Molting Spider

Arachnids lack some features of other chelicerates, including backward-pointing mouths and gnathobases ("jaw bases") at the bases of their legs; both of these features are part of the ancestral arthropod feeding system. Instead they have mouths that point forwards and downwards, and all have some means of breathing air. Spiders (Araneae) are distinguished from other arachnid groups by several characteristics, including spinnerets and, in males, pedipalps that are specially adapted for sperm transfer.
Taxonomy
Spiders are divided into two sub-orders, Mesothelae and Opisthothelae, of which the latter contains two infra-orders, Mygalomorphae and Araneomorphae. Over 40,000 living species of spiders (order Araneae) have been identified and are currently grouped into about 110 families and about 3,700 genera by arachnologists.