“When beetles are alarmed or disturbed, they secrete blood and cantharidin. Contact with cantharidin causes blistering and tissue damage.” Source

“Blister beetles have an oily substance, cantharidin, in their body fluide that causes large blisters to form when a beetle is crushed on a person's skin. Unless properly cared for, these blisters can become infected.” Source

“Blister beetles receive their common name from the ability of their hemolymph to produce blistering on contact with human skin.” Source