Any member of the insect order Coleoptera, with at least 250,000 species (the largest order in the animal kingdom), characterized by their special forewings, which are modified into hardened wing covers (elytra) that cover a second pair of functional wings. The order includes some of the largest and smallest insects and is the most widely distributed insect order. Beetles can be found in all environments except Antarctica and the peaks of the highest mountains. Most feed either upon other animals or upon plants, but some eat decaying matter. Many beetles are very dependent on particular features of their environment; some, for example, live only in the bark of a particular tree. It is this particularity that makes beetles useful for reconstructing ancient environments. Parts of the tough beetle exo-skeleton may be well-preserved in acidic or waterlogged conditions (as in peats silts and lake clays). The temperature preferences of beetles may be determined from the fossils making it possible to reconstruct climatic changes. Beetles can also be used to investigate changes in vegetation living conditions and food-storage problems.