The Blues: nature's trompe d'oeil #2

As promised in the last posting, I'm going to continue talking about beautifully blue animals and, as in our exploration of blue feathers, virtually none of the sky-coloured feet, beaks, skins, and insect parts that follow have any blue chemicals to thank for their hues. All are due to scattering of light by fine particles suspended in liquids, or by carefully ordered layers of collagen fibres, or thin layers of scales precisely spaced, or parallel ridges that reflect and emphasise blue light. The physics of most of it is beyond me I'm afraid - fortunately this is not a physics blog! However if you're mathematically minded, this might be of interest.

Bearing the general principles in mind, here is a tour of some of nature's blue bits.

Poison Dart Frog, family Dendrobatidae, Ecuador. (Don't try this at home, or anywhere else, incidentally! Local people seem to acquire an immunity to these stunning but potentially deadly little frogs. I've heard of visitors getting very sick from handling one.) Frog skin colours are very complex, with often three layers of different cells in the skin. In the case of blues, iridophores sit above melanin-filled melanophores; the iridophores reflect blue light back, like iridescent feathers that we discussed last time.

Reptile scales can be similarly constructed, to produce blues that can often be switched on and off by contracting the melanin cells below the light-scattering layers, especially in the case of displaying males.

Male Gippsland Water Dragon Itellagama (Physignathus) lesueurii, National Botanic Gardens, Canberra.

Unidentified skink, Cooktown, north Queensland. Any assistance gratefully received!

Male agamid, Queen Elizabeth National Park, Uganda.

Blue skin in mammals and birds is caused by arrays of collagen fibres, again above a melanin layer. I don't have pics of blue mammal parts (the best known of which are probably Mandrill faces and backsides, and the scrotums of Vervet Monkeys; oh well, I don't want to alarm your family filters anyway).

Southern Cassowary Casuarius casuarius, Wallaman Falls north Queensland. (Taken through a car window!)

Blue-faced Honeyeater Entomyzon cyanotis, Griffith, New South Wales.

Blue feet are very popular in one bird made famous by wildlife documentaries. Here it's just the skin again.

Blue-footed Booby Sula nebouxii, Puerto Ayora, Galapagos. The bluer the feet, the more attractive the owner to the only one who matters.

A bird's beak is covered with a thin layer of keratin rather than collagen; I can't find much about the role of keratin in producing blue beaks, but I see no reason why it wouldn't be a similar story to the collagen structures in skin.

Andean Ruddy Duck Oxyura jamaicensis, El Calafate, Argentina.

White Tern Gygis alba, Lord Howe Island.

Dusky Woodswallow Artamus cyanopterus, Canberra.

And of course there are numerous invertebrate examples.

Leaf Beetle, Chrysomelidae, possibly a cryptocephaline (the cylindrical leaf beetles) Whyalla, South Australia. I have no idea about this beautiful chewer in particular, but in some blue beetles at least layers of disc-shaped scales in the cuticles are responsible. (My thanks to Susan for helping with the family.)

In grasshoppers, the mechanism is apparently again a suspension of granules in the cuticle, above a dark background; I assume that this is how it works in this wonderful beast, but I don't know that it's been investigated.

Painted Locust Schistocerca melanoceraSierra Negro volcano, Galapagos. Like so many other Galapagos residents, they are found nowhere else.

Tropical Rockmaster Diphlebia euphoeoides, north Queensland.

Black-headed Skimmer Crocothemis nigrifrons, Canberra.

And of course numerous butterflies and moths flaunt blues, with a variety of variations on the themes described above, involving precisely spaced layers of scales. Some members of Papilio and Graphium swallowtails actually do have some rare blue pigments, but even these are emphasised by scale orientation.

Shining Oak Blue Arhopala micale, Cairns, north Queensland.

Satin-Green Forester (Pollanisus viridipulverulenta, Yeldulknie Conservation Park, South Australia. Its iridescence makes it flash from green to blue, depending on the angle. Don't sniff too deeply - its family, Zygaenidae, specialises in releasing cyanic acid in self-defence! (My thanks to Susan for putting me on the right track to identifying this one by recognising the family.)

Urania Moth, Manu National Park, Peru

Euchromia creusa, north Queensland. Here the blue is not (mostly) in the wings, but in body scales. Presumably the principle is the same however.

Finally, to a couple of animals which probably do have pigments - many marine animals do, including crustaceans, and I see no reason why these crabs would not.

Sally Lightfoot Crab Grapsus grapsus, Galapagos.

Soldier Crabs Mictyris longicarpus, Cullendulla Creek Nature Reserve, New South Wales

Well I'm about blued out for now, but I've enjoyed the journey; I'd love to hear if you have too, or if you have any comment to make. We're still to talk blue in plants, but we might have a break from blue before we tackle that one.