The largest phylum in the animal kingdom, including the well-known insects, spiders, ticks, and crustaceans, as well as many smaller, less well-known groups, and an abundance of strange forms known only as fossils. Arthropods make up about 95% of all species of animals. The estimated number of known species exceeds 1 million; of this number, most are insects. No one really knows the exact number of arthropod species. Some authorities estimate that it may range as high as 10 million.