The blood of flies, like many insects, is not actually red. Instead, it is typically a clear or pale yellowish color. Insects don’t have blood in the same way that vertebrates do; they have a fluid called hemolymph, which serves a similar purpose to blood but does not contain hemoglobin. Hemolymph is responsible for transporting nutrients and waste throughout the insect’s body.
Unlike vertebrate blood, which gets its red color from the iron-containing protein hemoglobin, the hemolymph of flies contains different proteins and compounds that do not color it red. This is why, when you see flies or other insects, their blood does not exhibit the familiar reddish hue that we associate with the blood of mammals or birds.