Arachnids, crustaceans, and insects do not contain blood but hemolymph instead. In the case of insects, hemolymph does not play an important role in oxygen transport. Some common uses are: creating a hydraulic system, autohemorrhaging, and gluing themselves to other animals. – WTF Fun Facts
A new study found that Horseshoe Crabs are a member of the arachnid family tree and are genetically more closely related to spiders than to other crabs.