A Brief Story of Insect Blood

The blood or haemolymph of blister beetles is similar to that of other insects in that it contains nutrients (taken from the food that the beetles eat) that are carried around to the various organs within the beetle's body where they are used to run the internal parts of the bug. The organs also dump their waste products into the blood, which carries them to a special organ that removes these unwanted materials so they can be excreted by the beetle. 

What's so Bad About Blister Beetle Blood?

The blood of blister beetles is unusual in that it contains the chemical cantharidin, a very toxic, nasty chemical that protects the beetle from some animals that would kill and eat the insect. Males manufacture their own cantharidin in large amounts but females cannot make their own protective chemicals.  Females do not need to do so because when they mate, the male transfers his cantharidin to his mate, which she then gets to use for herself and also to cover her eggs to protect them from being eaten by predators.

Besides blister beetles, there are a few other insects that make cantharidin. The false blister beetles and the cardinal beetles have cantharidin in their haemoloymph. All of these insects are great to observe, but should not be touched.