The neglected cousins: What do we know about the smaller arachnid orders?

An overview of the systematics of smaller arachnid orders (Opilioacariformes, Ricinulei, Palpigradi, Uropygi, Amblypygi, Schizomida, Solifugae and Pseudoscorpiones) is provided, along with data on numbers of recognized families, genera and species for each group. The micro-diverse orders, Opilioacariformes (1 family, 9 genera, 19 species), Ricinulei (1 family, 3 genera, 55 species), Palpigradi (2 families, 6 genera, 78 species), Uropygi (1 family, 16 genera, 103 species), Amblypygi (5 families, 17 genera, 136 species) and Schizomida (2 families, 34 genera, 205 species), are amongst the smallest of all terrestrial arthropod orders. The meso-diverse orders, Solifugae (12 families, 140 genera, 1,087 species) and Pseudoscorpiones (24 families, 425 genera, 3,239 species) - along with the Scorpiones (1,279 species) and Opiliones (c. 6,000 species) which are not dealt with in this contribution - are dwarfed by the three mega-diverse arachnid orders, Araneae (c. 36,000 species), Parasitiformes and Acariformes (with a combined total of c. 48,000).