1. Terrestrial Arthropods have adaptations for life on land, such as trachea or book lungs for breathing air

2. The body is composed of an unsegmented anterior region (prosoma), with a pair of Chelicera, a pair of leglike appendages (pedipalps), four pairs of walking legs, and a posterior region (opisthosoma); it is equipped with book lungs or tracheae, for respiration

3. Medical Definition of Arachnida : a large class of arthropods that are mostly air-breathing by means of trachea or book lungs, that include the spiders and scorpions, mites, and ticks, and that have a segmented body divided into two regions of which the anterior bears four pairs of legs but no antennae Other Words from Arachnida