The 65,000 species of arachnida include spiders, scorpions, ticks, mites and many other species. Because of the difficulty of observing their activities, their importance to man may easily be underestimated, particularly in terms of their function in controlling the size of insect populations. It has been estimated that an acre of woodland in the temperate zone may well contain 14,000 spiders. It is not known how many arachnid species are endangered by man's impact on the natural environment.