The class Arachnida includes the spiders, mites , ticks, scorpions and related organisms. Members of the class Arachnida are generally characterized by having two body parts. Their head and thorax are fused into a structure called the cephalothorax. There are four pairs of segmented legs attached to this cephalothoracic region. The second body region of the Arachnida is the abdomen. Arachnids do not have antennae (have you ever seen antennae on a spider?). Most spiders and scorpions are predatory species. The feeding habits of mite and tick species range from blood feeding on vertebrates, to plant feeders, to predatory species that feed on other mite species. Most arachnids are terrestrial (as opposed to aquatic).