The blue liquid seen coming out of certain bugs is blood. The color of the blood is determined by the metallic content. Mammals generally have iron-rich blood, which causes it to turn red when exposed to oxygen. Many insects and spiders, however, have blood that is known as hemocyanin, which uses copper to transport oxygen about their bodies and creates a blue tone when it is exposed to the air. There are several prime examples of insects that will bleed blue when squashed or cut.
The horseshoe crab is an underwater Arthropod -- the family group that includes insects, spiders and lobsters. The horseshoe crab crawls along the sea floor using its leg-like appendages, gathering and eating marine clams and worms. Because the horseshoe crab has copper-laden blood, its blood will appear blue when lost from its body.
The emperor scorpion is the one of the world's largest scorpions and, although a toxic stinging insect, a popular pet. An adult emperor scorpion averages 7.9 inches in length and will live up to 8 years in captivity. Under ultra-violet light, the nearly pitch-black scorpion will turn a glowing blue. The emperor scorpion, as an Arthropod, transports oxygen around its body through hemocyanin. The copper cofactor in hemocyanin will cause the blood to turn blue when exposed to the air.
The pillbug is a small, land insect very closely related to crustaceans such as shrimp. The pillbug is commonly referred to as the roly poly, potato bug or armadillo bug. When threatened, the pillbug will roll into a small ball, which serves as an easy identifier of the bug. Uncommonly for animals on land, the pillbug breathes through gills and transports oxygen through its body by means of copper-rich blood, which gives the blood a blue shade.
A tarantula is any of a group of large, hairy arachnids, or spiders. The tarantula, like the emperor scorpion, is popularly kept as a pet. Also like the emperor scorpion and other Arthropods, the tarantula has blue blood. The blue coloration of the blood is caused by the pigmentation of the oxygen-carrying portion of the blood. In this case, that is copper-rich hemocyanin that will cause the tarantula to bleed blue.