What distinguishes arachnids from insects?

Arachnids are distinct from insects primarily due to their anatomical structure, particularly the number of legs they possess. They typically have four pairs of legs, which amounts to eight legs in total. This characteristic is one of the key identifiers in distinguishing arachnids, such as spiders and scorpions, from insects, which have only three pairs of legs, totaling six.

In addition to leg count, arachnids usually exhibit two body regions: the cephalothorax and the abdomen, but this does not specifically set them apart in a quantifiable way as the leg count does. The presence of wings in arachnids is non-existent, as they do not possess wings at any stage of their life cycle unlike some insects, which have varying forms of wings. Therefore, the defining trait of arachnids having four pairs of legs highlights a fundamental difference in body structure that classifies them separately from insects.