Acarines live in practically every habitat, and include aquatic (freshwater and sea water) and terrestrial species. They outnumber other arthropods in the soil and detritus. Many are parasitic, and they affect both vertebrates and invertebrates. Most parasitic forms are external parasites, while the free living forms are generally predators and may even be used to control undesirable arthropods. Others help to break down forest litter and dead organic matter such as skin cells. Others still are herbivorous and may damage crops. Damage to crops is perhaps the most costly economic effect of mites, especially by the spider mites and their relatives, earth mites, thread-footed mites and the gall and rust mites.