Electronic Supplementary Material for: Foot, G., Rice, S.P., & Millett, J. Trap
colour of the carnivorous plant Drosera rotundifolia does not serve a prey
attraction or camouflage function. Biology Letters
ESM1: Details of colour analysis of artificial and real Drosera rotundifolia
traps.
Colour data was determined from photographs taken with a digital camera. Images
were calibrated using a standardised colour card (QPcard 201, QPcard AB, Sweden)
and mean leaf/trap RGB values determined using image analysis software (ImageJ
[1]). RGB values were converted to CIE 1976 (L*a*b*) colour space (CIELAB) using
formulas set out by [2]. This standard colour space includes all colours perceivable
to humans. Each colour is defined by its position in the colour space based on the
‘Lightness’ (L*) of a colour, its position between red and green (a*, negative=green,
positive=red) and its position between blue and yellow (b*, negative-blue,
positive=yellow). Thus CIELAB enables assessment of colour difference and how
red or green an object is by the value of the a* axis. Colour differences between D.
rotundifolia leaves and background (conspicuousness) were determined by
calculating the delta-E 1976 (∆E76) as follows:
Equation 1: ∆E76 =√(L*bg - L*lf )² + (a*bg - a*lf )² + (b*bg - b*lf )²
∆E76 is a measure of colour difference based on the Euclidian distance between two
colours in three dimensional space. Where L*, a* and b* represent the three axis of
CIELAB and the subscripts ‘lf’ and ‘bg’ signify the leaf and background colours.
References
1
Abramoff, M., Magalhaes, P. & Ram, S. 2004 Image processing with ImageJ.
Biophotonics Int. 11, 36–42.
2
IRO Group Limited 2013 EasyRGB. Colour Calc.