SUMMARY
The behaviour of the Australian bulldog ant Myrmecia nigriceps (FT.
Smith) has been studied by using moving targets characterized by sizedistance
equivalence in relation to a stationary zero-point. The attack
behaviour of freely moving animals demonstrated that the ants can
discriminate between different targets, in the range of 5-80 cm, using movement
parallax to extract information about the targets. By studying the
antenna response it was possible to demonstrate that the stationary bulldog
ant can utilize binocular disparity information and that this mechanism has
an effective range of about 90 mm.
INTRODUCTION
A general problem in visual space perception is concerned with how the brain
assesses sizes, shapes and distances of objects in the external world, when the physical
three-dimensional world is projected onto the two-dimensional retinal mosaic.
Insects are promising experimental animals for both behavioural and physiological
studies of these problems. In the present investigation the Australian bulldog ant was
chosen for the following reasons. Firstly, it has a large interocular base together with
a large binocular overlap (60 ° in total with a crossover of 30 °) thus providing opportunities
for good disparity information. Secondly, it is very sensitive to motion and
will rapidly turn towards moving objects, preparing for an attack. Thirdly, it is
extremely aggressive, rushing towards moving objects, attacking them by biting them
with the 4 mm long jaws and even stinging them.
In her study of depth-vision in this insect, Via (1977) drew attention to an important
methodological question by stating that a crucial requirement of a test of absolute
distance perception is that the animal should be able to respond differentially to
different size-distance combinations characterized by the same angular properties in
the absence of secondary cues (Ittelson, 1960). On the basis of such a criterion Via
concluded that this ant was unable to estimate distances correctly and that instead it
exhibited a size-distance ambiguity.
In view of the fact that several investigations concerned with other species have
demonstrated the operation of different cues in space perception, especially binocular
disparity and movement parallax (Wallace, 1959; Campan, Goulet & Lambin, 1976;
Cartwright & Collett, 1979; Cloarec, 1979; Eriksson, 1980; Rossell, 1980; Goulet,
Campan & Lambin, 1981; Burkhardt & de la Motte (1983) it seems probable that the
intact bulldog ant should be able to utilize some of these mechanisms.
METHODS
Experiment 1
Via's experiments were performed on fixed, isolated heads and the question arises
whether the responses of freely moving animals are also characterized by size-distance
ambiguity or whether these animals are able to discriminate between different sizedistance
combinations when the angular properties are initially the same. To find an
answer to this question an experiment was conducted in which the attack behaviour
of the ants was studied when a moving target, which belonged to a set of equivalent
size-distance combinations (Ittelson, 1960), was presented.
Stimulus
Five stimulus patterns were generated by moving black square targets one at a time
in such a way that the targets subtended the same visual angle and exhibited the same
angular motion properties when viewed from the starting point (see Fig. 1). The
properties of the five targets used in the experiment are shown in Table 1. The targets
were attached to a needle mounted on an X -- Y recorder governed by a Wavetek
function generator. The target moved sinusoidally to and fro with a frequency of 1 Hz.
The background was composed of untextured white cardboard (width 34 cm, height
50 cm) with a luminance of about 125 cd m~2, and the black targets had a luminance
of about 7cdm~2. The surrounding was an arena made of white cardboard
Ant attack behaviour 117
Table 1. Description of stimulus targets in relation to the zero-point
Stimulus SI S2 S3 S4 SS
Size(mm) 5x5 10x10 20x20 40x40 80x80
Distance(mm) 50 100 200 400 800
Amplitude(mm) 7-5 15 30 60 120
Motion frequency was 1 Hz, sinusoidal motion.
34 cm, length 90 cm, height 9 cm) placed over the X -- Y recorder according to the
design of Via (1977).
Conditions
The animals were collected by inserting a thin stick into the entrance hole of the
nest. Those which attacked the stick by biting and clinging to it were selected as
experimental animals. Since the ants in a pilot experiment seemed to exhibit
behavioural adaptation (implying a decline of the responses) it turned out to be
important to avoid artefacts due to adaptation. Therefore a 5 X 5 Latin square design
(Underwood, 1957) was used with the animals randomly distributed to the five
stimulus conditions in order to counterbalance progressive effects. Twenty-five
animals were tested in the experiment, each animal giving one response to each target.
In this way the first response to each target was obtained for five groups of five animals
and the effect of adaptation could be evaluated. The animal was placed in a small
white cardboard cage (diameter 50 mm) which was brought to the arena. The target
was set in motion and the animal was let out through a small opening in the cage (width
10mm, height 10 mm). The time interval between the stimulus presentations was
1 min. The motions of the ant and target were recorded with a television camera and
a video tape recorder.
Experiment 2
The purpose of this experiment was to make a comparative study of the behaviour
of freely moving animals under conditions of monocular and binocular vision. The
previous set-up was modified in such a way that the target (10 X 10 mm black square)
started its motion (moved to the left, see Fig. 4) when the animal came out of the cage.
The distance from the zero-point to the target was 5 -- 20 cm and the target moved with
constant velocity (10cms"1) perpendicular to that distance. The track as well as the
motion of the target were recorded with a television camera and a video tape recorder.
Experiment 3
Since it appears probable that the bulldog ant uses binocular disparity to estimate
distances, at least in 'near space', an experiment was conducted to determine the
maximum range of the mechanism in question. Therefore all other cues (Ittelson,
1960) except binocular disparity should either be occluded or held constant.
Consequently a differential response cannot be ascribed to these cues. However, if the
animal uses binocular disparity (either horizontal disparity or vertical disparity, or
both) then it should be able to respond differentially to the different targets. The
stimuli which fulfil these requirements to a reasonable degree were generated by black
squares moving in depth towards the eye in such a way that the optical changes were
the same for different sizes and distances in relation to a zero-point. Thus the stimuli
were generated according to the principle of 'equivalent distal configurations' (Ittelson,
1960), and presented to a fixed animal in order to occlude movement parallax
information.
Stimuli
The stimulus was one of a set of five different sized squares moved towards the eye
with a constant velocity in such a way that, from a zero-point between the eyes, each
stimulus appeared to be identical to the others (see Fig. 2). The target sizes, distances,
motion amplitudes, target velocities and horizontal disparities are shown in Table 2.
The motions were generated by a Wavetek function generator and an X -- Y recorder
which carried the target. In this way the different targets provided the same optical
information when viewed from the zero-point. The angular subtense of the targets was
18-9° at the starting distance, and 28-1 ° at the stopping distance. Thus the angular
change was roughly 30 ° s"1 (for an exact formula describing the angular velocities and
angular accelerations see Eriksson, 1982). The black target had a luminance of about
7 cd m~2, and the white, untextured background, which subtended a visual angle of
54 ° in height and 37 ° in width, had a luminance of about 125 cd m~z.
Responses
The bulldog ant exhibits a very reliable response to an object suddenly moving
Fig. 2. Design of binocular disparity experiment with size-distance equivalent stimuli.
Table 2. Description of stimulus targets in relation to the zero-point
Stimulus
Size(mm)
Starting
distance (mm)
Stopping
distance (mm)
Target
velocity (cms ')
Disparity
(a2 -- ai) degrees
SI
5X5
15
10
1-67
1609
Horizontal disparity is computed
animals).
S2
25X25
75
50
8-33
3-23
S3
45X45
135
90
1500
1-80
for an interocular base of 3-0 mm
S4
65x65
195
130
21-67
1-25
S5
85X85
255
170
28-30
0-96
(mean of measurements on experimental
Ant attack behaviour 119
So
0-80
0-60
0-40
~ 0-20
0
0 10 20 40
Distance (cm)
Fig. 3. Attack response probability as a function of target distance.
80
towards the animal, especially if the object is approaching obliquely from above. The
animal rapidly lifts the antenna towards the object. The response is visually guided
since it is equally well elicited when a sheet of glass is placed between the animal and
the object. In the present experiment the animal was fixed in wax via a cardboard
bridge from head to thorax and was tilted down about 20°. The antenna response as
well as a meter needle showing the target motion were recorded with a television
camera equipped with a micro-lens. The responses (motion of the antenna tip from
the downward, hanging position to the upward, maximum position) were analysed by
single-frame analysis. A small mirror was placed below the animal in such a way as
to produce an image of the jaw movements. This image was displayed on the television
screen together with the above mentioned information.
Procedure
Since a preliminary experiment using very short pauses between trials had demonstrated
behavioural adaptation with regard to both the antenna responses and the jaw
responses, the following procedure was used in order to obtain data which were not
biased by adaptation effects or other habituation effects. In order to fulfil these
requirements the experiment was conducted using a randomized 5 x 5 Latin square
design (Underwood, 1957). Five ants were used, each receiving five trials in each of
the five blocks (stimulus distances). The time interval was 10 min between blocks and
60 s between trials within blocks. A later check of the data revealed no systematic
changes due to presentation order.
RESULTS
Experiment 1
No significant adaptation effects could be established when the data in the Latin
square were analysed. Of 125 stimulus presentations the animals exhibited an attack
response (moving towards the target and biting it) in 53 cases. On the basis of the
120 E. S. ERIKSSON
hypothesis that the bulldog ant cannot discriminate different size-distance combinations
it is expected that the attack responses should be randomly distributed among
the five stimulus conditions. This, however, is not the case. A Cochran Q-test for
related samples (Siegel, 1956) reveals that the overall differences between the conditions
are statistically significant (P< 0-001). When the attack response probability
is plotted against target distance it is evident that the responses are related to target
distance (Fig. 3).
On average the animals attacked the smallest target in 79 % of the presentations and
the largest target in only 16% of the presentations. This is supported by an analysis
of the movements of the animals. Typically, with the target at shorter distances (5 and
10 cm) the animal comes out of the cage and without delay rushes directly towards the
target, biting it and sometimes even stinging it. In the group of five ants given the
smallest target first, all the animals attacked it.
In contrast, the animals reacted differently to the largest target. Here the typical
behaviour was characterized by hesitation and even what might be termed an
avoidance reaction. After being released from the cage the animals usually moved
rather slowly towards the target. But after a short while they hesitated and started to
move sideways (like the target) and finally even turned back, moving away from the
target. A few animals started rushing towards the target, then slowed down and turned
back. In fact, one of the animals only moved 10 cm, then started to reverse while still
watching the target and making small lateral movements! This animal, as well as the
ones showing the avoidance reaction to the largest target, usually demonstrated no
such responses to the smallest target. More exactly, the largest target elicited only 4
attacks out of 25 presentations, while the smallest target elicited 18 attacks. The
avoidance reaction was present in 16 cases out of 25 when the largest target was used
but only in one case out of 25 when the smallest target was used. These two differences
are statistically significant when tested with the Cochran Q-test for related samples
(P<0-001). The mean turning point was 47-5cm from the zero-point, i.e.
approximately half-way to the largest target.
From the results above it is clear that the reactions of the experimental animals are
not confused by size-distance ambiguity. Instead the animals, in one way or another,
were able to extract differential information for the different conditions. The results
strongly support the conclusion that the ants accurately perceived the different targets,
that the nearest target was experienced as small (or close or moving with low
velocity) and thus could safely be attacked, but that the largest targets were experienced
as large and/or fast moving and should be avoided. The experiment, however,
does not allow any conclusions to be drawn as to whether the ants were using binocular
disparity, motion parallax or both of these mechanisms. To answer these questions,
the following two experiments were conducted.
Experiment 2
Intact animals
When released from the cage, the intact animal usually started moving towards the
moving target in a characteristic path ending with an attack (biting and sometimes
stinging the target). A typical track showing an attack by an intact, binocular animal
is presented in Fig. 4.
Ant attack behaviour 121
Distance (cm)
Fig. 4. Attack trajectory for an intact, binocular animal. Solid line, position vector curve. Dashed
curved line, velocity vector curve; dashed straight line illustrates the interception strategy. Dots show
animal positions sampled during the locomotion; see text.
Motion parallax animals
Two different ways of investigating the role of motion parallax were tested.
(a) The left eye of each of a group of six ants was entirely covered with paint, thus
effectively destroying disparity information. These animals were tested as above using
different target distances (5 -- 30 cm), different target sizes (5 -- 40 mm squares), as well
as several other objects (including a grasshopper and a living ant tied to the moving
target holder). Out of a total of 24 trials only one attack (at 5 cm distance) was
recorded. The dominant behaviour was that of the ant disregarding the target and
slowly moving along different tracks unrelated to the target. This behaviour may be
due to the fact that by painting the whole left eye we may have destroyed not only the
binocular disparity mechanism but also motion parallax which reasonably should
require an interaction between the two optical flow fields in the two eyes.
(b) In order to test the idea that motion parallax requires an interaction between
the two eyes (but with binocular disparity occluded) we painted the left eye of two
animals in such a way that disparity was destroyed but the animal could still use the
lateral part of the eye (paint covering about 37 columns of facettes or about 30 ° off the
median plane; see Via, 1977, Fig. 9) as well as the corresponding areas below, above
and behind the lateral centre of the eye. In this way the left eye had an intact, roughly
circular area, the centre of which looked out to the left. Since the right eye of these
animals was intact they should be able to use movement parallax despite the lack of
disparity information.
When these animals were tested they both moved around quite normally and also
attacked the moving target in the same way as normal animals. Fig. 5 shows the track
of a successful attack ending with the ant biting the target.
From the outcomes of these two tests we conclude that binocular disparity is not
necessary in order to release attack behaviour. The data suggest that the proper attack
behaviour is possible on the basis of movement parallax information and that this
mechanism requires an interaction between the two eyes.
122 E. S. ERIKSSON
7-5 10
Distance (cm)
Fig. S. Attack trajectory for a binocular movement parallax animal. Solid line, position vector curve;
dashed line, velocity vector curve; see text.
Animals with frontal retinas
Some new ants were used in a study in which both eyes were painted symmetrically
in such a way that motion information from the lateral parts was occluded and only
information from the frontal (disparity) parts of the eyes was provided (see Via, 1977,
Fig. 9). Thus the animals should be able to perceive the target properly using these
frontal areas even if motion information from the lateral parts is missing.
The results showed that the attack behaviour could be elicited in these animals even
if they sometimes did not appear to see the target. The result from an attack of one
animal is shown in Fig. 6.
In summary, experiment 2 has demonstrated that normal attack behaviour is found
in animals when binocular disparity, but not bilateral motion information, is occluded
as well as in animals which only had the frontal retinal areas intact. From this outcome
we conclude that binocular disparity is not a necessary condition for attack behaviour
in the bulldog ant and that the data suggest the operation of a motion parallax mechanism
which requires interaction between the two eyes.
Analysis of attack strategies
In order to describe the attack behaviour in the present situation, i.e. the trajectory
over time in relation to target motion, three different attack strategies will be discussed.
The interception strategy
This strategy, which is used by some species, e.g. bats and hoverflies (Collet &
Ant attack behaviour 123
Land, 1978), implies that the animal correctly perceives the situation and from values
of distance to the target (D), target velocity (Vr), velocity of the organism (Vo) and
time (t) computes the predicted point of collision (Pc) (see Fig. 7).
I n this case the animal moves with, say, constant velocity (Vo = k) towards the predicted
point of collision. Thus the mathematical criterion for this strategy is given by:
dP0/dt = V0. (1)
The position vector (P0.i) of the animal at time (t) can be obtained by integrating the
velocity vector (Vo). Hence we obtain:
Po = f Vodt = / (Vo cos 0)dt + / (Vo sin 0) dt. (2)
12-5
u
i
"5.
.2
-o
2-5 7-5 10
Distance (cm)
Fig. 6. Attack trajectory for a binocular (frontal retinae) animal. Solid line, position vector curve;
dashed line, velocity vector curve.
o,t
D
Fig. 7. The interception strategy. Vo, velocity vector of the animal; Po, position vector over time;
PCl point of collision; VT, target velocity; D, distance; Ojnt, interception angle.
124 E. S. ERIKSSON
Consequently the position vector of the organism is given by:
Po, t = (Vot COS0)J + (V0t a'md)j. (3)
Using the Pythagorean theorem, it can be shown that the time to collision is given by:
fo2 -- VT
2). (4)
The predicted point of collision can be computed from target velocity (VT) and time
(t). Hence we find
Pc=VTto
Finally the interception angle (ftnt) is given by:
ftn, = arctan VT/D - [D2/(VO
2 -- V,2)]0s.
(5)
(6)
The target-locking strategy
According to this strategy the animal moves straight towards the target during the
attack. However, this strategy can be implemented by applying at least two different
mathematical criteria.
The position vector strategy. If we regard the position vector of the animal over
time, the criterion implies that the position vector from the starting point should be
directed towards the target all the time. This criterion (see Fig. 8) is then expressed
by:
6 = arctan V-rt/D. (7)
Fig. 8. Position vector strategy. Position vector of the animal (PQ) as a function of distance (D) and
target position vector (Pr) -
Ant attack behaviour 125
By inserting this value of 6 in vector equation (3) we obtain the following formula for
the position vector of the animal over time:
o,/ = [V<ycos(arctan Vt-t/D)-t]i + [Vcrsin(arctan Vt-t/D)-t]_;. (8)
Thus, if we know the velocity with which the animal moves as well as target velocity,
time and distance from animal to target, then we can compute the theoretical trajectory.
This theoretical curve (the position vector curve) has been computed by first
roughly estimating the animal's velocity from data and then adjusting the values of
Vo requiring that the curve should predict the empirical collision point. The obtained
curves are shown in Fig. 4 (VT=10cms~1, Vo = 17'6cms"'), Fig. 5 ( V T = 1 0
cms"1, Vo^ZScms"1) and Fig. 6 (VT= lOcms"1, Vo = 14-5cms"1).
The velocity vector strategy. A different, and apparently more meaningful
criterion, implies that the velocity vector should be directed towards the target all the
time. Mathematically this criterion is defined by:
0=arctan[(VTt -- y,)/(D -- x,)], (9)
where VTt gives the target position over time and Xt and yt are the coordinates of the
animal over time, see Fig. 9.
By using a computer and an iterative method, successive values for optional time
intervals (At) could be defined for 6 and hence the coordinates xt and yt could be
computed according to the following formulae:
0n = arctan[(nAtV, - y n - l)/(D-xn - i ) ] , (10)
Xn = Xn-l+ VoAtCOS0n, (11)
yn = yn-i+VoAtsin0n. (12)
Distance
Fig. 9. Velocity-vector strategy. Illustration of the animal position vector (Pa) generated by the
velocity vector (VQ) fixated at the target (T). PT position vector of target; xt, yt, coordinates of the
animal. The theoretical trajectory is shown by the solid line. The dashed line through the point of
collision (Pc) shows the trajectory according to the interception strategy.
126 E. S. ERIKSSON
4 r
aI
2
co
£ 1
0
10 50 90 130 170
Distance (mm)
Fig. 10. The antenna response as a function of target distance. Vertical bars show standard deviations.
By using different time intervals (At) it was found that no significant increase in
precision could be obtained with time intervals shorter than 0-03 s. The theoretical
velocity vector curves are shown in Figs 4, 5 and 6 (VT = 10cms"1, and Vo = 19-6,
29-4 and 12-75 cms"1, respectively).
A comparison between the different theoretical attack strategies indicates that the
bulldog ant utilizes a position vector strategy (Figs 4-6).
Experiment 3
The bulldog ant demonstrates a three-fold behavioural reaction to the approaching
targets under the present conditions. It starts moving its legs rapidly, especially if the
target is close to the animal. It opens its jaws and sometimes shows snapping
behaviour (Via, 1977). The snapping reaction never appeared when the three most
distant targets were presented but in some cases the reaction was elicited by the nearer
targets. However, the jaw reactions were too infrequent to provide reliable data. The
animal finally exhibits an antenna reaction which turned out to be a very good indicator
of target distance. The animals reacted every time the target was presented at
the shortest distance and the amplitude of the response was related to target distance.
(Fig. 10).
Obviously the antenna response is graded with regard to target distances up to about
90 mm. Thereafter the animals cannot discriminate between the different target distances
(or disparities) but give the same, small response to the three larger distances.
A statistical analysis (Friedman's two-way analysis of variance, see Siegel, 1956)
demonstrated an overall significant difference between distances (P<002). The
difference between stimuli S3 and S5 (see Table 2) is not significant, while the
difference between stimuli SI and S3 is significant (P < 0-025) as are the differences
between stimuli SI and S2 (P< 0-025), and between stimuli S2 and S3 (P<0-05)
according to the randomization test for matched pairs (Siegel, 1956). The bulldog ant
can therefore discriminate between different, short target distances (0-90mm).
Ant attack behaviour 127
In order to check whether the antenna responses were due to binocular vision or to
monocular optical changes, a new ant was tested using the same stimuli but with larger
motion amplitudes (the motion path distances in Table 2 were doubled, maintaining
the same stopping distances). Under binocular conditions the animal reacted every
time to the shorter distances but no response at all was evoked for the larger distances.
However, when the entire left eye was covered with a non-toxic water-soluble paint
no responses could be elicited under any condition despite several trials. The animal
was hanging as dead, exhibiting no antenna responses, no leg movements and no jaw
movements. In the final test a thin, sharp needle was used to remove the paint from
the eye. When the animal was tested again it now quite clearly demonstrated the
normal behaviour with antenna, legs and jaws. For about 10 stimulations with the
smallest target it responded every time in the normal way with the antenna. It is
concluded that the antenna reaction is due to binocular vision and not to monocular
factors or possible artefacts, either mechanical, thermal or chemical.
Theoretical analysis
In order to arrive at an exact estimation of disparity information present in the
experimental targets we computed the binocular disparities (ai -- zq) for the different
targets (Fig. 11).
The values for ai were computed according to the formula:
ai = arctan(S/2-B/2)/D (13)
and for m according to:
a2 = arctan(S/2 + B/2)/D. (14)
The disparity information then can be defined as a2~ai (Table 2).
S2
Fig. 11. Computation of binocular disparity. LE, left eye; RE, right eye; B, interocular basis
(3 mm); D, distance from eye to target; S, target size; ai, a2, binocular disparities.
128 E. S. ERIKSSON
If binocular disparity constitutes an effective stimulus, the reaction (the antenna
response) should be a function of the stimulus. A well established principle from
psychophysics states that the response is a power function of the stimulus, i.e.
R = aSn. (15)
Consequently, we can find out whether the response (R) is a power function of the
stimulus (S) by plotting the logarithm of the antenna response against the logarithm
of the stimulus (disparity). This implies that
logR = loga + nlogS. (16)
A plot of the data revealed that the log R-log S relationship was linear with an intercept
approximately equal to zero (loga) and a slope (n) equal to 0-48. Thus:
R = S048, (17)
which implies that the antenna reaction is approximately equal to the square root of
disparity. This theoretical curve (the solid line in Fig. 10) is drawn through the data
points and the correspondence is fairly good.
In order to find the maximum range of binocular disparity we need some measure
of threshold sensitivity. Using the fact that the interommatidial angle (A#) is 2° (Via,
1977), a calculation on the basis of the formulae above shows that a disparity threshold
of 2° corresponds to a maximum distance of 80 mm. A theoretical calculation for
maximum distance discrimination according to the formula b = 2a/n (Ogle, 1959),
or according to the formula by Burkhardt et al. (1973), yields estimates of 86 mm
(85-! and 85-9 mm respectively) for maximum distance discrimination. The formula
of Burkhardt et al. (1973) assumes that the optical axes are parallel. This may be a
correct approximation in several cases (Chmurzynski, 1963; Horridge, 1977;
Cloarec, 1979) and the correspondences between the different formulae are acceptable.
DISCUSSION
The present study provides evidence for depth perception in the Australian bulldog
ant Myrmecia nigriceps. Because insects have a fixed-focus system with a large depth
of focus and have immovable eyes, they cannot utilize the cues of accommodation and
convergence. Of several possible variables or cues influencing the perception (Gibson,
1950; Ittelson, I960), there are two which now seem to play a dominant role in
space perception, namely binocular disparity and motion information (movement
parallax).
Disparity theory
As pointed out by Via (1977), binocular vision does not necessarily imply depth
perception, i.e. stereopsis. However, certain species do use disparity information to
govern their behaviour (Maldonado & Levin, 1967; Maldonado, Benko §c Isern,
Ant attack behaviour 129
1970; Maldonado& Rodriguez, 1972; Rossell, 1980; Burkhardt&de la Motte (1983).
The present study provides further support for a disparity mechanism in insects since
the data show that the Australian bulldog ant can utilize disparity information in near
space (0 to about 90 mm).
Theoretical calculations by Burkhardt et al. (1973) and Horridge (1977), as well
as in the present study, demonstrate a good correspondence between theory and data
but also that binocular vision in insects has a restricted range. Hence, outside this
range other factors must be responsible for their visually guided behaviour, a conclusion
which has also been drawn by Goulet et al. (1981).
Movement parallax
Since insects exhibit behaviour which clearly implies that they perceive the size of
objects far beyond the range of binocular disparity, they must possess a far-range
visual mechanism which gives them correct information about the gross spatial
relationship in their environment (Horridge, 1977). A recent investigation, (Goulet
et al. 1981) demonstrated that crickets can discriminate distances up to about 1 m.
Motion parallax has also been shown to be effective in the grasshoppers (Wallace,
1959; Collett, 1978; Eriksson, 1980). The present study provides corresponding data
for the bulldog ant (experiment 1 vs experiment 3). Of special interest here are the
differential reactions (attack vs avoidance) to the smallest and largest targets. The
avoidance reaction was on average elicited 33 cm from the larger targets and in some
animals as far as 70 cm from the target. Obviously these distances are outside the range
of binocular disparity.
The mechanism which is probably responsible for far-range visual information is
movement parallax, a mechanism which implies that optical size-distance ambiguity
is resolved via body-state information in the moving animal (Eriksson, 1973, 1974).
Attack strategies
This study has demonstrated that bulldog ants do not utilize an interception
strategy during attack. Nor do they seem to utilize a velocity-vector strategy, which
may appear to be a very simple method (cf. Lanchaster & Mark, 1975). However, the
velocity-vector strategy implies a longer locomotion path than the other two strategies
and therefore it is not as economical as, for example, the interception strategy. On the
other hand, the interception strategy means that the predator has to start its motion
at a large interception angle, which in turn presupposes that the prey will continue to
move in a predictable manner. If the prey avoids the collision by a course change, then
the predator has to execute gross corrections and the interception strategy may be
rather inefficient, especially if the prey completely reverses its course.
A compromise between an interception strategy and a velocity-vector strategy
would be to use a reduced interception angle in order to avoid extreme course changes.
This is essentially what the position-vector strategy means and hence it may be an
optimal method for attack when the prey is able to change its course rapidly. The
present data (Figs 4, 5, 6) indicate that the bulldog ant utilizes an attack strategy
which can roughly be described as the position-vector strategy. However, since some
data indicate a goal gradient (animal velocity increases the nearer it is to the goal), it
may be possible that a velocity-vector strategy with variable locomotion velocity will approach a position-vector strategy with constant velocity. This question needs
clarification in future studies.
With regard to the neural mechanisms underlying attack behaviour, this study has
shown that the bulldog ant may utilize both binocular disparity and movement parallax
to gain information about the moving object. However, since in both cases motion
information is involved it is possible that there is one single neural principle operating
in these seemingly different cases. Such a principle would require synchronous
changes of optical stimulation over time in a way which is reminiscent of common
motion extraction according to the rules of neural vector analysis that appear to govern
the functioning of certain neurones in the optic lobe of the blowfly Phormia terraenovae
(Eriksson, 1984).
The next step, therefore, is to conduct neurophysiological studies on the bulldog
ant in order to discover if such neural mechanisms exist.
The author wishes to express his gratitude to Professor G. A. Horridge for his
encouraging support of the study, and to Dr Dan Nilsson for many stimulating
discussions.
Study published with permission.
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