Why is it worth flying at dusk for aquatic insects? Polarotactic water detection is easiest at low solar elevations
1 Biooptics Laboratory, Department of Biological Physics, Eötvös
University, H-1117 Budapest, Pázmány sétány 1,
Hungary,
2 Plant Protection Institute of the Hungarian Academy of Sciences,
Department of Zoology, H-1525 Budapest, P. O. B. 102, Hungary
3 International University Bremen IUB, School of Engineering and Science, P.
O. B. 750561, D-28725 Bremen-Grohn, Germany
* Author for correspondence (e-mail: gh{at}arago.elte.hu)
Accepted 20 November 2003
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Summary |
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Key words: dusk-flying water insects, aquatic habitat recognition, polarotaxis, reflection polarization, polarization sensitivity, 180° field-of-view imaging polarimetry
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Introduction |
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The aim of the present study is to show that the daily change in the
reflection-polarization pattern of water surfaces is a further important
visual ecological factor that may contribute to the preference for the
twilight period for habitat searching by polarotactic water insects. These
insects detect water by means of the horizontal polarization of light
reflected from the water surface (Schwind,
1991,
1995
;
Horváth and Varjú,
2003
). Using 180° field-of-view imaging polarimetry, we
measured the reflection-polarization patterns of two artificial surfaces
(water-dummies) in the red, green and blue spectral ranges under clear and
partly cloudy skies at different solar elevations. The water-dummies were
composed of a horizontal glass pane underneath which was a matt black or a
matt light grey cloth, which imitated a dark or bright water body,
respectively.
Assuming that polarotactic water insects interpret a surface to be water if the degree of linear polarization of reflected light is higher than a threshold and the deviation of the direction of polarization from the horizontal is lower than a threshold, we calculated the proportion, P, of the dummy surface detected polarotactically as water. We found that at sunrise and sunset P is maximal for both water-dummies, and at these times their reflection-polarizational characteristics are most similar. From this, we conclude that polarotactic water detection is easiest at low solar elevations, because the risk that a polarotactic insect will be unable to recognize the surface of a dark or bright water body is minimal. This partly explains why many aquatic insect species usually fly en masse at dusk. As the air temperature at sunrise is generally too low, dusk is the optimal period for polarotactic aquatic insects to seek new habitats.
The results presented here could be achieved using the 180°
field-of-view imaging polarimetry developed recently by Gál et al.
(2001a,b
)
and Horváth et al.
(2002
), with which the full
polarization pattern of horizontal reflecting surfaces can be measured. This
technique made it possible to measure the P-values of polarizing
surfaces in the entire lower hemispherical visual field of a hypothetical
flying polarotactic water insect. Previously, such measurements and the
derivation of P could not be performed, because earlier imaging
polarimetric measurements (e.g.
Horváth and Zeil, 1996
;
Horváth and Varjú,
1997
; Horváth et al.,
1997
,
1998
;
Kriska et al., 1998
;
Bernáth et al., 2002
)
were restricted to relatively small (
40°x50° maximum)
fields of view.
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Materials and methods |
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Our imaging polarimeter was a Nikon F801 photographic camera equipped with a Nikon-Nikkor fisheye lens with a 180° field of view, 8 mm focal length and 2.8 f-number. Fujichrome Sensia II 100 ASA colour reversal films were used as a detector. The fisheye lens possessed a filter wheel with linearly polarizing (HNP'B, Polaroid) filters with three different directions of the transmission axes. With this technique, polarization patterns can be measured in the red (650±30 nm, wavelength of sensitivity maximum ± half bandwidth), green (550±30 nm) and blue (450±50 nm) spectral ranges.
The polarimeter with down-facing fisheye lens was suspended on a holder
above the centre of the water-dummy in such a way that the vertical optical
axis of the lens pointed towards the nadir
(Fig. 1A). In order to minimise
the disturbance of the shadow of the holder on the dummy surface, different
holder orientations relative to magnetic north were chosen
(Fig. 1C). The distance between
the outermost surface of the fisheye lens and the glass surface was as small
as possible (7 cm) in order to measure the reflection-polarizational
characteristics of the water-dummies in a conical field of view as wide as
possible (160°). The fisheye lens was focused into infinity to record
the mirror image of the sky reflected from the glass surface. For a complete
measurement, three photographs were taken through the polarizers with three
different transmission axes. This took
10 s, during which the operator
triggered the expositions by a remote cord and turned the filter wheel of the
polarimeter three times. During measurements, the operator lay on the ground
below the level of the glass pane to avoid unwanted reflections
(Fig. 1A). After the
measurement of a water-dummy, it was replaced by the other dummy within
1
min and the procedure was repeated. This allowed us to measure the
reflection-polarization patterns of both dummies within a few minutes, i.e.
under almost the same illumination conditions and at the same solar elevation
(
s).
Measurements were carried out near the time of the summer solstice under
sunny, partly cloudy skies on 17 July 2002 and under sunny, cloudless clear
skies on 18 July 2002 near Kunfehértó in Hungary
(46°23' N, 19°24' E) from sunrise (04:49 h; local summer
time = Universal Time Conversion + 2 h) to an hour after sunset (20:37 h) at
the different s shown in
Fig. 1C. The maximum
s was 67° at noon (12:56 h). Because of disturbance by
early morning dewfall, reflection-polarization patterns at low solar
elevations are presented here only for the sunset and dusk period.
The evaluated reflection-polarization patterns are presented here in the
form of circular maps, the centre and perimeter of which are the nadir and the
horizon, respectively. The numerical values of the degree (d) and
angle () of linear polarization are coded by different shades of grey
and colours. In these maps, the water-dummies cover an approximately circular
area as wide as
160°. The azimuth angle (
) of a given direction
of view is measured clockwise from the solar meridian, and its nadir angle
(
) is measured radially in such a way, that
is proportional to the
radius (nadir:
=0°, horizon:
=90°). Note
that the polar system of coordinates used for the representation of the
reflection-polarization patterns is simply the mirror image of the celestial
polar system of coordinates. Although during measurements the direction of the
polarimeter holder relative to the fixed dummies changed as the sun moved
along its celestial arc (Fig.
1C), for the sake of a better visualization in Figs
2,
3,
4 we present all circular
pictures rotated in such a way that the actual solar meridian always points
vertically upwards, since these patterns are symmetrical to the
solar-antisolar meridian under clear skies.
|
The mirror image of the polarimeter, its holder and the remote cord, as
well as their shadows (Fig.
1A), moved counter-clockwise with respect to the solar meridian
over time (Figs 2,
3). Our aim was to compare the
reflection-polarization patterns of the two water-dummies. Therefore, for
comparative analyses, we excluded regions (chequered in Figs
2,
3) inwhich landscape near the
horizon, unwanted overexposure, disturbing shadows or mirror images of the
polarimeter, its holder and remote cord occurred in the individual pictures
taken at a given s. Thus, for both dummies at a particular
s, we obtained a time-dependent mask, the area of which was
inappropriate for comparative analyses and from which viewing directions were
not taken into account. Hence, in comparative analyses, only those viewing
directions were considered where the mirror image of the sky and the
polarizational characteristics of the reflected skylight could be registered
without any disturbance.
The reflection-polarization patterns of a perfectly black water-dummy
(Fig. 4), absorbing the
penetrating component of incident light, were calculated with the mathematical
method developed by Schwind and Horváth
(1993), Horváth
(1995
) and Gál et al.
(2001a
) for incident
single-scattered Rayleigh skylight.
Schwind (1985) showed that
backswimmers (Notonecta glauca) avoid a light source emitting
vertically polarized light. The same was demonstrated in dragonflies
(Horváth et al., 1998
;
Wildermuth, 1998
), mayflies
and many other water-loving insects (Schwind,
1991
,
1995
;
Kriska et al., 1998
;
Bernáth et al., 2001b
).
Polarotactic water insects consider any surface as water if (1) the degree of
linear polarization of reflected light (d) is higher than the
threshold of polarization sensitivity (dtr) and (2) the
deviation of the angle of polarization of reflected light from the horizontal
(
) is smaller than a threshold (
tr) in
that part of the spectrum in which the polarization of reflected light is
perceived. Therefore, an imaginary polarotactic water insect levitating above
the centre of our water-dummies was assumed to interpret as water those areas
of the dummies from which skylight is reflected with the following two
criteria: (1) d>dtr=5% and (2)
|
-90°|<
tr=5°. We
introduce the quantity `percentage P of a reflecting surface detected
as water', which is the angular proportion, P, of the viewing
directions (relative to the angular extension of 2
steradians of the whole
lower hemisphere of the field of view of the insect) for which both criteria
are satisfied. In other words, P gives the relative proportion of the
entire ventral field of view in which the water-dummies are considered
polarotactically to be water. The higher the P-value for a reflecting
surface in a given visual environment, the larger its polarotactic
detectability; i.e. the higher the probability that a water-seeking insect can
find it by polarotaxis. Thus, for the sake of simplicity, P is
subsequently called `polarotactic detectability'. Using the Mann-Whitney test
and the statistical program SPSS (version 9.0), the P-values
calculated for the grey water-dummy were compared with those of the black
dummy in the blue, green and red parts of the spectrum.
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Results |
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Column 2 in Figs 2 and
3 shows the patterns of the
degree of linear polarization (d) of skylight reflected from the
water-dummies in the blue part of the spectrum at different solar elevations.
The grey water-dummy is less polarizing than the black one. The light
reflected from it is almost unpolarized in many directions of view, and its
maximum d is only 30%. At the Brewster angle - at which the
surface-reflected light is totally and horizontally polarized (56° from
the nadir for glass) - very low d-values occur in many azimuth angles
(Fig. 2). The black water-dummy
is an effective polarizer, reflecting highly polarized skylight from many
directions of view. At the Brewster angle, a continuous annular region,
subsequently called the `Brewster zone', occurs with maximum d.
Depending on
s, two neutral points with unpolarized
reflected skylight appear within the Brewster zone perpendicular to the solar
meridian (Fig. 3).
Column 3 in Figs 2 and
3 shows the patterns of the
angle of polarization () of skylight reflected from the water-dummies
in the blue part of the spectrum. For the grey dummy, as the solar elevation
increases, the proportion of the nearly vertically polarized reflected
skylight with -45°<
<45° (shown in red and yellow) becomes
dominant over the nearly horizontally polarized reflected skylight with
45°<
<135° (shown in green and blue) perpendicular to the
solar meridian. However, from regions of the grey water-dummy towards the
mirror image of the sun, approximately horizontally polarized light is always
reflected. At near-zero solar elevations, this is also the case for regions
towards the mirror image of the antisun. From the Brewster zone of the grey
dummy, nearly vertically polarized light is always reflected perpendicular to
the solar meridian (Fig. 2).
From the black water-dummy, predominantly nearly horizontally polarized
skylight is always reflected irrespective of
s. However,
approximately vertically polarized skylight is reflected from 8-shaped regions
with long axes perpendicular to the solar-antisolar meridian within the
Brewster zone as well as from crescent-shaped areas near the horizon
perpendicularly to the solar meridian. From the Brewster zone of the black
dummy, horizontally polarized skylight is always reflected
(Fig. 3). Note that the mirror
images of the polarimeter, its holder and remote cord disturb the
-patterns only slightly. Therefore, in these regions, we omitted the
chequered pattern of these mirror images in the
-maps of Figs
2 and
3. These regions were, however,
not taken into account in comparative analyses.
In column 4 of Figs 2 and
3, the regions of the
water-dummies where the degree of polarization
d>dtr=5% and the angle of polarization
|-90°|<
tr=5° are
shaded in black, assuming that the imaginary polarotactic insect detects the
water in the blue part of the spectrum. At a solar elevation of 0°, the
grey water-dummy is interpreted as water only in areas towards the mirror sun
and mirror antisun and partly in the Brewster zone. As
s
increases, the area detected as water gradually decreases and the grey dummy
is considered as water only in small spots around the mirror sun and opposite
to it. At higher
s, the grey dummy is not interpreted as
water even in the Brewster zone (Fig.
2). The black water-dummy is always considered as water at or near
the Brewster angle. However, further away from the Brewster angle the black
dummy is not interpreted as water perpendicular to the solar meridian
(Fig. 3). Since quite similar
patterns were obtained in the green and red parts of the spectrum, we omit to
present them here.
Fig. 4 shows the patterns of
the degree and angle of linear polarization and the areas detected as water
for a perfectly black glass reflector (index of refraction
ng=1.5) - which absorbs all penetrating light - computed
for the same solar elevations as in Figs
2 and
3 and for incident
single-scattered Rayleigh skylight. The patterns in
Fig. 4 are very similar to
those in Fig. 3. Hence, the
reflection-polarizational characteristics of the black water-dummy approximate
those of a perfectly black glass reflector. The same patterns were also
computed for a perfectly black water reflector with an index of refraction
(nw) of 1.33, and we obtained practically the same
results. Hence, the slightly higher index of refraction of glass makes the
reflection-polarizational characteristics of glass surfaces only slightly
different from those of water: the degree of linear polarization of light
reflected from the glass is slightly higher and the Brewster angle of glass
(B=56°) is slightly wider than that of the water
(
B=53°), for example. Thus, the conclusions drawn from
the data obtained for the glass water-dummies also hold for flat water
surfaces.
In Fig. 5, the left column
shows the percentage, P, detected as water (polarotactic
detectability) calculated for the grey and black water-dummies under clear
skies as well as for the perfectly black glass (ng=1.5)
and water (nw=1.33) reflectors as a function of the solar
elevation in the blue, green and red parts of the spectrum. The polarotactic
detectabilities, P(s), of the perfectly
black reflectors are approximately the same in all three spectral ranges,
since the slight wavelength dependency of the refractive indices of glass and
water can be discounted in the visible part of the spectrum.
P(
s) of the perfectly black reflectors was
calculated for the full surface of the reflectors (broken curves) as well as
for the masked surface, i.e. for regions appropriate for comparative analyses
(individual data points displayed with triangles). The right column in
Fig. 5 shows the difference,
P, in the polarotactic detectability between the grey and
black water-dummies as well as between the perfectly black glass and water
reflectors. In Fig. 5, the
following are seen:
|
We obtained practically the same results for partly clouded skies.
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Discussion |
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If the polarization of light reflected from water is analyzed in the whole lower hemisphere of the visual field of a flying and water-seeking imaginary polarotactic insect, P is proportional to the chance of a water body being recognized as water in the optical environment. Thus, in the visible part of the spectrum, polarotactic water detection is easiest in the sunrise and sunset periods, when the reflection-polarizational characteristics of dark and bright waters are most similar, the P of bright or dark waters is maximal and the risk that a polarotactic insect will be unable to recognize the surface of a water body is minimal. This conclusion is also valid for a visual field of the ventral polarization-sensitive eye region of water insects, which may be narrower than the whole lower hemisphere because the areas detected as water are centred at or near the Brewster angle (see column 4 of Figs 2, 3 and column 3 of Fig. 4).
We used dtr=5% and
|tr|=5° as the thresholds of the
degree and angle of linear polarization for our imaginary polarotactic insect;
these are characteristic of the highly polarization-sensitive blue receptors
in the specialized dorsal rim area of the compound eye in the field cricket
Gryllus campestris (Labhart,
1980
). Since, in insects associated with water, the values of
dtr and
tr of polarization
sensitivity are unknown, and they could be species-specific, we set these
values arbitrarily. However, we also computed how the polarotactic
detectability of the water-dummies depends on these thresholds. We found that
by increasing dtr, P decreases monotonically, and
the increase of
tr results in the monotonous increase
of P. Since there were no sudden changes, local extrema, breaking
points or plateaus in the P(dtr) and
P(
tr) curves, we could not establish any
criterion for a threshold value that could be preferred. This fact has the
important consequence that the values of these two thresholds can indeed be
chosen arbitrarily, and the actual choice concerns neither the relative values
of P calculated for different
s nor the conclusions
drawn from them. Thus, the arbitrary use of dtr=5% and
|
tr|=5° is not a serious
restriction.
Under clear skies at a given s, the
reflection-polarizational characteristics of the water-dummies as well as real
water bodies depend on two components of returned light. The first component
is the light reflected from the glass/water surface. The direction of
polarization (e-vector direction) of this partially polarized component is
usually horizontal, and if the angle of reflection is equal to the Brewster
angle, it is totally polarized (d=100%). The second component is the
light originating from below the surface due to reflection from the underlying
substratum or the bottom of the water or to backscattering from particles
suspended in the water. This component is always vertically polarized due to
refraction at the surface (Horváth
and Pomozi, 1997
). The net polarization of returned light is
determined by the relative intensities of these two components. Since these
two components have orthogonal directions of polarization, their superposition
reduces the net degree of polarization. If the intensity of the first
component is greater than that of the second, the returned light is partially
linearly polarized with horizontal e-vector. When the second component is the
more intense, the returned light is partially vertically polarized. Finally,
if the intensities of these two components are approximately equal, the
returned light is practically unpolarized.
In the ultraviolet (UV), the second component of returned light originating
from below the water surface is considerably reduced in natural water bodies
due to the great absorption of UV light by the dissolved organic materials and
the low UV reflectivity of the bottom
(Schwind, 1995;
Bernáth et al., 2002
).
Thus, in the UV, the majority of natural water bodies possess similar
reflection-polarizational characteristics and
P(
s) as our black water-dummy in the blue
part of the spectrum (Figs 3,
5). Consequently, although we
could not measure the reflection-polarization patterns of the water-dummies in
the UV, our conclusions also hold for this part of the spectrum. This is
important because many water insects detect water polarotactically in the UV
(Schwind, 1985
,
1991
,
1995
).
Comparing the results of our measurements performed under clear skies (Figs
2,
3,
5) with those under partly
cloudy skies (data not shown), we could establish that the
P(s) of the water-dummies possess the same
qualitative features under clear and partly cloudy skies. The light emitted by
clouds is usually almost unpolarized
(Können, 1985
). If this
nearly unpolarized cloudlight is reflected from the horizontal glass surface
of the water-dummies, it always becomes partially polarized with horizontal
direction of polarization. Thus, clouds can enhance the relative proportion of
horizontally polarized reflected light in those regions of the reflector from
which nearly vertically polarized light would be reflected if the sky were
clear. Therefore, the consequence of clouds will be a slight increase in
P: the more extended the cloud cover, the larger is P.
Hence, under a cloudy sky, polarotactic water detection is slightly easier
than under a clear sky with the same
s.
For s>30°, P increases with
s in the case of the black water-dummy
(Fig. 5). Therefore, at high
s, the P of black waters could be as great as that
at
s
0° (sun on the horizon). This means that at high
s, the polarotactic detection of dark waters can be as easy
or even easier than at sunset. However, when
s is high (near
noon), the air temperature can be much higher, the air humidity much lower and
the wind speed much greater than at dusk, conditions that are disadvantageous
to small-bodied water insects (1-5 mm; e.g. Sigara sp.). These
insects possess such high surface-to-volume ratios and such thin chitinous
cuticle that they can become easily dehydrated during flights of tens of
minutes. Their flight can also be hindered by wind, which usually abates at
sunset when direct solar radiation quickly decreases to zero
(Landin and Stark, 1973
).
Consequently, only larger-bodied water-seeking polarotactic insects could take
advantage of the high P of dark waters at high
s.
This may be the reason why large- or medium-bodied (1-5 cm) aquatic insects
(e.g. Dytiscidae, Hydrophilidae, Notonectidae) are attracted to
horizontal black plastic sheets used in agriculture not only at dusk but also
at noon (Bernáth et al.,
2001a
,b
).
These beetles can also fly for a few hours during daytime at a higher
temperature and a lower air humidity due to their larger size, smaller
surface-to-volume ratio and thick sclerotized cuticle, which slows down the
dangerous dehydration of the body.
Schwind (1991,
1995
) used quite similar
water-dummies (composed of glass panes underlaid by different substrata) in
his multiple-choice field experiments to our dummies. He also assumed that
these dummies can imitate the reflection-polarizational and spectral
characteristics of real water surfaces.
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Acknowledgments |
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References |
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