Those found in the southern USA are considerably smaller and are generally secretive. This familiar and highly successful group certainly owes much of its success to its extensive use of silk and poison. Although most species do possess poison, the vast majority are not dangerous to humans and spiders are certainly among the most important animals in controlling insect populations. For instance, considerable research is being done on ways to manage crops so as to encourage spiders as an important means of pest control. Although all spiders use silk, not all build webs to capture their prey.
Harvestmen move about deliberately unless alarmed.
Many species are omnivorous. Ticks and mites vie with spiders as the arachnid order with the most species. They are world-wide in distribution and are found in nearly every imaginable terrestrial habitat and even some aquatic environments. However, be aware that a great many species are not parasitic. Acarology is often considered a separate discipline from the rest of arachnology although mites and ticks are certainly arachnids.