Arachnids are a class of joint-legged invertebrates that include creatures such as spiders, scorpions, ticks, and mites. They typically have eight legs and a body divided into two main parts: the cephalothorax and the abdomen. Unlike insects, which have six legs and three body segments, arachnids do not have antennae. Most arachnids are terrestrial and can be found in various habitats, from forests to deserts. They play important roles in ecosystems, often serving as predators that help control insect populations. Some arachnids, like ticks, can be parasites, feeding on the blood of other animals.