromus rufus (Walckenaer, 1826)
(Philodromidae). In the most primitive spiders, Mesothelae, ballooning is absent be-
cause of lifestyle and lack of convection and wind speed gradient in their natural
habitat (BELL et al. 2005). Although many families use aerial dispersal, there is scarce
data concerning the aerial dispersal of the primitive spiders from the suborder Or-
thognatha. DECAE (1987) states that this may be caused by their large size, little use
of silk, and hiding lifestyle. However, an example comes from young Australian
spiders (Missulena spp.). Members of that genus were observed flying, which prob-
ably helped them spread over the Australian continent (PRESTON-MAFHAM & PRES-
TON-MAFHAM 1984). Other reports also suggest that spiderlings of Atypus piceus
(Sulzer, 1776) and Sphodros atlanticus Gertsch et Platnick, 1980 show a tendency
Fig. 2. Erigone sp. ballooning on the beach at Ertebolle, Jutland, Denmark. Photo courtesy of MORTEN
D.D. HANSEN
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P. Szymkowiak, G. G(cid:243)rski and D. Bajerlein
to migrate by air (KRAUS & BAUR 1974, GERTSCH 1979, COYLE 1983). In the Hap-
logynae, ballooning is rare, observed in a few families. It happens probably due to
unsuitable habitat conditions. Ballooning occurs within the Entelegynae, but is wide-
spread only amongst the higher Entelegynae and prevalent in a few families of this
group. Though aerial dispersal is attributed to species rather than families, Aranei-
dae, Lycosidae, Salticidae, Tetragnathidae and Thomisidae are the richest in airborne
species (SALMON & HORNER 1977, GREENSTONE et al. 1987, BELL et al. 2005).
Despite low ballooning frequency, also many species of Lycosidae are usually listed
as undertaking this kind of aerial dispersal, and lycosids are often used both for
laboratory and field experiments (RICHTER 1967, 1970, BELL et al. 2005).
Although the weight of aeronauts is usually between 0.2 and 1.0 mg, GREEN-
STONE et al. (1985) reveal that spiders of up to 25.5 mg are also capable of aerial
dispersal. SUTER (1999) suggests that larger spiders do not fly or do this relatively
seldom, because of a few factors: (1) loss of energy when climbing to a point suit-
able for rising; (2) higher risk of being spotted by predators; and (3) decreased
number of places suitable for rising.
Seasonal dispersal activity
Spiders fly all the year round in the temperate zone, but they tend to prefer
certain periods when their dispersal greatly increases (BLANDENIER & F(cid:220)RST 1998).
The same authors mention 3 distinctive periods: mid-June, late July to early
August, and September. The results obtained when studying the intensity of flights
depends on species, the place where research is carried out, and whether it can be
carried out at various seasons of the year. Meioneta rurestris was predominantly caught
from the end of July till mid-October, while DUFFEY (1956) claimed that the periods
when the spiders were most active continued from February to April. Another
species, Araeoncus humilis, has 2 dispersal periods, when it is very active: the first
from late May until late July and the second from mid-September to mid-Novem-
ber. Porhomma microphthalmum is highly active in July and early August, whereas
the second, less active period starts in October and ends in December. Aerial disper-
sal of Erigone atra is characterized by almost permanent activity throughout the year,
but lower from late May to late November (BLANDENIER & F(cid:220)RST 1998). Abun-
dance peaks of aeronauts and their terrestrial populations usually overlap, but spe-
cies compositions sometimes differ (WEYMAN et. al. 1995, PEARCE et. al. 2005).
Reasons for aerial dispersal
Increase in temperature and decrease in humidity have a visible impact on the
intensity of dispersal. However, this phenomenon does not disappear even if the tem-
perature falls to 0(cid:176)C (RICHTER 1970). Dispersal of juvenile spiders just after leav-
ing the cocoon enables to avoid not only food shortage but also the aggression of
other species. The study conducted by LEGEL & WINGERDEN (1980) on Erigone
arctica (White, 1852) (Linyphiidae) confirms this hypothesis. Quantity and quality
of food, feeding history, and temperature conditions during juvenile development
may also be a limiting factor (BONTE & MAELFAIT 2001, BONTE et al. 2003). In
artificial conditions, 62.5% of poorly-nourished spiders in comparison to 22.5% of
well-nourished ones, dispersed by air. DUFFEY (1998) and BARTH (2002) reveal that
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overcrowding is another reason for ballooning. Spiders raised individually were not
dispersed, while 41% of those kept in groups did so (DUFFEY 1998).
The reasons why mites use aerial dispersal are diverse, but similar to those
revealed in other arachnids. Predatory mites sometimes leave their habitat because
of the shortage of their potential prey. Tetranychus urticae (Prostigmata) starts to
disperse after sources of food have become deficient. This increases the sensitivity
of mites to light and enhances their activity. An essential influence on the intensity
of dispersal of Aculodes dubius (Nalepa, 1891) (Prostigmata) is exerted jointly by
temperature and wind speed (the best conditions are at a speed of over 25 km/h and
a temperature higher than 18(cid:176)C). In the laboratory, raising the temperature from
12(cid:176)C to 24(cid:176)C and high illumination causes an 8-fold increase in the number of
migrating Aceria tulipae (Keifer, 1938) (Prostigmata). Probably the phototaxis of
this mite depends on its physiological state. If the conditions are favourable, it shows
negative phototaxis, and when they get worse, phototaxis is positive (NAULT & STYER
1969).
Evolutionary approach
While many ancestral lineages of mites had the ability to produce silk, now-
adays only spider mites (Tetranychidae) have the potential to disperse by balloon-
ing. This is because of an adaptation to undertaking the risk associated with bal-
looning dispersal: a high percentage of tetranychid ballooners are mated females
capable of parthenogenesis. Moreover, spider mites are polyphagous and widely
distributed in many types of habitats.
There is no adaptive advantage between silkless dispersal and ballooning in
mites. Also the effectiveness of ballooning and silkless aerial dispersal is similar (BELL
et al. 2005).
Many observations of aerial dispersal of arachnids have been published, but
no evolutionary analysis of this behaviour has been done. Only recently, BELL et al.
(2005) suggested that ballooning might have appeared in the early Devonian period
and co-evolved with silk. Perhaps in the course of time, along with the habitat chang-
es, the phenomenon of tiptoe and take-off behaviours has evolved. The appearance
and consolidation of ballooning behaviour can be explained in the context of the
Mixed Evolutionary Stable Strategy.
PHORESY
The nature and origin of phoresy
The most common way of passive dispersal among arachnids is dispersal by
using other animals, which is called phoresy. This phenomenon is mainly observed
among mites. Except Ixodida, this phenomenon has developed independently in many
groups within Acarina: Prostigmata, Astigmata and Mesostigmata (CROSS & BOHART
1969, SHVANDEROV 1975, BINNS 1982, HUNTER & ROSARIO 1988, HOUCK &
O(cid:146)CONNOR 1991) and accidentally in Oribatida (NORTON 1980). Mite carriers belong
to various systematic groups, but insects are most popular among them, especially
members of Coleoptera, Diptera and Hymenoptera (HUNTER & ROSARIO 1988). In
a few recorded cases the carriers were crustaceans, myriapods, reptiles or mammals
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P. Szymkowiak, G. G(cid:243)rski and D. Bajerlein
(FAASCH 1967, DOMROW 1981, MIKO & STANKO 1991, RIGBY 1996, MERTINS &
HARTDEGEN 2003). Sometimes mites use other phoretic mites for dispersal, and this
phenomenon is called hyperphoresy (ATHIAS-BINCHE 1994). Phoresy was rarely
observed in pseudoscorpions. This group of arachnids disperses similarly to mites,
by using insects.
A wide range of relationships between passively dispersed arachnids and their
carriers can be observed: from neutral to parasitic. The former concerns Pseudopyg-
mephorus atypoides Rack, 1982 (Prostigmata), whose females were found on the
head of a female spider, Atypoides riversi O.P.-Cambridge, 1883. This mite species
is probably phoretically associated with one of the spider(cid:146)s potential prey species. A
mite may move onto a spider body to avoid mastication and digestion by its en-
zymes when the spider ingests a prey that is the mite(cid:146)s host. This does not necessar-
ily have to be a case of phoresy, because of a small percentage of spiders observed
as mite carriers (VINCENT & RACK 1982). The relationship of Pygmephorus costa-
ricanus Rack et Eickwort, 1979 (Prostigmata) with the Agapostemon nasutus Smith,
1853 (a bee, which builds underground nests) may serve as a perfect example of
commensalism.
FARISH & AXTELL (1971) defined phoresy as (cid:145)a phenomenon in which one
animal actively seeks out and attaches to the outer surface of another animal for a
limited time, during which the attached animal ceases both feeding and ontogenesis,
such attachment presumably resulting in dispersal from areas unsuited for further
development, either of the individual or its progeny(cid:146).
Settlement in new areas gives both adults and their offspring a chance for
successful colonization. Phoretic mites usually use the r-type life strategy. Fast de-
velopment and high reproduction allow an easy establishment of new colonies in
new areas, as well as settlement in ephemeral niches. The lack of phoretic mites in
plant monocultures and plant communities at the climax stage is a very characteris-
tic feature because of stability of environmental conditions (BINNS 1982, ATHIAS-
BINCHE 1984, EVANS 1992).
Frequency of migration episodes and host/phoront specificity played a major
role in the evolution of phoresy. For those reasons, the phenomenon became either
facultative or obligatory. In the case of seasonal obligatory migrations, host/phoront
specificity is not so high as in cyclic obligatory migration. Finally, phoresy may
evolve towards a diversity of trophic relationships between organisms, e.g. commen-
salism, mutualism, predation, or parasitism (ATHIAS-BINCHE 1994). Generally the
phoretic stage does not feed on its host, but it was observed that Macrocheles muscae-
domesticae (Scopoli, 1772) (Mesostigmata), being phoretic on a housefly, feeds
on nematodes found on the fly(cid:146)s body (IGNATOWICZ 1975), and Antennophorus
grandis Berlese, 1904 (Mesostigmata), attached to an ant(cid:146)s head, irritates its mouth
opening, which results in the throwing out of a drop of a substance, which is then
eaten by the mite (KRANTZ 1978). Analogous, close relationships as those mentioned
above, are characteristic for Echinomegistus wheeleri Wasmann, 1902, Micromegistus
bakeri Tr(cid:228)gardh, 1948 (Mesostigmata) and carabid beetles (Coleoptera) (KRANTZ 1978).
Urodiscella philoctena Trouessart, 1902 (Mesostigmata) scrapes off and eats the
secretion of the ant that carries the mite (KRANTZ 1978). An unintentional damage
done by the mites to their hosts, was described by CHMIELEWSKI (1977) as (cid:145)trans-