Arthropod Relatives

This subcategory groups arthropods that fall outside the atlas's existing insect, arachnid, crustacean, and mollusk subcategories: horseshoe crabs and sea spiders from the chelicerate lineage that also includes arachnids, and myriapods such as millipedes and centipedes, along with non-insect hexapods such as springtails. Horseshoe crabs are living fossils little changed in body plan for hundreds of millions of years, using book gills to breathe and blue, copper based blood to fight infection. Millipedes and centipedes both have many body segments each bearing legs, but centipedes are fast moving predators with a venomous first pair of legs while millipedes are slower detritus feeders with two leg pairs per segment. Springtails are tiny, widespread soil and leaf litter animals related to but distinct from true insects.