|MadSci Network: General Biology|
The color of blood differs according to the presence or absence of respiratory pigments and the type of pigment used by an organism. Animals use the respiratory pigment hemoglobin. Hemoglobin transports oxygen (or carbon dioxide) using iron, thus animals have blood that is colored red. Many arthropods and some molluscs use hemocyanin which gives their blood a faint bluish color due to the presence of copper. Insects on the other hand, do not use respiratory pigments to transport oxygen, rather they have branching tubes called trachae that suck in air. Insect blood, therefore, does not have a color due to a respiratory pigment. The color of insect blood, colorless, yellow, or green, likely comes from the pigments of food the insect eats (e.g. chlorophyll in plants). I hope this answers your question!
Try the links in the MadSci Library for more information on General Biology.