Many species are parasites or are specialised to live in one or a few hosts. A single species of tropical tree is, on average, a host to 162 host-specific beetle species. Since there are 50,000 tropical tree species, and beetles amount to 40% of total insect species, and there are also tree-specific species below the canopy, it is possible to estimate the total number of arthropod species living in tropical forests. The number is 30 million. This contrasts rather strongly with the total of 1.4 to 1.8 million species which have already been described. It seems textbooks have underestimated the number of species in existence by a factor of about 20.