Lum: Behavior in the Pseudoscorpion Garypus
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RAC
ABS
The general behavior of the supralittoral pseudoscorpion Garypus californicus
was studied along the shoreline of Pacific Grove, California, where it is commonly
found. Behavior, observed in the field and the lab, was analyzed and broken into
components which were deseribed and figured. The components were divided into
solitary and social behaviors, which were further divided into more specific categories
containing specific actions and postures. 116 encounters between adult organisms
were analyzed and treated as sequences of behaviors. An "intruder" either randomly
entered or was placed in the area of an initially stationary "host". Threatening
gestures made by the two participants usually resulted in one organism leaving or a
fight. In 101 cases the end result was the intruder leaving. Significant amounts of
clustering behavior were found and correlate well with the initial posture of the host.
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INTRODUCTION
Garypus californicus Banks, 1909, is a 4-5 mm long pseudoscorpion that occurs
along the coastline from Trinidad, California, to Isla Asuncion and Isla de Guadalupe,
Baja California (Lee, 1979). It has also been reported in the Pomona-Laguna,
California area (Moore, 1917), from San Nicolas Island, California (Cockerell, 1940),
and from Palo Alto, California (Banks, 1909). The geographical distribution of the
animals appears to have been governed by the oceanic currents of the Pacific Ocean
due to the use of rafting as a means of dispersal (Lee, 1979). It is a nocturnal animal
that constructs nests for brooding, hibernation, and molting (Weygoldt, 1969). Little
else is known about G. californicus.
Although many studies have been made on the complex mating behaviors
exhibited by pseudoscorpions, very few general behavioral studies have been made, and
none on this species. In the present study an attempt was made to analyze the
behavior of G. californicus into simple components that could be described, figured,
and later treated as sequences of postures and actions during the recording of such
activities as nest building, brooding, feeding, and fighting.
MATERIALS AND METHODS
Specimens were collected at various points along the high intertidal zone at
Mussel Point and Asilomar Beach State Park, Pacific Grove, California, during the day
and early evening between 19 April and 13 May, 1982. Organisms were captured and
placed in small bottles for later laboratory analyses. Studies were carried out at the
Hopkins Marine Station of Stanford University in Pacific Grove, California.
Lum: Behavior in the Pseudoscorpion Garypus -Page 4-
Pseudoscorpions were maintained in the laboratory in terraria (0.5-5.0 gallons).
Each terrarium contained 1-3 cm of sand, a dish filled with wet sand, and rocks,
pieces of wood, and shell for concealment. This arrangement mimicked the natural
high intertidal habitat of the pseudoscorpions. Each terrarium was covered by a fine
mesh netting to prevent escape. The animals were fed weekly with collembola
collected from the beach. For closer observations, 60 mm petri dishes were lined
with wet filter paper and a 5 mm layer of sand grains 0.7-1.0 mm in diameter.
Animals apparently in good condition (moved about and ate occasionally) were kept in
the laboratory for up to four weeks.
Observations of behavior were made both in the field and in the laboratory. In
the field, rocks in the high intertidal and supralittoral zone were overturned and
observations were recorded. The rocks were then carefully returned to their original
positions. In the laboratory, observations of the animals in the terraria and petri
dishes were made using both a hand lens and a dissecting microscope. High speed
behaviors were recorded on videotape through a dissecting a microscope, and analyzed
at slow playback speeds. A total of 26 animals was observed. In analyzing behavior,
a component was termed an action pattern if (1) it was performed by more than one
individual, (2) it was observed more than once during the study, and (3) it varied in
form about a typical or modal performance.
Methods used in studying interactions between individuals are given in the
section dealing with adult G. californicus behavior during encounters.
Lum: Behavior in the Pseudoscorpion Garypus -Page 5-
RESULTS
HABITAT
G. californicus can be found in a narrow band along the coastline, extending
from the higher high water mark up to several meters above the initial appearance of
vegetation. Individuals were found living under rocks or driftwood on dry or slightly
moist substrates. No specimens were found on moist or wet substrates. Substrate
size varied greatly from 0.5 mm diameter sand grains to 12 mm diameter gravel,
although organisms were most commonly found on 1-3 mm diameter sand grains. Rock
size did not appear to matter as long as the rock was not deeply buried and there was
sufficient room under the rock for movement. Organisms commonly found with
pseudoscorpions are centipedes, mites, spiders, amphipods, beetles, beetle and fly
larvae, pill bugs, collembola, and ants.
Discussion. G. californicus were always found above or right at the point of the
highest drift. The pseudoscorpions were situated where they very rarely got splashed
or submerged. (Lee, 1979), has reported they can only tolerate submergence for an
hour, but may survive longer periods if there are air bubbles to press their spiracles
against. Many females were found in brood nests. Often times spiders were found
living very close to the pseudoscorpions. The spiders also constructed nests out of
silk and gravel which were sometimes hard to distinguish from the ones made by
pseudoscorpions.
PSEUDOSCORPION ACTION PATTERNS AND POSTURES
The postures and general behavior observed in G. californicus can be divided
into solitary behavior patterns and social behavior patterns. Solitary behavior
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patterns include non-locomotory action activities and postures, locomotory actions,
grooming actions, feeding actions, nest building actions, and brooding actions and
postures. Social behavior patterns include action patterns between adult
pseudoscorpions, action patterns between juvenile animals, and action patterns
associated with brooding and care of the young. Mating behavior was not seen.
Many postures can be distinguished by the position and behavior of the
pedipalps. In the descriptive nomenclature used, the word "chelae" refers to the
pedipalp chelae, not chelicerae. The "wrist angle" is the angle formed by the
pedipalp tibia and tarsus (fig. 1). The "elbow angle" is the angle formed by the
pedipalp femur and tibia (fig. 1). The angle of the palpal femur to the main body axis
is also shown in fig. 1.
SOLITARY BEHAVIORS
NON-LOCOMOTORY ACTION PATTERNS AND POSTURES
[Legs are extended essentially as in fig. 2 unless otherwise noted)
RESTING/RELAXED (fig. 2a)
Femurs of pedipalps drawn in toward body, bringing elbows above
opisthosoma; angle of elbows 30 degrees; chelae closed, tips nearly touching
each other.
RESTING/ALERT (fig. 2b)
Femurs of pedipalps nearly parallel to prosoma, with femurs held at an
angle of 0-20 degrees to main body axis; angle of elbows 100-120 degrees;
chelae open.
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COWERING
From any previous position, pseudoscorpion immediately assumes the
resting/relaxed position.
THANATOSIS (fig. 2c)
Body motionless, feigning death; legs fully folded up and held together
underneath body; pedipalps drawn close to body with elbows nearly touching
dorsally; chelae closed, tips of two chelae nearly touching each other and just
anterior to the chelicerae.
Discussion. The resting positions are the usual positions G. californiçus
assume when not active. The resting/alert posture is used when the animals
are in an exposed position where they are not surrounded by cover in at least
two directions. The resting/relaxed posture is most frequently used when the
pseudoscorpions are well concealed or clustered with other pseudoscorpions.
When disturbed (as in lifting the cover of a petri dish), the
pseudoscorpions cower and remain motionless. If not further disturbed, they
resume activities in 1-3 minutes. When animals are greatly disturbed, they
immediately assume the thanatosis position. In this position the
pseudoscorpions are very hard to distinguish from the substrate or debris. In
1-3 minutes a pseudoscorpion slowly extends its legs and grasps the substrate if
on its stomach. If it is on its back, it quickly rights itself. In both cases,
after a few preliminary investigative movements, it quickly runs off.
Remaining motionless probably offers the pseudoscorpions protection from
predators who depend more on movement than touch or taste.
-Page 8-
Lum: Behavior in the Pseudoscorpion Garypus
TERNS AND POSTURES
LOCOMOTORY ACTION PA
[The method of locomotion was observed to be the same as in other eight
legged animals (Kaestner 1927
FORWARD WALK (fig. 3a)
Femurs of pedipalps extended laterally, held 80-90 degrees from main
body axis; angle of elbows 120-140 degrees; chelae open and pointing forwards;
bottoms of movable fingers of chelae are constantly bounced along substrate
when walking.
FORWARD RUN (fig. 3b)
Similar to foward walk, but with pedipalps extended farther forward;
femurs of pedipalps held 100-120 degrees from main body axis; chelae are
bounced along substrate much less frequently than in forward walk, and held
higher.
TURN (fig. 3c)
Both chelae point in direction of turn; leading pedipalp femur held 70-80
degrees from main body axis, elbow angle 160-170 degrees, chela open; trailing
pedipalp femur held 40-70 degrees from main body axis, elbow angle 80-90
degrees, wrist angle 90-100 degrees, chela closed or partially open.
SIDEWAYS WALK (fig. 3d)
Similar to turn, but with trailing chela also open and pointing forward;
femur of trailing pedipalp held parallel to prosoma; trailing pedipalp wrist angle
80-90 degrees; walking motion is similar to that of a crab.
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BACKWARD RUN/WALK (fig. 3e)
Femurs of pedipalps held against side of body; angle of elbows 70-100
degrees; chelae closed.
RIGHTING RESPONSE (figs. 3f and 3g)
Starting with pseudoscorpion lying on its back; legs are kicked actively;
if they contact any surface, it is grasped and the animal rights itself; failing
this, one pedipalp is extended anteriorly and the closed chela is pushed against
substrate; at the same time the back is arched, lifting prosoma above substrate:
opposite pedipalp is used as counterbalance and is pushed off the substrate
dorsal to the prosoma and thrown anteriorly and laterally to bring the animal to
its stomach; if unsuccessful, process is repeated alternating pedipalp functions
until righted.
INVESTIGATIVE MOVEMENTS
After stopping locomotion, pedipalps are moved anteriorly, posteriorly,
laterally, and dorsally; open chelae are touched to nearby objects and
substrate.
Discussion. During forward walking, the constant bouncing of the chelae on
the substrate allow the pseudoscorpion to judge its surroundings. Fig. 4 shows
the many sensory hairs on the pedipalp chelae. When walking alongside a large
object, the inside pedipalp is bounced on the side of the object as the other
pedipalp bounces on the substrate. The position of the pedipalps in the
forward walk facilitates the quickest forward motion while still sensing what
lies directly ahead.
By pointing both chelae in the direction of motion in the turn, the
pseudoscorpion can both have a clearer view and use its pedipalps to sense
Lum: Behavior in the Pseudoscorpion Garypus -Page 10-
what is in front of it. In the sideways walk, the turning posture is modified so
that what is directly ahead and to the side can both be sensed.
Both the sideways and backward walk/run are seldom used except when
the animal is trying to escape. This may be because the sideways walk is an
inefficient means of locomotion and the inability to sense where it is going in
the backward walk. However, the backward walk is very quick, and is as fast
or faster than the forward run.
Usually a pseudoscorpion rights itself within seconds after falling on its
back, but after many futile tries the animal becomes exhausted and rests with
both pedipalps flexed and chelae held behind prosoma. After a short rest,
attempts to right are continued until exhaustion or animal is righted.
Frequently while walking, pseudoscorpions stop and make investigative
movements with their pedipalps. These movements probably help orient them
as course changes often follow these stops.
GROOMING ACTION PA
TTERNS AND POSTURES
IThe chelae of the pedipalps are the most important sensory organs of the
pseudoscorpion and must frequently be groomed to retain optimal functioning
(Weygoldt, 1969). The tarsi of the legs also contain many hairs and which are also
groomed, although not as frequently as the palpal fingers.)
GROOMING OF CHELAE (fig. 5a)
One finger of chela is slowly pushed, from tip to base through
chelicerae; other pedipalp is held in the resting/alert position.
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GROOMING OF LEGS (figs. 5a and 5b)
Body is braced against immovable object; one pedipalp is held in
resting/alert position while other is used to hold body above substrate; all four
Tegs on the side of the bracing pedipalp are lifted off the substrate and the
tarsi rubbed against each other.
Discussion. The grooming of the chelicerae is always done by first grooming
the fixed finger of one chela, then the movable finger of the same chela. The
other chela is then groomed, fixed finger before movable. If interrupted
before completion of the actions, the next time grooming is attempted, the
process is repeated in totality.
The hairs on the legs are probably sensory, but aren't as important as
those on the chelae. During grooming of the legs, the tarsi are rubbed
together for 10-30 seconds, after which the animal may walk off or groom the
legs on the other side.
FEEDING ACTION PATTERNS AND POSTURES
The pseudoscorpion feeds on small soft-bodied arthropods such as flies.
collembola, beetle larvae, and adult beach-flies emerging from puparia (Evans, 1980).
and has also been seen preying on hard-bodied arthropods (Lee, 1979). In the
Taboratory the pseudoscorpion was observed eating collembola and small spiders.
GRASP (fig. 6a)
May involve one or both chelae; grasped object is held between movable
and fixed fingers of chela.
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CHELA TO CHELICERAE TRANSFER (fig. 6b)
Object grasped by chela is brought to and grasped by chelicerae:
pedipalp chela then releases object.
FEEDING METHOD
Prey is quickly grasped and poison injected by the poison teeth at chela
fingertips; prey is either held until it stops struggling or is dropped and later
grasped by chela; prey is transferred from chela to chelicerae, which pierces
the body for reception of digestive enzymes from gut; prey is sucked dry and
either dropped to the ground or transferred to the chela and then dropped.
Fig. 4 details the poison teeth at the chelae tips.
Discussion. Pseudoscorpions often miss at their first attempted grasp at prey.
If prey is still within range, more grasps are attempted until successful of prey
is out of range. They do not chase their prey, but wait motionless until prey
approaches and then quickly grasps it. Most of the time the prey is then
dropped and ignored until the poison takes effect and prey is immobilized
before consumption. More than one prey can be killed at one sitting.
NEST BUILDING AND BROODING ACTION PATTERNS AND POSTURES
[Brood nests are built using any solid substrate for a wall. Where small
particles (such as sand or gravel) are abundant, they are used in the construction of
the other walls of the brood nest.)
BROOD NEST CONSTR UCTION
Particle to be used in construction is grasped, transferred to chelicerae.
then positioned in wall of nest and immediately glued into place by silk
extruded by spinnerets located at the ends of the moyable fingers of the
Lum: Behavior in the Pseudoscorpion Garypus -Page 13-
chelicerae. Fig. 7 gives a detailed photo of the spinnerets and hairs
surrounding the chelicerae.
SILK EXTRUSION RELATED MOVEMENTS
When silk is exuded, movements of the prosoma facilitate aiming of silk
application.
LAYING OF EGGS AND FORMING OF BROOD SAC
Not observed in the lab.
BROODING POSITION
Brood sac with eggs remains attached to genital pore; legs grasp the side
or top of brood nest; pedipalps positioned as in resting/relaxed posture.
Discussion. Brood nests are built one layer at a time, like an igloo. At least
one side of the nest is solid, usually the bottom of the rock. The other sides
are constructed by gluing one particle into place at a time and then building
over it. This continues until the pseudoscorpion is completely enclosed. The
actual construction takes from 1-2 days, followed by 1-2 days of lining the
walls with silk. In the lab a pseudoscorpion was observed to line a structure
consisting of two well-slides placed face to face with silk and form a brood
nest.
After the lining the walls, the female grasps the side or the top of the
nest, and about 2 days later forms a brood sac filled with 16-25 eggs. The
female feeds the eggs nutritive fluids from her body and the eggs grow
(Weygoldt, 1969). The female does not eat or move while brooding except
when the enlargement of the eggs forces her to change positions. If disturbed
while brooding, female either repairs small holes, or drops her brood sac, leaves
the nest, and runs away.
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SOCIAL BEHAVIORS
lOnly thise actions and postures which differ from previously mentioned ones
are included in this section. Mating, caring of the young, and behavior between
juveniles were not seen.)
NON-AGGRESSIVE BEHAVIORS BETWEEN ADULTS
WALKOVER
One pseudoscorpion walks over the back of another.
CLUSTER
One pseudoscorpion settles down beside another and assumes one of the
resting positions.
Discussion. The walkover most frequently takes place when the psedoscorpion
being walked on is in the resting/relaxed position. If it occurs in any other
position, aggressive behavior usually follows. Pseudoscorpions are often found
clustering together in groups of two or more. This may have some survival
value.
AGGRESSIVE BEHAVIORS BETWEEN ADULTS
THREATENING GESTURE (fig. 8)
The pedipalps, initially in the resting/alert position, are rapidly extended
anterior to the prosoma so the open chelae almost touch each other; pedipalps
very rapidly return to the resting/alert position; motion may be repeated many
times.
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CHELA-TO-CHELA GRASE
The chela of one animal is grasped by the chela of another
pseudoscorpion.
LEG PINCH
One leg of a pseudoscorpion is grasped by the chela of another.
FIGHT
One or more chela-to-chela grasps and/or leg pinches are made between
two pseudoscorpions.
FIGHT-OR-FLIGHT RESPONSE
After a threatening gesture is made by one pseudoscorpion to another,
the threatened animal either runs away or stays and fights.
QUICK SPIN
Pseudoscorpion rapidly spins around to face the source of a stimulus
from the side or rear.
CHASE
One pseudoscorpion walks or runs after another while making threatening
gestures and/or leg pinches.
Discussion. Threatening gestures are used as a warning and as a challenge to
other animals who enter an area occupied by a pseudoscorpion. Gestures
précede many fights, although, rarely, gestures are made after a fight. The
chela-to-chela grasp is often the first contact of a fight. When a
pseudoscorpion's chela is grasped, it struggles violently. Once free, it may
then run away or continue fighting. An aggressing animal approaching another
from the side or rear initiates a fight by giving a leg pinch. The pinched
animal does a quick spin and a fight ensues.
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A fight between two pseudoscorpions never ends fatally. This may be
due to immunity to its own poison, or the non-injection of poison, although the
mechanism of grasping appears the same as that in feeding. After a loser
Teaves the scene of a fight, the victor sometimes gives chase, but chases are
made only for short distances.
ADULT G. CALIFORNICUS BEHAVIOR DURING ENCOUNTERS (fig. 9)
METHODS
In studying interactions between idividuals, only adult pseudoscorpions were
used. There were three parts to the experiment. In one part four females and one
male were simultaneously placed in a 60 mm petri dish lined with wet filter paper and
à 5 mm layer of sand grains. A second part was carried out with two females in a
dish, and the third involved one female and one male in a dish. Encounters were
either random interactions or set up by placing one individual 2-3 mm in front of
another. When one pseudoscorpion, the "intruder", moved or was placed within 3 mm
of an initially stationary "host", an encounter occurred. Data from each encounter
were recorded on a pre-designed chart which included most combinations of prior
observations. New combinations were added to the chart when they occurred. An
encounter ended when (1) one animal moved moved at least 6 mm from the point of
initial interaction, (2) both animals left, or (3) both settled down within 6 mm of each
other. Relative sizes were estimated during each encounter.
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RESULTS
This experiment was performed to test general social behaviors during
encounters, and differences between male and female interactions. The first
experiment involving four females and one male had a total of 76 encounters. The
second involved two females and 20 encounters, and the third involved one male and
one female with 20 encounters. No significant difference was noted between
male/female interactions and female/female interactions. For this reason all further
numbers refer to the combined total of 116 encounters.
Following the initial placement or entry of an intruder, one of four events
occurred. (1) In 32 trials the intruder was ignored, and in 25 of those the intruder
left. In five others the intruder walked over the host and left, but in two cases a
fight resulted after the intruder walked on the host, and in both cases the intruder
left after the fight. (2) In 34 trials the first action made was either a chela-to-chela
grasp or a leg pinch, (3) In 46 trials a threatening gesture was made by one or both
animals. (4) In the remaining 4 trials the intruder and host clustered together.
Where the first action was a grasp, twice as many first grasps were done by
the host than the intruder. In 21 cases the first grasp resulted in the intruder
leaving, while the other 13 resulted in a fight. All but two of the intruders left
after the fight.
In most of the encounters which began with a gesture, the host gestured and
the intruder followed suit. In 22 cases the intruder left after the gestures, but in
another 22 a fight resulted. After the fight, the intruder left 15 of the times, while
the host left 4 times, and both clustered another 4 times (numbers do not add up
because "both leave" is counted twice).
101 out of the 116 encounters ended with intruder leaving, while in only 11
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interactions the host left. 37 encounters resulted in fights and 10 encounters ended
with clustering.
DISCUSSION
The initial posture of the host was an important factor in determining the
result of the interaction. If the host was in the resting/relaxed position, the intruder
clustered or was ignored a large percentage of the time , whereas if the host was in
the resting/alert posture aggressive behavior usually resulted. This supports the
belief that the resting/relaxed posture is more of a sleeping state and weak stimuli
are not sufficient to elicit any response. However in the resting/alert posture, the
pseudoscorpion is receptive to all stimuli as in the fight-or-flight response. There
were probably more aggressive interactions in the lab than in the natural habitat
because more animals were in the resting/alert state due to the absence of places for
concealment in the dish and the higher population density. But, the ratios for the
two different types of aggressive interactions are still accurate.
Thère is some question of whether or not pseudoscorpions show territoriality.
Many times they are found clustered closely together without apparent signs of
territoriality, yet the frequent fighting when one's area is intruded upon contradicts
this. It may be that when a pseudoscorpion is active or in the resting/alert position.
it does not let other animals in its area, but when it sleeps it does.
In those cases where threatening gestures were made, if the host was the only
one to gesture, the intruder usually left. If the intruder gestured at any time, a fight
usually ensued. It appears that the intruder takes the gesture as a challenge and
"decides" whether to fight or run away. No correlation between size and decision
was noted.
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When the intruder was significantly larger (at least 1.5 times) than the host.
there was a higher probability the host would lose the fight and leave. In no cases
where the host was of comparable size or larger did it leave. The host seems to have
an advantage over the intruder, and this advantage does not depend on length of
residency prior to the encounter. Length of residency varied from a few seconds to
ten minutes.
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Lum: Behavior in the Pseudoscorpion Garypus
ACKNOHLEDGEMENT
I wish to thank Dr. Donald P. Abbott for the tremendous amount of assistance,
advice, and patience he had during the course of this study and the preparation of
this manuscript. Special thanks to Chris Patton for assistance in making the excellent
el ec tron micrographs, and Alan Baldridge for his help in collecting the literature used.
Finally, thanks are also due to all the faculty, TA's, and students of the Problems in
Marine Biology Class (Spring, 1982) at Hopkins Marine Station of Stanford University
for their comments, suggestions, and moral support.
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Lum: Behavior in the Pseudoscorpion Garypus
LITERATURE CITED.
Banks, N. 1909. New Pseudoscorpionida. Canad. Entomol. 41: 303-307.
Chamberlin, J.C. 1921. Notes on the genus Garypus in North America
(Pseudoscorpionida-Celiferidae). Canad. Entomol. 53: 186-91.
Cockerell, T. D. A. 1940. The insects of the California islands. Proc. of the Sixth
Pacific Sci. Congress of the Pacific Sci. Assoc. 4: 283-295.
Evans, W. G. 1980. Insecta, Chilopoda, and Arachnida: insects and allies, pp. 641-
658 and fig. 28.2, Plate 190. In R. H. Morris, D. P. Abbott, and E. C.
Haderlie, Intertidal invertebrates of California. Stanford, Calif. Stanford
University Press. 690 pp.
Lee, V. F. 1979. The maritime pseudoscorpions of Baja California, Mexico
(Arachnida: Pseudoscorpionida). Occas. Pap. Calif. Acad. Sci. 131: 1-38.
Moore, W. T. 1917. Record of two pseudoscorpions from the Claremont-Laguna
Region. Jour. Ent. Zoo. 9: 26-29.
Savory, T. H. 1966. False scorpions. Science. 214: 95-100.
Weygoldt, P. 1969. The biology of pseudoscorpions. Cambridge, Mass,: Harvard
University Press. 145 pp.
Lum: Behavior in the Pseudoscorpion Garypus -Page 22-
CAPTIONS
Fig. 1. Pseudoscorpion anatomy.
(a) Ventral view. (b) Dorsal view.
Non-Locomotory postures.
Fig. 2.
(a) Resting/Relaxed. (b) Resting/Alert. (c) Thanatosis.
Locomotory postures.
Fig. 3.
(a) Forward walk. (b) Forward run. (c) Left turn. (d) Sideways walk.
(e) Righting response (ventral view). (f) Righting response (lateral view).
Scanning electron micrograph of the ventral view of pedipalp chela showing
Fig. 4.
sensory hairs and poison teeth. (0 )
Fig. 5.
Grooming postures.
(a) Grooming of chela. (b) Grooming of legs.
Fig. 6.
Feeding postures.
(a) Grasp. (b) Chela to chelicerae transfer.
Scanning electron micrograph of the chelicerae showing spinnerets and hairs.(200
Fig. 7.
Threatening gesture.
Fig. 8.
Fig. 9. Sequences of behavior during encounters in adult Garypus californicus. First
number in parentheses refers to number of interactions in the experiment
with four females and a male. Second number refers to experiment involving
two females, and third refers to interactions in the male/female experiment.
The relative width of arrows indicates the combined total of all experiments.
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