Why is the blood of a cockroach white?
Male cockroaches have colorless blood, while the female cockroaches may occasionally have orange blood. This is due to the absence of hemoglobin in their blood (the same cell that makes human blood red). Human beings use hemoglobin to carry oxygen, but cockroaches do not breathe the way we do. Cockroaches use a series of tubes called tracheae for breathing (and exhaling) and in fact they can breathe even in absence of a head. Cockroaches, like other insects, have an open circulatory system and their blood is also known as hemolymph (or haemolymph). It flows freely inside the body, touching all the internal organs and tissues. About 90% of this blood is watery fluid and the remaining 10% is made up of hemocytes. The oxygen is delivered by the tracheal system and not the circulatory system in a cockroaches (or most of the other insects).
Cockroaches have white blood because cockroaches lack hemoglobin in their blood. Hemoglobin is primarily made up of iron and is what gives human blood its red color. Cockroaches don't have blood vessels like a mammal does, and their blood is not used to carry oxygen. Instead, they have an open circulatory system, with body cavities full of hemolymph (the insect version of blood). Hemolymph (blood) is not responsible for carrying oxygen like ours is. Instead of oxygenating blood with their lungs and then dispersing it throughout the body like ours, cockroaches "breathe through their skin;" they have a system of tubes, called tracheae, that deliver oxygen throughout the body. The tracheae are oxygenated through special pores on the cockroach's skin. Hemolymph also is important for the cockroach's immune system because of cells called hemocytes responsible for protecting the roach from pathogens. About 90% of hemolymph is plasma: a watery fluid; usually clear, but sometimes greenish or yellowish in color.