Each summer is a time for vampires, at least the dwarf variety known as mosquitoes. Sun worshippers are out in force, some concerned about contracting skin cancer, others blissfully ignoring such fears. Each scantily clad human body presents mosquitoes with an all-you-can-eat buffet beyond their wildest insect equivalent of dreams. One summer, I overheard one of my new nursing students tell a friend that mosquitoes only take red blood, not blue blood. Blue blood? The comments made this scientist pause.

Blue blood? Yes, many people believe we have blue blood in our bodies. However, no known vertebrate, animals with backbones, is known to have such a substance. There is a small group of green-blooded lizards found in New Guinea, the coloration coming from high concentrations of bile pigments. That level of bile in human blood would be extremely toxic, but the lizards thrive. Ongoing research is trying to find out how they survive, and the bile salts may protect the lizards from malaria.

Then there are a few species of Arctic and Antarctic fishes, whose blood is a clear, colorless material. Instead of bile or other pigments, they circulate a complex substance that keeps their blood from freezing in the otherwise deadly cold water.

But thats it, really. A few clear-blooded fishes and some green-blooded lizards. Other than that, all vertebrates have red blood, and that includes people. Green-blooded Mr. Spock, of course, is both fictional and half alien (which begs a host of genetic problems I shall not dwell on here).

So why has the mistaken belief persisted for so long that we humans have both blue and red blood? The myth began back in the days of landed European aristocracy. In a time when it was seen as a sign of low status to be a laborer, and only people who had to work for a livingtending farms, erecting buildings, making roadsgot significant exposure to the sunlight. Having a tan was a sign of ones lower societal station. How very different today, where tans are associated with leisure and vacation time in warm climes! As for the blue blood, one need look at the blood vessels as seen in thin, pale areas of a light-skinned persons body. The vessels do indeed appear blue, and the effect is much more pronounced in those alabaster-skinned damsels of romance novels. Alas, the blue coloring is really the reflected light bouncing off of the vessel walls and skin, not the blood within. But in stratified European society, the lighter ones complexion, the further removed one was from labor, and hence the bluer ones blood.

If we could actually peer into a human body, we would see only a very subtle difference in blood color dependent upon where the blood was going. Blood that leaves the heart and brings oxygen to the body is bright red and travels in arteries. That blood then picks up carbon dioxide and travels back to the heart through veins. This returning blood is darker, some might say maroon, others brick, but still decidedly red.

To mosquitoes, though, blood by any shade is a meal.

Dr. Sprackland is the Director of the Virtual Museum of Natural History at www.curator.org.