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1) Proctor, M., Yeo, P. & Lack, A. (1996) Natural History of pollination. Harper Collins
2) Thompson,
W. R. et al. (1972) Flavanols: Pigments Responsible for Ultraviolet
Absorption in Nectar Guide of Flowers. Science, Vol. 177, pp.528-530
3) Faegri, K. and van der Pijl, L. (1971) Principles of Pollination Ecology. Pergamon press
4) Eisner, T. et al. (1969) Ultraviolet video-viewing: the television camera as an insect eye. Science, Vol 166 pp1172 -1174
5) Kevan, P.G. (1976) Fluorescent nectar (Technical Comment). Science, Vol. 194, pp. 341 - 342
6) Thorpe, R.N. et al. (1976) reply to Kevan. Science, Vol 194, pp. 342
7) Hart, J.W. (1988) Light and plant growth. Unwin Hyman
8) Kevan,
P.G. (1978) "Floral colouration, its colorimetric analysis and
significance in anthecology". in The pollination of flowers by insects.
Ed. Richards, A. J., Academic press
9) Proctor, M. and Yeo, P. (1975) The pollination of flowers. Collins
10)
Singarajah, K. V. (1988) Spectral sensitivity of motion-sensitive units
of the butterfly ventral nerve cord. Journal of Insect Physiology, Vol
34, No. 11 , pp1005-1012
11) Bernard, G. D. (1979) Red-absorbing visual pigment of butterflies. Science, Vol 203 pp 1125 - 1127
12)
Struwe, G. (1972) Spectral sensitivity of the compound eye in
butterflies (Heliconis). Journal of Comparative Physiology, Vol 79 pp
191-196
13) Swihart, S.L. & Gordon, W.C. (1971) Red photoreceptors in butterflies. Nature Vol. 231 pp 126-127
14) Kay,
Q. O. N. (1976) Preferential pollination of yellow-flowered morphs of
Raphanus raphanistrum by Pieris and Eristalis spp. Nature, Vol.261
15)
Horridge, G. A., Marcelja, L. & Jahnke, R. 1984 Colour vision in
butterflies. Journal of comparative physiology, Vol 155:529-542
16)
Nekrutenko, Y. P. (1965) 'Gynandromorphic Effect' and the Optical
Nature of Hidden Wing-pattern in Goneopteryx rhamni L. (Lepidoptera,
Pieridae). Nature, Vol 205 pp.417 - 418
17)
Silberglied, R. E. and Taylor, O. R. (1973) Ultraviolet differences
between the sulphur butterflies, Colias eurytheme and C. philodice, and
a possible isolating mechanism. Nature, Vol 241:406-408
18)
Rutowski, R. L. (1978) The courtship behavior of the small sulphur
butterfly, Eurema lisa (Lepidoptera: Pieridae). Animal Behavior 26,
892-903
19) Thornhill, R. and Alcock, J. (1983) The evolution of insect mating systems. Harvard university press
20) Gibson, D. A. (1992) in "Biomedical photography". Ed. J. P. Vetter, Butterworth-Heinemann
21)
Eguchi, E. et al. (1982) A comparison of electrophysiologically
determined spectral responses in 35 species of Lepidoptera. Journal of
Insect Physiology, Vol.28, No.8, pp 675-682
22) Kodak limited (1972) Ultraviolet & fluorescence photography. Kodak limited
23)
Frolich, M.W. (1976) Appearance of vegetation in ultraviolet light:
absorbing flowers, reflecting backgrounds. Science, Vol. 194, pp.839 -
841
24) Davies, A. (1993) There to bee seen. British Journal of Photography 5/8/1993
25)
Kevan, P.G. et al. (1973) A grey-scale for measuring reflectance and
colour in the insect and human visual spectra. Ecology, Vol 54:924-926
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Theory
Ultraviolet light meets visible light at 400 nm (20).
The region of ultraviolet which is to be used in this project is within
long wave ultraviolet, Long wave ultraviolet ranges from approximately
320nm to 400nm, but as the lens limits this, the range used is 365nm to
400nm.
All theories of colour vision assume that there are 3 types of
photoreceptor involved. However, butterflies can have 2, 3 or 4 types (1, 15, 21), and 5 types have been reported in the Japanese Yellow Swallowtail Butterfly, Papilio xuthus (10, 21).
Materials
The photographs were taken on a Canon A1 camera fitted with a Canon
50mm lens through Vivitar extension tubes. Visible light was eliminated
by a 3 inch square Kodak Wratten ultraviolet filter No. 18a. This was
mounted on the camera via a Cokin P-system filter holder and a gelatin
holder. The ultraviolet photographs were taken on Kodak T-Max 400 film.
Film
All photographic emulsions are inherently sensitive to ultraviolet
light to varying degrees, but all sufficiently so for use in
ultraviolet photography of long wavelength ultraviolet rays. Another
limitation is that some colour films contain filters to cut out
ultraviolet light between the gelatin layers, as a result it is
considered that the best results will be from the use of black and
white film.
Filters
An ultraviolet transmission filter is required in order to
eliminate visible light from the image. An appropriate filter for this
purpose is a Kodak 18a filter which is available in a 2 inch square or
3 inch square format. The 3 inch square version can be mounted in a
gelatin holder with the Cokin P system filter holder, for use on most
camera formats. This needs to be sealed around the edges to prevent any
extraneous light from affecting the images produced. All ultraviolet
transmission filters of this type are made from optical glass, as
gelatin absorbs ultraviolet light.
Lenses
Different wavelengths of light focus on different planes when
transmitted through a lens. As a result there must be an alteration in
focus from what appears correct in visible light. This can be overcome
by focusing visually and changing focus by a preset amount (only of
practical use with a standardized set-up). Depth of focus can also be
affected by the use of short focal length lenses, and use of the
smallest possible aperture. The system can be limited by the optical
properties of photographic lenses, which tend to prevent transmission
of wavelengths below 365nm (for photography below this point quartz
lenses become necessary) (20).
Methods
The systems described can be standardized by the production of a gray-scale which is effective in the ultraviolet region (25). This is of greatest use in setting up and standardising a system.
Film processing
Development of the ultraviolet images was in Kodalith developer, at
24 C, for 11 minutes, with constant agitation for the first and last
whole minutes, and for 10 seconds in every 30 seconds in between.
Ultraviolet photography outside
From my own tests (using Kodak T-Max 400), there is a 7 stop
difference in exposure value between images taken with Kodak's 18a
filter and images taken without any filter. There are 2 methods of
accommodating for this, the simpler of these is to increase exposure of
light to the film, and the alternative is to "push" the film.
Increasing the exposure of light to the film increases the density of
the negative. Pushing the film (extending the development of the film,
either by an increase in developing time, developer temperature, or
both), increases the density of the negative, however if the film is
being pushed to give the standard density then, it has been
under-exposed and there will be a loss of detail, there will also be an
increase in contrast. Ultraviolet light produces images with a low
contrast, so the film will need to be pushed. The best results come
from a combination of 4 stop increase in exposure and pushing 3 stops
using T-Max developer.
There is at present no viable method for metering available
ultraviolet radiation at any particular point in time (ultraviolet
flash units, and ultraviolet light meters are available (22)
but are prohibitively expensive). Therefore, once a reasonable estimate
has been made, bracketing should ensure that a negative of appropriate
density is obtained.
The use of flash as a standard source of ultraviolet radiation was
considered, initial attempts did not show any results. This may have
been due to a filter which is used on some flash units to cut out
ultraviolet. This information is not always readily available.
In the production of the images the background must be chosen as
carefully as is possible under the circumstances, as many images
produced show a darkened background. This is often not the case in
natural situations, with highly ultraviolet reflective backgrounds
occurring in nature. Some flowers absorb ultraviolet, but to an animal
with ultraviolet vision it will stand out from the rest of the plant,
as parts of the foliage often reflects ultraviolet (particularly hairs)
(23).
Ultraviolet photography in the studio
An alternative method for producing similar images to those
produced from ultraviolet transmission photography, is to collect a
flower which is known to have an ultraviolet pattern and to press and
dry it. After only a few hours the, previously ultraviolet, pattern
will fluoresce under ultraviolet light.
Using a copy stand with ultraviolet light is useful for determining
the best exposure range. If a quick release base and platforms are used
on the cameras, with the same lens being changed from one to the other,
the images should come out in register. Registration marks also help in
bringing the images together.
The main problems encountered in the studio are wilting of the
flowers and movement of the flowers. This is mostly a risk during the
change over from ultraviolet tubes to daylight tubes.
Producing false colour images
Three methods of false colour imaging for mimicking the visual system of a honeybee have been documented (24).
In this case a different method to those described has been used. Using
a copy stand in the studio and a tripod in the field, a Cullman quick
release base was mounted securely. The cameras each had bases mounted
in the same place to ensure that the images from the different cameras
would come into register when overlaid.
As a quality slide scanner was not available, photographs were
taken on colour print film (Fuji super HG). I printed the images of the
ultraviolet up to 10 x 8 inch on Agfa Multicontrast paper. Both the
black and white and the colour prints were scanned on a flatbed scanner
into Adobe PhotoShop (via HP Picturescan). Once the images were on the
computer, the images were brought to a standard size.
For the false colour images representing bee vision, the colour
image was kept in RGB mode and the image was compiled by copying: the
Green information into the Red channel; the Blue information into the
Green channel; and the ultraviolet information into the Blue channel.
Subjects
Flowers pollinated by butterflies have the highest proportion of
nectar guides, reported at 83%. Therefore, taking photographs at a
butterfly house provides both a good source of butterflies and of
flowers. Permission was given to take photographs at the White post
butterfly house, Nottinghamshire.
The use of a butterfly houses can cause problems due the the high
level of humidity. The lenses should be attached to the camera before
entering the butterfly house, with the film preloaded, to prevent
moisture affecting the inside of the camera. The outside of the camera
will mist over, the humidity condensing on the lens and other surfaces,
this will go away of its own accord as the camera and lens equal the
atmospheric temperature (this may require some patience). Other
problems include the tendency for butterflies to move when approached,
this tendency seems to increase with both temperature and light.
Butterflies need the warmth of the sun to get their wings to a working
temperature, with sunlight being one of the best sources of ultraviolet
light, the result is that the butterflies are easiest to photograph
when it is overcast, but longer exposures are required.
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Biological adaptations of flowers
For a plant to survive from one generation to the next, it should
do all it can to help its pollinator to thrive and produce healthy
offspring. This needs to be done without reducing its' own ability to
produce viable seed. Nectar is rich in sugars, a high energy food for
insects. In addition to these sugars, butterfly flowers provide a
relatively high proportion of amino-acids (1).
Amino acids are the building blocks of cells, and insects which take
advantage of this, where available, probably increase their chances of
survival and reproduction. To the flower this would make the insect
more likely to be a pollinator, generation after generation. This has
most significance amongst insect species which will feed mainly on a
few types of flower.
Communication with insects
Apart from colour, plants have many ways of attracting potential
pollinators including scent, reflectance, size, outline, surface
texture, temperature and motion. Although colour is thought to be the
most important factor for insect pollinators. Plants which do not
depend on insect or bird pollination are unlikely to have showy or
scented flowers (3).
Colour
In order to attract the potential pollinator to that particular
blossom, availability of nectar has to be advertised to the butterfly.
This is displayed in the colour of the petals. The colour of the nectar
guide of Aesculus hippocastanum changes from yellow to red when nectar
is no longer in production. This happens after the flower has been
pollinated and the ovules have begun to develop. The flowers are then
less regularly visited by insects(1, 3).
Once pollination has taken place the flower may wilt and discolour
rapidly as it is of no further use. The exception being if it is a
small flower which is a part of an inflorescence (3).
Nectar Guides
Further help is available in the form of nectar guides, also
referred to as "pollen guides" or "honey guides". These are usually a
visual contrast, either in colours which we can see or against
ultraviolet. These guides may not function to attract the insect, but
act as guides for closing in on the target once the flower has been
chosen (1).
The lines towards the nectar may be a structural adaptation as in the Thistle (Cirsium vulgare - see Fig.
1), or more commonly, the lines pointing to the nectar are a pattern on
the petals, leading towards the centre from all angles. While others
such as Dog Violet (Viola riviniana - Fig. 3, right), have their
guide lines as marks on the petals which are visible from the direction
of approach. The flowers of the Dog violet hang down, and so there are
guides on the lower petals are where the insect lands, no guides are
needed on the upper petals.
Fig. 3 Dog Violet - Viola riviniana
Bullseyes
One of the best known flowers with the bulls eye effect in ultraviolet is the black-eyed Susan, Rudbeckia hirta , which contains compounds absorbing strongly between 340nm and 380nm (2).
The petals of the Black-eyed Susan, a large daisy-like flower, appear
plain yellow to humans while appearing to have a very dark centre to
insects.
The use of ultraviolet by flowers
Apart from pollen guides, plants will use ultraviolet to their
benefit in other ways, such as ultraviolet pollen, ultraviolet nectar,
fluorescent pollen or fluorescent nectar. They can also use a
contrasting background to make the flowers stand out against a
different level of ultraviolet reflection from leaves or leaf hairs.
Ultraviolet light is also of use, to insects, for the identification of
plants. This is most apparent where many plants which appear similar to
humans, grow together (e.g. many composites). Yet these are presumably
distinguishable to insects.
It has also been proposed that there may be differences in
ultraviolet reflectance as a flower matures, to prevent competition
within a species (4).
Some flowers have been recorded as having fluorescent nectar, however,
butterfly pollinated flowers have not been found to use this tactic.
The significance of fluorescent nectar is still under debate, but
occurs regularly enough to assume that it is not present merely by
chance, and so must have some function (5, 6).
Most blue light receptors of plants are reactive to long wave
ultraviolet, to around 370nm. In general, ultraviolet is not thought to
have a significant effect on plants. Adaptive features, to protect
against damage from ultraviolet radiation, are likely to be present in
plants growing at high altitudes. Ultraviolet levels are raised at high
altitudes because atmospheric scattering and absorption has had less
distance to reduce the ultraviolet content of solar radiation (7).
Fig.4 Himalayan balsam (Policeman's helmet) - Impatiens glandulifera.
Visual system of insects
Bees are very widely studied insects with regard to their visual
system. They can detect three colours ultraviolet, blue and yellow (8), no bees investigated can see red(9).
This has been interpreted in the trichromatic theory. A pentachromatic
visual system (i.e. the eyes contain 5 different types of cell which
react to different bands of light), has been reported in Papilio xuthus, the Japanese yellow swallowtail butterfly (10). Butterflies vary widely in their sensitivity of light, and are considered to have the widest visual range of any animal (11). Atrophaneura alcinous has a visual range from at least 400nm(See Appendix 1, Note 1) to 700 nm, while Heliconis sara has a range from 310nm to 650nm (12).
The ability to see red is rare in the insect kingdom but appears to be quite common in butterflies (11), and is now known to be an essential part in the release of courtship behaviour of some species(12, 13).
It had previously been thought to be that red, orange and even yellow
colouration of butterflies served merely as a warning for potential
predators (11).
Butterflies seem to prefer yellow in their feeding behaviour (8, 13),
with Pieris and Eristalis spp having a preference for the
yellow-flowered morph of Raphanus raphanistrum (wild radish), over the
white coloured morph of the same plant (14) (See Appendix 1, Note 2).
Sensitivity to the polarization of light has been reported in some cells of a few butterflies (15).
It occurs mostly in the upper lenses of the compound eye, and is most
likely used for spatial orientation in overcast conditions.
Ultraviolet patterns and behavioural effects of colour on butterflies
Fig.5 Butterfly vision simulation of Cleopatra - Goneopteryx cleopatra.
Not all butterflies feed on nectar, there are some species which
specialize in feeding on tree sap and/or fruits, so they will have
adapted to that way of life, and are unlikely to have the same visual
responses as nectar feeders (1).
The male and the female of a species of butterfly are often very different, this is a necessity for quick recognition(4, 16, 17).
Quick recognition during courtship is important, as they normally rely
on a rapid, erratic escape flight to defend against predation. The
courtship, however, involves an ascending flight with a conspecific,
this is conspicuous and predation would tend to select against it.
To the human eye many butterflies appear the same, but the
butterflies themselves can often identify each other quite easily from
ultraviolet markings. For example the males and females of Eurema lisa
, the small sulphur butterfly, differ only in the ultraviolet region,
the males being strongly ultraviolet reflective and the females
unreflective in ultraviolet. The need for significant differences in
appearance exists only in butterflies which are palatable to birds.
Those which are not palatable have a much reduced risk of being
attacked and so can spend much more time in identification and
courtship (18).
In addition to this sexual selection can occur within species, the
males choosing the female on the basis of age. The younger females have
less ultraviolet than the older individuals, and it is the younger ones
which are preferred. The male preference for younger females in Pieris
rapae is due to the female laying approximately half their lifetime
production of eggs in the first quarter of their life (19).
The ultraviolet patches on some butterflies are directionally iridescent (17),
as a result they appear to flicker in flight. This flickering is
thought to have an important role in butterfly behaviour and
communication (4).
Butterflies tend to avoid the colour green in their feeding behaviour, possibly being effectively invisible (3),
but are attracted to it during egg laying. The next generation need to
be placed near a good source of food as caterpillars have a voracious
appetite. The green photoreceptors are instead used for the detection
of movement (10).
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The content for this section was produced by Eddie Aicken BSc
(Hons) Biological Imaging who produced this web site as part of BSc
(Hons) degree in Biological Imaging, at the University of Derby.
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
. Eddie graduated in June 1997 and is now working as a medical photographer.
Introduction
Over time, many plants have adapted their structure and their
flowers to be more readily pollinated by insects. The more successful
ones survived and continue to evolve to their pollinators needs. The
results are flowers which are often brightly coloured and/or scented to
advertise the availability of nectar. The nectar is the motivating
factor for the potential pollinator to visit the plant. The nectar is
placed to ensure that the insect brushes past the pollen or the
anthers, increasing the chances of pollination as they feed. It is then
no surprise to note that insects behavioural patterns and visual
systems are exploited by plants. This is seen in the visual cues, which
flowers provide, for the insect to close in on the nectar.
There are two main ways in which the flower can provide visual
cues. The first is the use of lines of contrast converging on the
nectar containing region (Nectar guides see Biology page). The Thistle,Cirsium vulgare
(Fig.1, right), is a good example of this, it is a roundish flower with
dark lines pointing towards the centre, regardless of the direction of
approach. The other type of visual cue, referred to as the "bulls eye
effect", is a dark area in the centre of the flower. A common example
of this is the Michaelmas Daisy, Aster novae-belgii (Fig.2, right), which also has lines on the petals leading towards the bulls eye.
These visual cues are intended for the eyes of pollinators, which
can see ultraviolet light, as well as other colours. Ultraviolet
patterns are invisible to most animals including humans, they often
mirror patterns which we can see. They also occur on plain, apparently
patternless, flowers.
Fig.1 Common Thistle - Cirsium vulgare
Fig.2 Michaelmas Daisy - Aster novae-belgii
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