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Although the word "arachnid" is often considered synonymous with "spider," spiders are far from the only arachnid lineage. The class Arachnida includes 11 diverse sets of animals, 10 of which aren't Araneae, i.e., spiders. Three orders—Acari, Scorpiones and Opiliones—include familiar animals like mites, ticks, scorpions, and daddy longlegs.
Arachnids, Araneae and Beyond
Spiders, members of the class Araneae, are the largest group of arachnids, but they've got plenty of peers. All arachnids have six pairs of appendages, four of which are legs, no backbone and two body segments, one of which is a fused head and thorax. Arachnids are distributed across every continent except Antarctica, and appear in the fossil record more than 400 million years ago, which puts them among the first animals to make the transition from water to land. Eleven orders of arachnids are around today: Acari, Amblypygi, Araneae, Opiliones, Palpigradi, Pseudoscropionida, Ricinulei, Schizomida, Scorpiones, Solifugae and Thelyphonida. Three additional orders are extinct: Haptopoda, Phalangiotarbida and Trigonotarbida. Three of the extant orders—Acari, Scorpiones and Opiliones—represent some of the most well-known non-spider arachnids.
Acari, the Ticks and Mites
It's hard to tell how many species of arachnids exist, in part because the taxonomic systems are always changing and in part because some species are so small and have yet to be documented. The second-largest documented group, behind spiders, is Acari, which includes mites and ticks. These animals have colonized nearly every land and marine environment and climate. Some of them eat plants or other small animals, while others act as parasites of plants or other larger animals. Many mites and ticks have a complex symbiotic relationship with the organisms on which they live. Some also spread disease. Most members of Acari are very small—mites, which are smaller than ticks, can be as little as 1/64 inch—and lack the primary body segmentation present in other arachnids.
Scorpiones, the Scorpions
Members of the arachnid order Scorpiones are scorpions. Their primary physical distinctions include claws and a 12-segment body that includes an upwardly curved tail. Scorpions live around the world, mostly in warm, dry regions and the tropics. Most range from half an inch to 10 inches long. The business end of their tail has a stinger that they use to instantly kill their prey—insects and spiders—which they hunt at night. They hide in burrows or under rocks during the day. Scorpions give birth to live young who are born one at a time over a period of weeks and cling to their mother's back until their first molt. All of them glow under UV light. Multiple species of arachnids look like true scorpions but in fact belong to other orders.
Opiliones, the Harvestmen and Others
One of the most recognizable arachnids that's not a spider looks an awful lot like a spider: harvestmen, or daddy longlegs. These animals are members of the arachnid order Opiliones who live in temperate and tropical regions on every continent except Antarctica. The long, stilt-like legs of harvestmen cause many to mistake them for spiders, but they lack the ability to spin silk, have a less distinct waist between their primary body segments, and don't produce venom. Most eat insects and plants. Some Opiliones are likewise dull-colored but have short legs. Others are brightly colored and have bizarre eye decorations. Despite a persistent urban legend, harvestmen aren't poisonous and can't bite people. Some of them can produce a nasty-smelling substance, however, when they're disturbed.
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