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© 1999 Macmillan Magazines Ltd
2. Garrett, S. L., Adeff, J. A. & Hofler, T. J. J. Thermophys. Heat
Trans. 7, 595–599 (1993).
3. Garrett, S. L. US Pat. No. 5,647,216 (1997).
4. Swift, G. W. Proc. DOE Natural Gas Conf. (Fed. Energy Tech.
Cent., Morgantown, West Virginia, 1997).
5. Ceperley, P. H. J. Acoust. Soc. Am. 72, 1688–1694 (1982); US
Pat. No. 4,114,388 (1979).
6. Swift, G. W. J. Acoust. Soc. Am. 92, 1551–1563 (1992).
7. Swift, G. W., Gardner, D. L. & Backhaus, S. J. Acoust. Soc. Am.
105, 711–724 (1999).
8. Reid, R. S., Ward, W. C. & Swift, G. W. Phys. Rev. Lett. 80,
4617–4620 (1998).
9. Smith, R. W. M., Keolian, R. M., Garrett, S. L. & Corey, J. C.
J. Acoust. Soc. Am. 105, 1072 (1999).
10.Chen, R.-L. & Garrett, S. L. in Proc. 16th Int. Congr. Acoust.
Vol. 2 (eds Kuhl, P. K. & Crum, L. A.) 813–814 (Acoust. Soc.
Am., Woodbury, New York, 1998).
11.Hofler, T. J., Adeff, J. A. & Atchley, A. A. J. Acoust. Soc. Am. 101,
3021 (1997).
12.Yarr, G. A. & Corey, J. C. US Pat. No. 5,389, 844 (1995).
can, and do, strike at a target using visual
information from just one eye.
The stronger refraction provided by the
cornea (as opposed to the lens) means that,
in each animal, the nodal point — the point
within the eye at which the lines connecting
points in the scene and corresponding points
in the image intersect — lies well in front of
the centre about which the eye rotates
. As a
result, when the eye rotates, the images of
objects at different distances move by differ-
ent amounts on the retina (Fig. 2). This
means that rotation of a single eye can pro-
vide information about the relative distances
of different objects. The animal can even
estimate absolute distances if the amount of
eye rotation is known. By gauging the dis-
tances of objects by rotating just one eye,
without using binocular stereopsis, and
without having to move the whole head as
some insects do
, the chameleon and the
sandlance minimize their chances of being
detected by potential prey
. At present,
however, such a means of distance estima-
tion by these animals is only a theoretical
Despite belonging to such different
families, the chameleon (a reptile) and the
sandlance (a fish) seem to have converged on
a common set of design principles for their
visual systems. So, in these two animals, eye
design cannot be predicted by evolutionary
origin — rather, it has been crafted almost
exclusively by environmental constraints.
But the visual system of the sandlance seems
to differ from that of most other vertebrates
(including the chameleon) in that, after the
eye has moved to a particular position, it
does not stay there. Instead, Fritsches and
find that the gaze drifts slowly back
towards the central viewing direction, pre-
sumably because of restoring forces exerted
by the muscles that rotate the eye. This drift
challenges the accepted idea
that animals
strive to keep a still image of the world on
news and views
VOL 399
27 MAY 1999
volution has generated a bewildering
variety of life forms on this planet.
Curiously, it has also occasionally pro-
duced animals from entirely different fami-
lies that seem to have converged on a similar
design plan — apparently as a result of simi-
lar lifestyles and environmental pressures.
One instance of this, reported by Pettigrew et
and Fritsches and Marshall
in Current
Biology, is vision in the chameleon and in the
sandlance Limnychthyes fasciatus, a small
teleost fish
The chameleon catches insects with a
quick flick of its long tongue
. The sandlance
lies concealed in underwater sand beds with
only its eyes protruding above the surface,
then lunges at tiny, unsuspecting copepods
(aquatic crustaceans) that stray too near
Pettigrew et al. and Fritsches and Marshall
now show that there are extraordinary paral-
lels between these two behaviours, and
between the visual systems that mediate
To a casual observer, the dart of the
chameleon’s tongue is very similar in its
trajectory to the lunge of the sandlance. The
sandlance is, in effect, a chameleon’s tongue
with eyes at the tip
. Moreover, both the
chameleon and the sandlance move the two
eyes independently and alternately (Fig. 1)
— while one eye remains motionless, the
other scans the environment in a saccadic
(jerky) fashion, presumably seeking prey.
The two eyes alternate when looking around
the environment, and each covers about half
the panorama. Scanning alternately with the
two eyes reduces the chance of alerting the
prey by tell-tale eye movements. Plus, given
that the two eyes are not yoked together to
look in the same direction, it would be
sensible not to move them simultaneously.
Otherwise, it could be difficult for the ani-
mal to decide whether the two eyes were
viewing the same target.
In both animals, the cornea of the eye —
rather than the lens — provides the bulk of
the refracting power. This extends the effec-
tive focal length of the eye, giving it a higher
angular resolution
. Moreover, the cornea
(not the lens) enables the eye to change its
focus and view objects at different distances.
This property makes the sandlance very
different from most other fish, which focus
mainly through the lens
. Pettigrew et al.
show that, in both the sandlance and the
chameleon, the refracting power of the
cornea is adjusted by a fast-acting, striated
‘cornealis’ muscle. There is also evidence that
the chameleon perceives depth by monitor-
ing how much one eye is focused, rather than
by combining the images from both eyes
. The sandlance probably adopts
a similar tactic
, meaning that both animals
When one eye is better than two
Mandyam V. Srinivasan
Figure 2 How the sandlance and the chameleon could gauge depth by rotating a single eye. a, When
the eye faces straight ahead, objects A and B are imaged at the same place on the retina. b, When the
eye rotates, however, the image of A moves by a larger amount than that of B, revealing that A is closer
than B. This method of estimating distance is possible only if the nodal point of the eye (N) is located
well in front of the centre of rotation of the eye (C), as is the case in these animals.
Figure 1 Keeping an eye on things —
independent eye movements in the chameleon
and sandlance. The activity of the two eyes
alternates, each covering about half the
panorama. Pettigrew et al.
and Fritsches and
show that there are many other
parallels between the visual systems of these two
animals, which are surprising given that they
come from such evolutionarily distinct families.
. D
. P
© 1999 Macmillan Magazines Ltd
their retina most of the time, to minimize
blur and to detect stationary or moving
objects more easily.
So why does the sandlance’s gaze drift?
One possibility is that there is simply not
enough visual texture in the murky, under-
water world in which it lives to help stabilize
the eye
. Another intriguing idea is that the
slow drift is exploited by the sandlance to
infer the distances to objects (Fig. 2). But, as
Fritsches and Marshall point out, another
advantage in drifting back to the central
position is that the eye is then best positioned
to move to the next target of interest, which
may appear anywhere in the visual field. This
strategy is akin to that used by tennis players,
who return to mid-court after dispatching
each ball. Indeed, the sandlance eye may
be a drifter with a purpose, and not just an
aimless wanderer.
Mandyam V. Srinivasan is at the Centre for Visual
Sciences, Research School of Biological Sciences,
Australian National University, PO Box 475,
Canberra, Australian Capital Territory 2601,
1. Pettigrew, J. D., Collin, S. P. & Ott, M. Curr. Biol. 9, 421–424
2. Fritsches, K. A. & Marshall, N. J. Curr. Biol. 9, R272–R273
3. Land, M. F. Curr. Biol. 9, R286–R288 (1999).
4. Pettigrew, J. D. & Collin, S. P. J. Comp. Physiol. A 177, 397–408
5. Harkness, L. Nature 267, 346–349 (1977).
6. Land, M. F. Nature 373, 658–659 (1995).
7. Wallace, G. K. J. Exp. Biol. 36, 512–525 (1959).
8. Kral, K. Behav. Processes 43, 71–77 (1998).
9. Walls, G. L. Vision Res. 2, 69–80 (1962).
Gone away, gone ahead,
Echoes roll unanswered.
Empty, open, dusty, dead.
Why have all the Weyrfolk fled?

Where have dragons gone together
Leaving weyrs to wind and weather,
Setting herdbeasts free of tether;
Gone, our safeguards, gone, but whither?

Have they flown to some new weyr
Where cruel Threads some others fear?
Are they worlds away from here?
Why, oh why the empty weyr?

-- "The Question Song", Anne McCaffrey
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