Influence of cues from the anterior medial eyes of virtual prey on Portia fimbriata, an araneophagic jumping spider
1 Department of Biological Sciences, University of Sussex, Brighton BN1 9QG,
East Sussex, UK
2 Department of Zoology, University of Canterbury, Private Bag 4800,
Christchurch, New Zealand
* e-mail: d.harland{at}biols.sussex.ac.uk
Accepted 11 April 2002
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Summary |
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Key words: Portia fimbriata, jumping spider, Salticidae, prey capture, vision, optical cue, anterior median eye, cryptic stalking, virtual prey
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Introduction |
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Humans and other large animals that rely on spatially acute eyesight
dedicate large tracts of the brain to visual processing. In insects, spiders
and other animals with body size considerably smaller than that of humans,
brain size is by necessity also relatively small. Small brain size means that
the insect or spider has at its disposal vastly fewer neurones to devote to
processing input from the eyes. Yet, in the face of size constraints, eyes
that are adapted for resolving moderately fine spatial details have evolved in
certain insects, honeybees (Srinivasan and
Zhang, 1998), dragonflies
(Sherk, 1978
) and hoverflies
(Collett and Land, 1975
) being
especially notable examples. The eyes of most spider species are only poorly
adapted for high spatial acuity. The most notable exceptions to this rule are
the jumping spiders (Salticidae), which have complex eyes (Land,
1969a
,
b
,
1985b
) and exceptionally
intricate vision-guided predatory strategies
(Forster, 1982
;
Jackson and Pollard,
1996
).
Salticids have eight eyes. Three pairs positioned along the sides of the
cephalothorax (called the `secondary eyes') have a combined field-of-view
close to 360° laterally and serve primarily as movement detectors (Land,
1971,
1985a
). However, it is a pair
of proportionately much larger, forward-facing anterior median eyes (called
the `principal eyes' or `AM eyes') that are specialized for high spatial
acuity over a much smaller field of view (approx. 1-2°; Land,
1969a
,
b
; Blest et al.,
1981
,
1990
;
Blest and Price, 1984
).
Salticid AM eyes, which are structurally very different from the compound eyes
of insects, provide spatial acuity that is typically an order of magnitude
finer than that of insect compound eyes
(Land, 1985b
; see also
Land, 1997
).
The use of different prey-capture tactics against different types of prey
(`predatory versatility'; Curio,
1976) appears to be widespread in salticids
(Richman and Jackson, 1992
)
and is especially pronounced among araneophagic species (i.e. a minority group
of salticids for which the preferred prey are other spiders;
Jackson, 1992
). Pronounced
predatory versatility may make araneophagic salticids especially suitable for
studying how visual information is processed by animals that are subject to
severe size constraints: by adopting prey-specific prey-capture behaviour, the
salticid effectively tells the researcher how it has classified the prey (see
Harland and Jackson, 2000a
,
2001
).
Portia fimbriata from Queensland, Australia, the most extensively
studied of the araneophagic salticids
(Jackson and Wilcox, 1998),
preys on spiders and on insects, both of which may be captured away from webs,
in webs that P. fimbriata build itself and in webs built by other
spiders (Jackson and Blest,
1982
). Away from webs, P. fimbriata preys especially on
salticids belonging to other genera, deploying against them a special tactic
termed `cryptic stalking' (Jackson and
Blest, 1982
).
The appearance and walking gait of P. fimbriata are unlike those
of any other salticid. Resting in a web, P. fimbriata resembles a
piece of detritus (Wanless,
1978), detritus mimicry when walking being preserved by a slow,
choppy gait (each leg moves jerkily and out of phase with the movement of the
other legs). Resting in a web, P. fimbriata adopts a special posture,
called the `cryptic rest posture' (Jackson
and Blest, 1982
). In this posture, with the legs close to the body
and the palps retracted beside the chelicerae, the outlines of the appendages
are hidden.
When cryptic stalking is adopted, the Queensland P. fimbriata
retracts its palps, as in the cryptic rest posture, and exaggerates the slow,
choppy character of its gait. When salticids detect movement, they sometimes
turn to face the cryptically stalking P. fimbriata, but P.
fimbriata freezes until the salticid once again faces away
(Jackson and Blest, 1982).
While stalking a spider of any type other than a salticid, or while stalking
an insect, the Queensland P. fimbriata does not consistently retract
its palps, nor does it consistently freeze when faced. Other studied species
of Portia (P. africana, P. albimana, P. labiata and P.
schultzi) and P. fimbriata from sites other than Queensland
(Northern Territory of Australia, Malaysia, Sri Lanka) attempt to capture
salticids, but they do not adopt cryptic stalking and they are considerably
less successful than the Queensland P. fimbriata. When cryptically
stalked by a Queensland P. fimbriata, salticids typically give no
evidence of recognizing the presence of a predator
(Jackson and Blest, 1982
).
When stalked by other Portia species, however, they may flee or else
turn on the Portia, attack it and drive it away
(Jackson and Hallas,
1986
).
Cryptic stalking by the Queensland P. fimbriata appears to be an
example of local adaptation to a locally abundant type of prey
(Jackson, 1992).
Jacksonoides queenslandicus Wanless is by far the most abundant
salticid on tree trunks, boulders and rock walls where P. fimbriata
hunts (Jackson, 1988
), but
many salticid species are present in the rain-forest habitat of the Queensland
P. fimbriata. From standardised tests carried out on 114 salticid
species (Harland and Jackson,
2001
), the optical cues that trigger cryptic stalking are known
not to be unique to J. queenslandicus. In these tests, only optical
cues were available to P. fimbriata (prey enclosed in a small glass
vial within a large cage). Despite testing with species that differed
considerably in appearance (including beetle mimics, species with unusual body
shapes and species with a wide variety of camouflaging markings), all except
Myrmarachne spp. (ant mimics) triggered cryptic stalking by P.
fimbriata. This suggests that some commonly present salticid features
serve as cryptic-stalking cues.
The specific optical cues used by P. fimbriata to identify
salticids have been investigated experimentally using odourless lures made
from dead prey on which various combinations of features were altered
(Harland and Jackson, 2000b).
P. fimbriata adopted cryptic stalking only against intact salticid
lures and modified lures on which the large anterior median (AM) eyes were
visible. Ordinary stalking was usually adopted when the AM eyes were not
visible on the lure. There was no evidence that cues from the legs,
cephalothorax and abdomen of prey salticids influenced the choice of stalking
style of P. fimbriata.
Here, we investigate in greater detail the cryptic-stalking cues provided by salticid AM eyes. This is a step towards a long-term goal of clarifying the perceptual processes governing the predatory behaviour of araneophagic salticids. Our rationale for concentrating on cues from AM eyes is that a fine-grain understanding of how P. fimbriata discriminates between salticid and non-salticid spiders may elucidate more general low-level mechanisms underlying acute vision.
Unlike in the earlier study (Harland
and Jackson, 2000b), here we used computer-generated virtual lures
instead of physical lures. Clark and Uetz
(1990
,
1992
,
1993
) pioneered the use of
video playback and simple video-derived animated lures in research with
salticids. Our methods differ from theirs because, instead of playing back
video recordings, we use a computer-generated three-dimensional animation
projected as a two-dimensional image onto a small screen. Specifically, we
investigate whether a virtual salticid lure will elicit cryptic stalking and
whether, as in the study based on physical lures, the
presence-versus-absence of AM eyes influences the decision of P.
fimbriata to use cryptic or ordinary stalking. We then extend the earlier
study by considering some initial questions about the specific cues from AM
eyes that may influence the predatory decisions of P. fimbriata. In
particular, we investigate the influence of cues from the size, number,
position and shape of the AM eyes.
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Materials and methods |
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Lures
In the previous study (Harland and
Jackson, 2000b), the physical lures used were constructed from
dead specimens of Jacksonoides queenslandicus. Appendages and
sometimes entire body parts were removed from these lures. To us, a salticid's
AM eyes appear as dark glossy hemispheres protruding from the anterior surface
of the carapace, one on either side of the body's sagittal plane. `Removal' of
the lure's eyes in the previous study was achieved by carefully obscuring the
hemispheres with red paint.
In preliminary studies, it was exceedingly difficult to control for other potential influences on the response of P. fimbriata when altering details of the principal eyes on a physical lure, but this goal is readily achievable using computer-generated virtual lures. For a standard, we developed a virtual lure depicting an intact adult J. queenslandicus female (Fig. 1A). Next, for investigating how specific abstracted features of the AM eyes influenced the behaviour of P. fimbriata, we made six experimental virtual lures by systematically altering the appearance of the standard lure (Fig. 1B-G).
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Lures were drawn and modified using commercially available computer software packages on a standard IBM PC clone computer (450 MHz Pentium II with 128 MB RAM, running Microsoft Windows 98). Using as references scanned taxonomic drawings and captured video-stills of dissected parts of specimens through a microscope, each part of the body was carefully sculpted with Macromedia Extreme 3D2. The finished virtual three-dimensional lure was rendered in monochrome as a standard (bitmap) movie file and animated so that it rotated by 360° about its vertical axis. Macromedia Director 7 was used for writing a test harness program that presented the rendered movies of the various lures to P. fimbriata and allowed us to manipulate the behaviour of the lures on screen. Using the test harness, we could move each lure horizontally and vertically across the screen and rotate it smoothly by 360° around its vertical axis. Smooth rotation of the lure's entire body was chosen over more realistic motion involving the legs because our specific interest was the optical cues provided by form in the absence of potential cues from prey-specific motion. On command, rotating lures suddenly stopped in place or else suddenly stopped rotating and then immediately faced forward.
Apparatus
Lures, rear projected from a computer projector, were presented to P.
fimbriata on a white screen (23.5 mm wide x 17.5 mm high). The
image from the projector was reduced by using an array of lenses, with the
brightness of the image being controlled by using neutral-density filters
placed behind and in front of the screen. We adjusted image scale to where the
projected lure's carapace width was the same (to the nearest 0.1 mm) as that
of a typical adult J. queenslandicus female. During testing,
illumination came from the projector screen (approximately 6751x at platform
surface) and from fluorescent tube ceiling lights 1.5 m above the
platform.
The testing platform (80 mm long x 65 mm wide) was placed on a table in front of, and level with, the bottom of the projector screen (Fig. 2). It was 160 mm above the table surface. The platform was a rectangular wire frame (brass welding wire, diameter 1.5 mm) over which were stretched multiple layers of non-sticky (structural) silk threads taken in the field from webs of Badumna longinquus (Desidae), a spider that is common on buildings in Christchurch, New Zealand. Silk threads were stretched over the frame in quantities sufficient to make a dense matting with no holes large enough for P. fimbriata to pass through. As a precaution against possible chemical traces left by previously tested P. fimbriata, the web platform was washed in a bath of 80 % ethanol, then allowed to dry for at least 30 min between tests.
|
Having P. fimbriata on a silk-covered platform facilitated
testing. Although J. queenslandicus is not a web-building species,
P. fimbriata in nature encounters J. queenslandicus both in
and away from webs, web-based encounters being common
(Clark and Jackson, 2000)
because J. queenslandicus often enters other spiders' webs, including
the webs of Badumna spp.
(Jackson, 1988
).
Testing protocol
Before each test, an individual of P. fimbriata was transferred
from its cage to a small plastic Petri dish (diameter 35 mm). From the Petri
dish, P. fimbriata was introduced into a narrow opaque tube (internal
diameter 13 mm, length 45 mm) attached to the end of the platform opposite the
screen (Fig. 2). A small
soft-tipped paintbrush was used to coax the spider into and out of the Petri
dish.
The tube faced the projector screen and virtual lure, with one end of the tube touching the web. P. fimbriata was introduced into the end of the tube that was away from the screen, and this end was then stoppered to ensure that P. fimbriata had to exit from the end of the tube that faced the screen. The top and the two sides of this end of the tube were fringed with human hair (fringe approximately 7.5 mm long, held in place with tape), but the bottom was left free. Being reluctant to move over the hair fringe, P. fimbriata almost always walked out from the bottom of the tube and onto the web. In rare instances when P. fimbriata left via the top or a side of the end of the tube and failed to walk onto the web, the test was aborted.
Whenever P. fimbriata failed to leave the tube after 15 min had elapsed, a small soft-tipped brush was used to direct it gently until it was facing the screen, after which it was allowed an additional 15 min to emerge. Should it still fail to emerge, the test was aborted. Before P. fimbriata left the tube, the lure was moved erratically back and forth on the screen. Once P. fimbriata walked out of the tube and onto the platform, erratic lure movement continued until P. fimbriata oriented towards it. Once P. fimbriata turned to face the lure, erratic movement ceased and the lure was moved smoothly to the centre of the screen. Once stalking began, the lure was rotated by 45° (whether left or right was decided at random) and the style of stalking of P. fimbriata was recorded.
Two specific tests were carried out next. The lure was moved 10 mm (whether to the left or the right was decided at random) to ensure that P. fimbriata was actually stalking the lure, rather than just walking towards the part of the screen where the lure happened to be situated. If P. fimbriata continued moving towards the lure (i.e. changed in its path appropriately), we recorded this as confirmation of stalking. The test continued whenever stalking was confirmed, but it was aborted whenever stalking was not confirmed.
When P. fimbriata had stalked to within 50 mm of the lure, a second test was carried out to determine whether P. fimbriata would freeze when suddenly faced by the lure: a lure that was initially facing 45° to the side was suddenly (i.e. without any in-between steps) made to face directly towards P. fimbriata. Whether P. fimbriata froze (i.e. stopped all movement) within 0.5 s of being faced was recorded. The `freezing test' was repeated at 5 s intervals until P. fimbriata touched the glass (projector screen) or until a total of three freezing tests had been completed. Instances in which P. fimbriata touched the glass after only one freezing test were rare, and testing was aborted whenever this happened.
Portia fimbriata sometimes freezes when its prey (including
non-salticid prey) stops moving, even if the prey does not turn and face the
P. fimbriata (Wilcox et al.,
1996). To distinguish freezing when the prey simply stops moving
from freezing when faced by a salticid, the lure was kept stationary for 2 s
after being rotated by 45° to the left of P. fimbriata but before
being made suddenly to face P. fimbriata. If P. fimbriata
froze during the 2 s pause, the lure was kept stationary until stalking
resumed (i.e. the lure was not made to face the P. fimbriata).
Experimental design
A paired design was adopted: each individual of P. fimbriata was
tested once with an intact lure (called `control') and once with a modified
lure (called `experimental'), with testing order (intact lure then modified
lure or modified lure then intact lure) decided at random for each individual
P. fimbriata. The P. fimbriata was returned to the small
plastic Petri dish and placed out of sight of the testing apparatus during the
interval (10-15 min) between the first and second test.
We chose not to consider the question of whether or not any stalking at all
was elicited by the experimental lures (stalking tendency) because it is the
stalking style that provides unambiguous evidence that P. fimbriata
has `classified' a lure as a salticid (see Harland and Jackson,
2000b,
2001
). Therefore, sequences in
which P. fimbriata stalked neither the control nor the experimental
lure were terminated.
Paired frequency data were analysed using 2 McNemar tests
for significance of changes (see Sokal and
Rohlf, 1995
).
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Results |
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Influence of the number of AM eyes
A lure with only one AM eye (Fig.
1C) was made by removing the left AM eye from a copy of the
standard virtual lure. Results (frequency of adopting cryptic stalking and of
freezing) in tests with the lure that had only one AM eye were not
significantly different from the findings with the standard lure
(Fig. 3A) (Tables
1,
2).
Influence of the size and relative size of AM eyes
Two lures (Fig. 1D,E) were
made. On one, the diameters of the AM eyes were reduced to that of the
anterior lateral (AL) eyes (Fig.
1D). On the other, the AL eyes were enlarged to the diameter of AM
eyes (Fig. 1E). The position of
the centre point of each AM and AL eye was preserved for both these lures
P. fimbriata often adopted cryptic stalking against the control lure, but only once against the lure that had small AM eyes (P<0.001). P. fimbriata also froze when faced by the control lure more often than when faced by the lure with small AM eyes (P<0.001) (Fig. 3B) (Tables 1, 2). However, the frequencies with which individuals adopted cryptic stalking against, or froze when faced by, the lure with enlarged AL eyes were not significantly different from these frequencies in tests with the control lure (Fig. 3B) (Tables 1, 2).
Influence of the position of the AM eyes
A lure was made with one AM eye removed, the other eye being repositioned
horizontally so that it was, Cyclops-fashion, in the centre of the spider's
`face' (Fig. 1F). The frequency
with which individuals adopted cryptic stalking when faced by the Cyclops-like
lure was not significantly different from this frequency with the control lure
(Table 1). However, individuals
more often (P<0.001) froze when faced by the control lure than
when faced by the Cyclops (Fig.
3C) (Table 2).
Influence of AM eye shape
A lure was made by replacing both round AM eyes of the standard lure with
square eyes, each side of the square being equal to the diameter of the
standard's AM eye (Fig. 1G).
The central region of each square AM eye was rendered so that it appeared to
bulge out to the same extent as the normal AM eye, thereby preserving the
specular spots and preserving the shape of the eye when viewed from the
side.
Compared with the lure that had square AM eyes (Fig. 3D), the control lure more often elicited cryptic stalking (P<0.001) and freezing when faced (P<0.001) (Tables 1, 2).
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Discussion |
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For initiating cryptic stalking and provoking the freezing response, the presence of two AM eyes was not necessary. Even a lure with one AM eye removed and the other AM eye in its normal off-centre position sufficed. Size, however, did matter. A lure with AM eyes reduced in size triggered cryptic stalking and the freezing responses less often than intact lures.
Eye shape also appears to matter. When the lure had square-edged AM eyes, P. fimbriata rarely appeared to classify it as a salticid at all (i.e. cryptic stalking was usually not initiated and freezing was usually not provoked when faced). Evidently, the shape of the edges of the AM eyes is an important salticid-identification cue.
The anterior lateral (AL) eyes face more-or-less forward and are visible face on. On the standard lure, the AM eyes are large relative to the AL eyes, suggesting that, for a P. fimbriata trying to decide whether to initiate cryptic stalking and whether to freeze when faced, a relevant cue might be the size of the AM relative to the AL eyes. Our findings, however, did not support this hypothesis. When we used a virtual lure on which we had enlarged the AL eyes while leaving the AM eyes at their typical large size, there was no evidence that the response of P. fimbriata was different from its response to the standard lure.
Yet P. fimbriata cannot be using absolute AM eye size as a salticid-detection cue. The salticids on which P. fimbriata preys in nature vary considerably in body size, which means that they vary considerably in absolute AM eye size as well. Furthermore, the distance between P. fimbriata and its salticid prey varies when cryptic stalking is initiated, which means that the image size of an AM eye will vary considerably even for salticid prey of similar body size.
Although our findings suggest that P. fimbriata does not pay attention to AM eye size relative to AL eye size, AM/AL relative size was not the only change brought about when we altered AM eye size on our virtual lures. For example, reducing the size of the two AM eyes, while preserving the position of the centre of each, altered the positions of the AM eye edges relative to each other, relative to the AL eye edges and relative to the edges of the carapace. These or other parameters may have influenced the reactions of P. fimbriata to the experimental lures.
Findings from testing with a Cyclops-like lure (a single AM eye centred between the two AL eyes) suggest that the horizontal position of the AM eyes influences the decision of P. fimbriata to freeze when faced by salticid prey (i.e. P. fimbriata more often froze when a lure had two AM eyes instead of a single centrally positioned AM eye), but not the decision of P. fimbriata to initiate cryptic stalking. In nature, P. fimbriata may often initiate cryptic stalking prior to getting a clear face-on view of the salticid prey. Horizontal positioning of an AM eye may not be so readily discernible in many instances, and it might not be optimal for P. fimbriata to base an early decision on difficult-to-discern details. However, deciding to freeze or not appears to depend on information about specifically whether a salticid is facing or not.
Findings from testing with the Cyclops may have other implications regarding how P. fimbriata determines AM eye size. Centring the AM eye on the face horizontally altered the horizontal component of distances between the edge of the AM eye and other facial features. In particular, the distances between the edges of the AL eyes and left and right sides of the carapace increased. This was also true for the lure with reduced AM eyes. However, P. fimbriata tended to adopt cryptic stalking against the lure with one horizontally centred AM eye, but not against the lure with reduced AM eyes. This suggests that the horizontal component of distances between the edge of the AM eye and other facial features is not used to determine AM eye size. However, the story might not be so simple.
`Cryptic stalking' is a term used for a predatory tactic that has distinct components: palp posture, walking gait and a tendency to freeze when faced by the prey. Using a single term, `cryptic stalking', for this collection of integrated components may be convenient, but should not been envisaged as implying that all components are governed in the same way by the same cues. The findings from using the Cyclops-like lure particularly strongly suggest that the cues for initiating cryptic stalking differ from the cues for freezing. It seems likely, however, that initiation of cryptic stalking primes P. fimbriata (i.e. renders P. fimbriata ready to freeze when the appropriate freezing-eliciting cues arrive).
From the perspective of P. fimbriata, detecting conditions under
which freezing is appropriate (i.e. detecting that a salticid is facing) may
be more demanding than detecting conditions under which cryptic stalking is
appropriate (i.e. simply detecting the presence of a salticid). The presence
of large, round AM eyes alone would not suffice as a freezing-elicitation cue
because a salticid's AM eyes are visible over a wide range of orientations,
not merely when the salticid prey is face on. For example, the AM eyes are
still easily seen when the salticid is facing 45° to the left or the right
(Fig. 1), but a salticid at
45° does not elicit freezing (Harland
and Jackson, 2000b). Findings from using a lure with one
horizontally centred eye suggests that, when P. fimbriata is deciding
whether to freeze, the horizontal distance between the edge of the AM eye and
the visible edge of the carapace may be an especially reliable cue. For
example, when an intact prey salticid that is facing P. fimbriata is
turning away to its left (Fig.
4), the distance between the right edge of the prey's right AM eye
and the right edge of its carapace increases rapidly as more of the right side
of the carapace comes into view. P. fimbriata might measure this
distance by first fixing its AM retina on the pattern made by the curved edge
of the salticid prey's right AM eye and then moving its retina horizontally
(thereby, moving its viewpoint over the image of the salticid) until the edge
of the carapace comes into view. Comparing the information gathered in this
way with either the distance of the spider from P. fimbriata (i.e.
its range) or its size (i.e. as established by other cues) would give an
accurate measure of how close the salticid is to facing P. fimbriata
head on.
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This hypothesis might explain puzzling findings from a previous study
(Harland and Jackson, 2001).
Pachyballus cardiforme Berland & Millot is, at least to the human
eye, an especially convincing beetle mimic
(Fig. 5). Almost half of the
P. fimbriata tested with Pachyballus cardiforme classified
it as prey (i.e. they stalked it). However, out of the seven P.
fimbriata that stalked Pachyballus cardiforme, only one adopted
cryptic stalking. The remaining six P. fimbriata that stalked
Pachyballus cardiforme adopted only some of the components of cryptic
stalking and did not consistently freeze when faced. Perhaps, as a consequence
of its resemblance to beetles, the AM eyes of Pachyballus cardiforme
are horizontally centred on a wide face (i.e. the distances between the sides
of the AM eyes of Pachyballus cardiforme and the edges of the
carapace are, compared with other salticids, relatively large even when
facing). This may cause problems for the perceptual processes of P.
fimbriata and account for anomalous reactions to this unusual
salticid.
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Perhaps the most important conclusion from this exploratory study is that
computer-generated virtual lures are applicable in research on visual
processes underlying the complex predatory behaviour of araneophagic jumping
spiders. Clark and Uetz (1990)
established that salticids respond to television (i.e. video tapes). Our study
has shown that araneophagic salticids respond to entirely man-made
computer-generated special-effects creations.
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Acknowledgments |
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