This document summarizes key facts about arachnids. It notes that arachnids have four pairs of legs and an external skeleton. They are found in many habitats around the world, both aquatic and terrestrial. Their favorite activities involve hunting and trapping insects in webs. Adaptations for terrestrial living include modifications to their respiratory, circulatory, and excretory systems. Their appendages are also adapted for more efficient walking. When selecting mates, spiders prefer those with many legs, an active daytime schedule, and a similar body size.