By Catriona Daly
Junior Sophister Functional Biology Student
Polarised light is
hugely important in nature. It can affect the growth of plants or the daily rhythm of insects
(Csabai et al., 2006; Shibayev and Pergolizzi, 2011). Some animals, such
as cuttlefish or insects, use polarised light for navigation (Cartron et al.,
2012). Humans also have some
perception of polarised light. In 1884, Haidinger noticed that if one looks at
light at a particular
angle, blue-yellow brushed shapes appear on the centre of the fovea (Le Floch et al., 2010). However, in day to day life, we do not notice polarised
light. Interestingly, some scientists believe that we need to consider how
manmade structures will affect animals with polarised vision. Wildermuth and
Horvat (2005) noted that buildings or objects can reflect polarised light in a
way that confuses animals, particularly insects. There are several human
inventions which are disorienting to animals, namely cars, asphalt and glass
buildings. We have a responsibility to make sure that the human world is as
animal friendly as possible.
Polarisation is the direction of vibraton of
the electric field vector of light. Light can be unpolarised,
linearly polarised at a particular angle or circularly polarised either clockwise
or anti-clockwise (Kleinogel and White, 2008). Many animals rely on polarised
light for navigation (Cartron et al., 2012). Turcsányia et al. (2009)
showed that mayflies detect water bodies by the horizontal reflection of
polarised light. Unfortunately, other man-made surfaces can give off similar
horizontally polarised light. Wildermuth and Horvat (2005) discovered that
dragonflies or Libuella depressa can mistake a car bonnet for a body of
water. Male Libuella depressa become territorial and fiercely defend the
car as they would a normal habitat. This misconception was not limited to
males. Stevani et al. (2000) found that female dragonflies were
ovipositing on cars and damaging the clearcoat. Kriska et al. (2006)
noted that red and black cars are most at risk as the degree of linear
polarisation from red or black car bonnets is high. Car owners who live near
tracts of water are recommended to buy 'green' cars in colours that will not
lead to misplaced mayflies. Light coloured cars are best, though a very dirty
car will also have a lower degree of polarisation, allowing for water as well as
mayflies to be saved.
A second example of an insect 'trap' is an
asphalt road (Kriska et al., 1998). The dark, smooth surface mimics the
homogeneity of a body of still water. An experiment was conducted with mayflies
or ephemeroptera. It found that long roads with an open sky above are a
prime site for mayfly swarming and breeding en masse. The higher temperature of
the asphalt even allows reproductive activity to go on longer. Eggs are then
deposited on the road, 6- 9 thousand per fly, and all are destroyed. Mayflies
have a very short sexual maturity and may not be able to breed again (Brodskiy,
1973). To confirm that the reflected polarised light was the reason for this
strange choice of oviposition, tests were carried out on the attractiveness of
coloured cloths. It was found that shiny dark material, which also reflected
light in a similar way to water, was the most attractive (Kriska et al., 1998).
They recommended light coloured markings on the road to help insects identify
it correctly.
Finally, insects are also prone to mistaking
glass buildings for bodies of water. Kriska et al., (1998) noted that
polarotactic Hydropsyche pellucidula or caddis flies were being lured to
glass buildings. They were baffled to find that the insects would alight on the
vertical glass panes and proceed to copulate. A large amount of time was spent
overall on the surface of the buildings with flies leaving for a few seconds at
a time and returning quickly. This was repeated for several hours. The caddis
flies did not lay their eggs on the buildings but returned to horizontal water
bodies to oviposit (Reich and Downes, 2003). This behaviour surprised
researchers as it indicated that the flies understood that the surface they
were on was not horizontal, yet they remained on the surface. They concluded
that the flies would land on the glass and, with less reflection visible from
up close, recognise that it was not horizontal. Once they left, the attraction
mechanism would kick in again. Kriska et al., (1998) found that at
certain angles, an insect flying toward a vertical glass surface can interpret
it as a flat body of water. The angle at which optimum polarity was observed
was a specific angle known as Brewster's angle (Roldan-Carmona, 2012). When
light hits a transparent surface at this angle, the reflected light is
perfectly polarised. This explains why caddis flies are particularly attracted
to glass surfaces. It was found that anything that breaks up the homogeneity of
the surface, such as blinds or open windows, decreases the amount of polarised
light. Owners of large glass buildings could deter caddis flies with some light
coloured blinds or curtains.
It is clear that polarised light plays a huge
role in insect vision. Thousands of insect eggs are destroyed due to incorrect
navigation by polarised light. Insects recognise large dark areas as water
because, in nature, that is exactly what is there. Humans have changed the face
of this earth and they have a responsibility to make sure at it is still
amenable to animals and their habits. By following the few simple
recommedations in these experiments, people living near water can make a huge
difference to the lives of insects.
References
Brodskiy,
A. K. (1973). The swarming behaviour of mayflies. (Ephemeroptera). Entomology
Review, 52 ,33-39.
Cartron
L., Darmaillacq A.S. , Jozet-Alves C., Shashar N. and Dickel L. (2012)
Cuttlefish rely on both polarized light and landmarks for orientation. Animal
Cognition, 15, 591-596.
Csabai
Z., Boda P., Bernath B., Kriska G. and Horvath G. (2006) A 'polarisation
sun-dial' dictates the optimal time of day for dispersal by flying aquatic
insects. Freshwater Biology, 51, 1341-1350.
Kleinlogel
S. and White A. G.(2008) The Secret World of Shrimps: Polarisation Vision at
Its Best. Public Library of Science One, 3, e2190.
Kriska
G., Horvath G. and Andrikovics A. (1998) Why do mayflies lay their eggs en
masse on dry asphalt roads? Water-imitating polarized light reflected from
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2273-2286.
Kriska
G. ,Csabai Z. ,Boda P., Malik P. and Horvath G. (2006) Why do red and
darkcoloured cars lure aquatic insects? The attraction of water insects to car
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Le Floch
A. , Ropars G, Enoch J. and Lakshminarayanan V. (2010) The polarization sense
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Reich P.
and Downes B. J. (2003) Experimental evidence for physical cues involved in
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Roldan-Carmona
C. ,. Giner-Casares 1. 1. , Perez-Morales M. , Martin-Romero M. T. and Camacho
L. (2012) Revisiting the Brewster Angle Microscopy: The relevance of the polar
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Shibayev
P and Pergolizzi R. G. (2011) The effect of circularly polarised light on the
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Stevani
C. V., Porto J. S. , Trindade D.1. And Bechara E. J. H. (2000) Mechanism of
automotive clearcoat damage by dragonfly eggs investigated by surface enhanced
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I. , Szentkiralyib F. , Bernathb B. and Kada rb F. (2009) Flight of mayflies
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Wildermuth
H. and Horvat G. (2005) Visual deception of a male Libellula depressa
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