Traps in Nature

Traps in Nature To see the environment through the eyes of a biting fly, I took photographs of some representative traps in the field under different conditions, using special filters to imitate the sensitivity of the fly retina. I then digitally combined the individual black and white images to provide false-colour representations of traps as they might appear in the ultraviolet (represented with red) and blue regions of the spectrum. Technical details are provided at the end of this page. From these scenes, it should be obvious why traps in the environment stand out to a fly with the ability to see in the blue and the ultraviolet. For each scene, I start with a normal black & white photo, followed by a photo taken with a blue filter. I have also provided one example of an image taken with an ultraviolet filter. The final false colour images are adjusted to give equal weight to the blue and ultraviolet regions of the spectrum (view the larger images for better picture quality). Each scene contains a white rectangle at the base of the trap as a field reference for brightness. This is a piece of white Tyvek® which has an even reflectance of about 95% across all wavelengths.
DRY cloth, direct sun and shade at 9 am Forest clearing with dark green vegetation Nzi trap made from Kenya cotton drill and white polyester netting, with a plastic bag at the top
Normal B&W Film	Blue Filter (400-500 nm)

Ultraviolet Filter (< 400 nm)	False Colour (uv+Blue)
	600 x 400 pixels (118 k)
WET cloth, mostly in shade at 9 am Woodland with bright and dark green vegetation Nzi trap made from VF opaque polyester cloth and white polyester netting, with a plastic bottle at the top
Normal B&W Film	Blue Filter (400-500 nm)

Normal Colour Film	False Colour (uv+Blue)
600 x 400 pixels (96 k)	600 x 400 pixels (94 k)
DRY cloth, light cloud cover at 10 am Woodland with bright green grass in background NG2G trap made from Ethiopian plain cotton cloth and white synthetic netting, with a plastic bottle at the top
Normal B&W Film	Blue Filter (400-500 nm)

Normal Colour Film	False Colour (uv+Blue)
600 x 400 pixels (127 k)	600 x 400 pixels (87 k)
600 x 400 pixels (94 k)	For comparison, a colour photo taken about an hour earlier, nearby in the same habitat. This is a Cotton Nzi trap in mostly diffuse sunlight, rather than in cloud. The cotton is a darker blue than the polyester/cotton, but it also appears dark in this photo as the trap is still slightly wet from morning dew.

Technical Details

The photographs were taken with an AF Micro Nikkor 60 mm f2.8 lens in late August, 1997 near Chanka, Ethiopia. I used Ilford Delta Professional ASA 400 B&W negative film and Kodachrome 64 colour slide film. Images representing the blue region of the spectrum were taken with a Kodak Wratten Gelatin No. 98 filter (peak transmittance between 430 and 440 nm). A visually-opaque B+W 403 (Schott UG1) uv-pass filter was used for the ultraviolet images. My camera equipment produced a halo in the centre of the ultraviolet images, presumably due to an imperfection in the filter I used.

A good web site on ultraviolet photography is Naturfotograf. The technical reference below is also a good introduction to this field.

Williams, A.R. & Williams, G.F. (1993) The invisible image - A tutorial on photography with invisible radiation, Part I: Introduction and reflected ultraviolet techniques. Journal of Biological Photography 61, 115-132.

For an introduction to visual pigments and colour perception in insects (unfortunately with poor coverage of the literature on biting flies) is:

Briscoe, A.D. & Chittka, L. (2001) The evolution of color vision in insects. Annual Review of Entomology 46, 471.