This week’s odonate of the week is Libellula pulchella, the twelve-spotted skimmer.
[EDIT: Apologies, this post was a tad premature (I’m still getting used to the WordPress interface!). Here’s the full post:]
The black pigmentation is melanin, but the appearance of white pigment is caused by the Tyndall Effect. The Tyndall Effect or “Tyndall scattering” is caused by light interacting with very small, transparent particles. This kind of colouration is known as “structural colouration” as opposed to “pigment colouration”. Pigment colouration involves a certain chemical that absorbs light at a range of frequencies. The frequencies that are not absorbed are reflected and those frequencies give the chemical an appearance of colour. In structural colours, there are certain structures which influence the passage of light. The size and configuration of these structures affects what wavelength (and, therefore, colour) of light is reflected.
A better-studied example of Tyndall scattering in insects is in the caterpillars of the moth genus Malacosoma. In the photograph is a caterpillar of Malacosoma neustria (photo by Lilly M, via Wikicommons) and below that is a diagram of the different parts of the cuticle (the insect’s skin, from Byers, 1975).
Note that in the left side of the drawing pretty much all of the light that hits the cuticle is absorbed. This leads to little light being reflected and so the cuticle appears black. In the centre drawing the size of the microtubules on the surface lead to scattering (reflection) of some wavelengths of light, predominantly blue in this case. Finally, white areas are caused by a combination of scattering at the microtubules as well as reflection from deeper in the transparent cuticle.
In Libellula pulchella the situation is very similar, but rather than having these small tubes arising from the surface there is a coating of grains on the top of the wing. These can be scraped off which leads to the wing becoming transparent. Interestingly, the Tyndall effect also relies on there being a difference between the refractive index of the granules or tubes and the surrounding medium (usually air). If you pour a liquid onto the wing that as a refractive index closer to that of the granules or tubes (around 1.5 in L. pulchella) then the wings appear transparent. Removal of the liquid returns the Tyndall scattering and the wings appear white again (Mason, 1926).
Byers, J.R. (1975) Tyndall blue and surface white of tent caterpillars, Malacosoma spp., Journal of Insect Physiology, 21: 401-415.
Mason, C.W (1926) Structural colour in insects: I, Journal of Physiological Chemistry, 30: 383-395.