Black Beauties- Super Black Butterfly Scales
Alison Sutton Fernandes0225014
Why Butterflies? Butterflies have
irridescent colours formed by photonic crystals.
But what about the intense black areas on the wings?
Wing scales with very low reflectance (>0.5%)
Possibilities of emulating them with other materials.
http://www.thaishop4you.com/buttrfly_big_view/bf163.htm
Surface Reflections Any interface that involves a change in
refractive index gives rise to surface reflections. Surfaces like black cardboard and paint, even though they appear black still reflect about 4%.
To a simple approximation, these surface reflections are governed by Fresnel equations. For air (ni) and chittin (nt):
R= ((nt-ni)/(nt+ni))2 = ((1-1.56)/(1+1.56))2
= 4.8%
In butterfly scales, you get values as low as 0.4%.
The Role of the Butterfly Wing Scale
The material the butterfly wing is made from, chitin, is effectively transparent. Yet when it adopts certain structures it can cause interference and diffraction of light rays to produce a range of colours.
In the case of black scales the main role of the upper part of the wing scale appears to be to collimate the light- to transmit it to an absorbent membrane beneath, and minimise surface reflections. It is this part of the Scale I hoped to investigate.
Begun investigations with 17 samples and a range of methods to see what different solutions there were and which were most effective.
High Resolution Optical Microscope
Typical Scale Structure The arrangement of scales
on the wing resembles that of shingles on a roof. In most species two distinct layers are present- ground and cover scales.
Typical scale dimensions are of the order 75micm by 200 micm. (scales come off as a fine dust). Underside tends to be plain and featureless, while interior and external visible top surface exhibit interesting microstructure.
Honeycomb Structure
Cross Ribs
Parides Hecuba
Two butterflies of the Parides family (Hecuba and Rotuse) instead of honeycomb structure had microribbing extending across between the ridges, effectively blocking the inner layers below.
Resulted in some of the lowest reflectances recorded.
Fractured Scales
Other Methods
SEM: Upper limit to resolution Difficulty seeing inner structure Hard to establish exact size of features
Alternatives: Embedded in Resin TEM
Cary SE Spectrophotometer Measures the reflectance of a sample
over a range of wavelengths using an integrating sphere.
Zero calibrated using a light trap– extremely absorbing.
Samples must be of sufficient size (limited to 5 species).
Beam must be carefully positioned. Scales easily lost.
Reflectance (%) of 6 butterf ly types over full w avelength range of CARY
-10
0
10
20
30
40
50
60
70
80
90
100
0 500 1000 1500 2000 2500 3000
Wavelength (nm)
Ref
lect
ance
(%
)
P. erlaces xanthias
P. ulysses ulysses
P. lysander
P. gambrisius
P. sesostris
Reflectance (%) of 6 butterfly types over visible wavelength range using CARY
0
0.5
1
1.5
2
2.5
3
3.5
4
350 400 450 500 550 600 650 700
Wavelength (nm)
Re
flect
an
ce (
%)
P. erlaces xanthias
P. ulysses ulysses
P. lysander
P. gambrisius
P. sesostris
Parides sesostris
Microspectrophotometer
Spectral information from single scales.
Problems: Drifting dark current Limited integration
time Very small area Surrounding reflections
and extraneous light Lambertian assumption Equipment failure
Cary Visible Region Reflectance
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Paridessesostris
Parides lysander Paridesgambrisius
Parides ulyssesulysses
Parides erlacesxanthius
Species
Ref
lect
ance
(%
)
Microspectrophotometer Visible Region Reflectance on Single Scales
0
0.5
1
1.5
2
2.5
3
Parides sesostris Parides lysander Parides gambrisius Parides ulyssesulysses
Parides erlacesxanthius
Species
Ref
lect
ance
(%
)
Microspectrophotometer
Attempted to Average Pixel Intensities
Microspectrophtometer Visible Region Reflectance based on Pixel Intensites
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Paridessesostris
Parideslysander
Paridesgambrisius
Paridesulyssesulysses
Parideserlacesxanthius
Species
Re
fle
cta
nc
e (
%)
Cary Visible Region Reflectance
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Paridessesostris
Parides lysander Paridesgambrisius
Parides ulyssesulysses
Parides erlacesxanthius
Species
Ref
lect
ance
(%
)
Conclusion
Scales with the honeycomb structure were on average significantly less reflective than those with crossribbing.
Suggests honeycomb more effective in minimising surface reflections and collimating light.
The microribbing appeared even more effective. All scales exhibited extremely low reflectances
Why Colour and Black? Camouflage. Sex Attractant. An absorber, attenuator, or deflector for
ultrasonics to defeat echolocations by bats. Signalling Identification- seen from a large distance,
distinguishable from background. Eyespots- scare away predators. Effective use of light. When ample light is
available to species, pigments are generally found. When light becomes scarce, more structural colour used (light is not lost and absorbed, but a lot reflected back).
Thermoregulation Butterflies bask to gain sufficient body temperatures for
flight activity. (Berwaerts, 2001) Butterflies with fully spread wings did warm more
efficiently. (Heinrich, 1986) Descaled wings reached lower temperatures.
(Berwaerts, 2001) Butterflies can develop different scales colours
depending on the season they are born in. Behavioural factors, such as wing orientation seem
more important. (Polycyn, 1986) The changes in reflectance are not great. Reflective in the infra-red region In some cases difficult to tell if behaviour adapts to wing
colour or wing colour adapts to behaviour.
Other ResearchMoth eyes (Hutley et al): Minimise Surface Reflections Triangle like projections on surface Gradually decreasing diffractive index
A similar type of structure is used to absorb sound wave in recording rooms without creating interference through reflections.
Thin films also attempt this method, by layering films of slightly decreased refractive index to lower surface reflections.
Application Structures could be scaled for specific
applications. You would create selective surfaces (since reflection in infra-red region is v. high).
Basic computer modelling has already confirmed a peak below 1% for a simple honeycomb structure.
Important to use nature as inspiration, not as blueprints.
Needs of an individual organism likely to be very different form our own.
References Berwaerts, K., Van Dyck, H. & Matthysen, E., (2001), Effect of
manipulated wing characteristics and basking posture on thermal properties of the butterfly Pararge aegeria, Journal of Zoology, 255(2), pp. 261-267
Ghiradella, H., (1994), Structure of Butterfly Scales- Patterning in an Insect Cuticle, Microscopy Research and Technique, Apr 1 1994, 27 (5), pp. 429-438
Heinrich, B., (1986), Comparitive thermoregulation of four montane butterflies of different mass, Physiological Zoology, 59(6), pp. 616-626.
Lawrence, C. & Large, M. C. J., (), Optical Biomimetics, , Lewis, H. L., (1973), Butterflies of the World, Harrap, London Leo, B., (1999), Mysteries of a Butterfly Wing, Microscope, 47
(2), pp. 79-92. Polycyn, D. & Chappell, M. A., (1986), Analysis of Heat Transfer
in Vanessa Butterflies: Effects of Wing Position and Orientation to Wind and Light, Physiological Zoology, 59(6), pp. 706-716
Acknowledgments
OFTC: Dr. Maryanne Large, Dr. Leon Poladian, Shelly Wickham
Applied Physics: Professor David McKenzie, Dr. Stephen Bosi
EMU: Tony Romeo, Dr. Ian Kaplin, Anne Simpson-Gomes
Tamar Ziv, James Griffin