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Animals use blood to transport various elements, such as water and food, throughout the body. In human beings, blood is also used to carry oxygen from the lungs to the tissues. The oxygen is attached to a substance called hemoglobin, which has a characteristic red color. This is why our blood is red.
Insects have blood too, but it is not used to carry oxygen. Instead, oxygen is distributed via an elaborate system of tubes and air sacs. Thus, insect blood does not have hemoglobin and is not red. In general, insect blood is either colorless, pale yellow or green.