Micro whipscorpions (Palpigradi)

These small (1-3 mm) delicate arachnids are found in tropical leaf litter and caves. They have a thin, pale, segmented integument, and a segmented abdomen that terminates in a whip-like flagellum. Fewer than 100 species have been described. 

Hooded tickspiders (Ricinulei)

These small (3-10 mm) cave and soil-dwelling arachnids can be found in the African and New World tropics. The male third leg is modified as a sperm transfer organ. Fewer than 80 species have been described. 

Short-tailed Whipscorpion (Schizomida)
These small (3-13 mm) tropical soil and cave-dwelling arachnids are moderately fast runners. Reproduction is via a spermatophore attached to the substrate. More than 300 species have been described. 

Windscorpions (Solifugae)

Also known as camel spiders, sun spiders, and solifuges, among other names, these nocturnal, fast-running hunters are notable for their large chelicerae. They occur mostly in arid habitats. They range in size from 20-70 mm. There are more than 1,100 described species. 

Whip Scorpions (Thelyphonida)

These are robust tropical predators with raptorial pedipalps and a long whip-like tail. They possess anal glands from which they spray defensive chemicals including acetic acid. Because of this, they are also known as vinegaroons. They range in size from 25-85 mm. There are about 100 species described. 

Mites and Ticks (Acari)

With more than 50,000 described species, this is the most diverse and abundant arachnid order. Large numbers of species remain undescribed. They are ubiquitous in terrestrial habitats and include vectors of animal and plant diseases, and parasitic forms. The monophyly of mites is far from certain, with 2-3 distinct lineages. They range in size from less than 0.1 to 16 mm.