What does arachnida mean?
The definition of arachnida
A*rach”ni*da, n. pl.
Etym: [NL., fr. Gr. (Zoöl.)

One of the classes of Arthropoda. See Illustration in Appendix.

Note: They have four pairs of legs, no antennæ nor wings, a pair of mandibles, and one pair of maxillæ or palpi. The head is usually consolidated with the thorax. The respiration is either by trancheæ or by pulmonary sacs, or by both. The class includes three principal orders: Araneina, or spiders; Arthrogastra, including scorpions, etc.; and Acarina, or mites and ticks.